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A Workshop Agenda and Abstracts WORKSHOP AGENDA REDUCING STRESS FRACTURE IN PHYSICALLY ACTIVE YOUNG SERVICEMEMBERS A Symposium Sponsored by Committee on Body Composition, Nutrition, and Health of Military Women Food and Nutrition Board December 10, 1997 National Academy of Sciences Auditorium Washington, D.C. Agenda 8:30 A.M.–8:40 A.M. Welcome on Behalf of the Committee on Body Composition, Nutrition, and Health of Military Women Barbara O. Schneeman, Chair
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8:40 A.M.–8:50 A.M. Welcome on Behalf of the Food and Nutrition Board; Allison A. Yates, Director, Food and Nutrition Board 8:50 A.M.–9:00 A.M. Welcome on Behalf of the Military; LTC Karl E. Friedl, U.S. Army Medical Research and Materiel Command, Fort Detrick, Frederick, MD I. Stress Fracture Incidence In Military Training 9:00 A.M.–9:30 A.M. Stress Fracture among Physically Active Women in the General Population; Peter Brukner, Stanford University, Stanford, CA 9:30 A.M.–9:50 A.M. Physical Training Interventions to Reduce Stress Fracture Incidence in Navy and Marine Corps Recruit Training; CDR Richard A. Shaffer,* Naval Health Research Center, San Diego, CA 9:50 A.M.–10:10 A.M. Rehabilitation of Stress Fractures in Army Basic Trainees; CPT Paul Durant Stoneman, Fitness Training Company, Fort Jackson, SC 10:10 A.M.–10:25 A.M. Part I Panel Discussion; Moderated by Anne Looker 10:25 A.M.–10:40 A.M. Break II. Body Composition (Weight, Bone Mineral Content, Muscle Mass), Genetics, And Stress Fracture 10:40 A.M.–10:10 A.M. Procollagen Gene Mutations as a Predisposing Factor for Stress Fracture; Eitan Friedman, * Chaim Sheba Medical Center, Tel-Hashomer, Israel 11:10 A.M.–11:30 A.M. Structural Indices of Stress Fracture Susceptibility in Female Military Recruits; Thomas J. Beck, * The Johns Hopkins University Outpatient Center, Baltimore, MD * Recipients of Defense Women's Health Research Program grants.
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11:30 A.M.–11:50 A.M. Quantitative Ultrasound and Other Risk Factors for Stress Fracture during Basic Training in Female U.S. Army Recruits; Donald B. Kimmel,* Merck Research Laboratories, West Point, PA 11:50 A.M.–12:05 P.M. Part II Panel Discussion; Moderated by Steven B. Heymsfield 12:05 P.M.–1:00 P.M. Lunch available in refectory III. Diet And Physical Activity 1:00 P.M.–1:30 P.M. Calcium Intake and Exercise Level: Synergistic Effects on Bone; Bonny L. Specker, South Dakota State University, Brookings 1:30 P.M.–2:00 P.M. Calcium and Iron: Foods vs. Supplements; Connie M. Weaver, Purdue University, West Lafayette, IN 2:00 P.M.–2:20 P.M. Dietary Calcium and Related Nutrient Intakes in Military Men and Women; LTC John P. Warber, U.S. Army Research Institute of Environmental Medicine, Natick, MA 2:20 P.M.–2:40 P.M. Effects of Prolonged Inactivity on the Musculoskeletal System with Evaluation of Countermeasures; Steven R. Smith, Pennington Biomedical Research Center, Baton Rouge, LA 2:40 P.M.–2:55 P.M. Part III Panel Discussion; Moderated by Nancy F. Butte 2:55 P.M.–3:05 P.M. Break IV. Hormonal Function (Amenorrhea, Pregnancy) And Bone Health 3:05 P.M.–3:35 P.M. Effect of Modulators of Bone Turnover on Changes in Markers of Bone Turnover; Michael Kleerekoper, Wayne State University, Detroit, MI 3:35 P.M.–4:05 P.M. IGF-1, Muscle Mass, and Bone Density; Clifford J. Rosen, St. Joseph Hospital and Maine Center for Osteoporosis Research and Education, Bangor
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4:05 P.M.–4:25 P.M. Dietary Energy Requirements in Physically Active Men and Women: Threshold Effects on Reproductive Function; Anne B. Loucks,* Ohio University, Athens 4:25 P.M.–4:45 P.M. Fitness, Bone Density, and Injury and Illness in Postpartum Soldiers; COL Joseph R. Dettori,* Madigan Army Medical Center, Tacoma, WA 4:45 P.M.–5:00 P.M. Part IV Panel Discussion; Moderated by Gail E. Butterfield 5:00 P.M.–5:30 P.M. General Discussion; Moderated by Barbara O. Schneeman 5:30 P.M. Closing Remarks; Barbara O. Schneeman Reception for committee and military liaison panel members and speakers immediately following in the Rotunda Dinner for committee and military liaison panel members and speakers following reception in the Members Room Presentation: The Art and Science of Longitudinal Studies of Healthy Young People; Tom Lloyd, The Pennsylvania State University College of Medicine, Hershey Medical Center
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WORKSHOP ABSTRACTS The abstracts appear in the order in which they were presented during the workshop on "Reducing Stress Fracture in Physically Active Young Service Members" which was held on December 10, 1997, in Washington, D.C. STRESS FRACTURE AMONG PHYSICALLY ACTIVE WOMEN IN THE GENERAL POPULATION Peter Brukner, M.B., B.S., FACSP, FACSM, Stanford University, Stanford, CA 94306-6175 and Olympic Park Sports Medicine Centre, Melbourne, Australia It is often suggested that women sustain a disproportionately higher number of stress fractures than men. Military studies consistently show that female recruits have a greater risk of stress fracture than to male recruits, with relative risks ranging from 1.2 to 10. This finding of an increased risk in women persisted even when training loads were applied gradually to a moderate level and when age and race were controlled. Possible reasons for a gender difference in stress fracture risk include differences in bone density, bone geometry, gait, biomechanical features, body composition, and endocrine factors, particularly estrogen status. In contrast, a gender difference in stress fracture rates is not as evident from athletic studies. These either show no difference between male and female athletes or a slightly increased risk for women, up to 3.5 times that of men. It is possible that a gender difference in stress fracture risk is lessened in athletes, as female athletes may be more conditioned to exercise than female recruits. Twin studies have indicated that up to 90 percent of the variation in bone mass can be attributed to genetic factors. A significant relationship between a family history of osteoporosis and yearly change in bone density has been demonstrated in runners and nonrunners. Although it is feasible that some individuals may be genetically predisposed to stress fractures when exposed to suitable environmental conditions, such as vigorous exercise, there are few data to evaluate the role of genetic factors in predisposing an athlete to this injury. Results of studies investigating the relationship between bone density and stress fracture risk have been contradictory. In a 12-mo prospective cohort study, female athletes who sustained tibial stress fractures had 8.1 percent less bone mass at the tibia/fibula. This finding was not evident for the male athletes. A large prospective study in the Israeli army and a cross-sectional study in athletes failed to corroborate a relationship between lower limb bone density and stress fractures. Dietary factors, in particular calcium levels, may contribute to the development of stress fractures via influences on bone density and bone remodeling. In animal studies, a calcium-deficient diet decreased the ability of bone to adapt to mechanical strain, while high dietary calcium intake had a favorable effect on bone biomechanical properties. In humans, some studies have found a positive relationship between dietary calcium intake and bone mass, while others have noted small gains in bone mass resulting from calcium supplementation. There is conflicting evidence to show that low calcium intakes are associated with an increased risk for stress fracture in athletes. Other compounds such as protein, total energy, phosphorus, fiber, sodium, alcohol, and caffeine could potentially affect bone health and therefore stress fracture risk. At present, no associations have been found between these and the incidence of stress fractures in athletes. Dietary behaviors and eating
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patterns may differ in those with stress fractures. Studies have suggested that disordered patterns of eating may be associated with a higher risk for stress fracture. Whether this association is causal or due to some other factor is not clear. Anthropometric characteristics, such as height and weight, and soft tissue composition, such as lean mass and fat mass, could theoretically affect stress fracture risk directly by influencing the forces applied to bones or indirectly via effects on bone density and menstrual function. No studies have compared muscle mass or muscle strength, particularly peak force production and fatigability, in athletes with and without stress fractures. No studies have reported differences in height, weight, body mass index, or fat mass among athletes who have and have not sustained a stress fracture. PHYSICAL TRAINING INTERVENTIONS TO REDUCE STRESS FRACTURE INCIDENCE IN NAVY AND MARINE CORPS RECRUIT TRAINING CDR Richard A. Shaffer, MSC, USN, Ph.D., M.P.H., Naval Health Research Center, San Diego, CA 92186 Introduction Stress fractures during military training have a high fiscal and operational impact in Navy and Marine Corps populations. The incidence of reported stress fractures in males ranges from 0.2 percent in Navy recruits to 4.5 percent in Marine Corps recruits. The reported female incidence ranges from 0.7 percent in Navy recruits to 9.6 percent in Marine officer candidates. The estimated annual impact among 2,000 female Marine recruits is $1,850,000, with 4,120 lost training days. Ongoing Research Two studies are currently under way among female Marine Corps recruits to predict and prevent stress fractures during training. The goal of the first study is to develop and evaluate modifications to the physical training curriculum to reduce stress fractures. The second study is a collaborative Defense Women's Health Research Project (DWHRP) effort with The Johns Hopkins University to develop structural indices of stress fracture susceptibility. Both of these studies were follow-on studies to an initial DWHRP effort to determine risk factors for stress fracture among female recruits. Conclusions Reported stress fractures have a higher impact on female training programs than on male programs. A significant factor affecting the increased rate of documented stress fractures in military women is differences in symptom reporting. Approximately 50 percent of stress fractures in military women are located on the femur and pelvis. These fractures result in greater rehabilitation time, disability, and operational costs compared to stress fractures below the knee.
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Stress fractures among women during military training are strongly associated with decreased levels of pretraining fitness and amounts of physical activity. Other specific risk factors are currently under investigation in these populations. In male military recruit populations, stress fractures have been reduced by 50 percent, without a decrease in graduation fitness, through the implementation of a safe and effective physical training curriculum. This curriculum was based on the fitness and activity attributes of the incoming recruit population. In female Navy recruits, a similar curriculum was designed that resulted in a 49 percent decrease in lower extremity overuse injuries. Physical training modifications, based on identified risk factors and military training goals, are under development for female Marine Corps recruits. STRESS FRACTURE EXPERIENCE AT FORT JACKSON CPT Paul Stoneman, M.S., USA, Fitness Training Company, 120th Adjutant General Battalion, Fort Jackson, SC 29207 Introduction Fort Jackson is the largest integrated Army basic combat training (BCT) site. Over 40,000 soldiers were trained at Fort Jackson in FY1997. The physical demands of BCT can result in injury. The stress fracture is of particular concern because that type of injury has prolonged healing time, severely restricts the training ability of the injured soldier, and creates a host of administrative problems for the command. Physical Training and Rehabilitation Program In 1995, a Physical Training and Rehabilitation Program (PTRP) was created at Fort Jackson to help deal with the problem of soldiers injured in BCT who require extended recovery and rehabilitation. The majority of these soldiers have stress fractures of the lower extremities. Injured soldiers are pulled from the training unit and reassigned to PTRP for rehabilitation until they either return and complete BCT or are separated from the Army. Experience and Interesting Observations Our experiences at PTRP agree with the general findings of studies of injury patterns in military trainees: Stress fractures require prolonged recovery, often 60 days or more. Women are more at risk for injury, including stress fractures. Women are slightly less likely to recover and graduate from BCT once injured.
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Interventions Recently at Fort Jackson, a task force was formed to investigate injury prevention and management. Another group looked at the remedial physical training policy. A database is being developed to help track injuries sustained in BCT. Other actions include a trial of placing equipment, such as stationary cycles and strength-training equipment, in the training units. This equipment will be used by soldiers with specific needs, such as the less-fit soldier or the injured soldier. It has been shown that less-fit soldiers, either male or female, are more likely to be injured. Perhaps having such equipment in the training units will help the injured stay fit, return to training quicker, and provide resources for "cross-training" of the less-fit soldier. IS THERE A GENETIC BASIS FOR STRESS FRACTURES? Eitan Friedman, M.D., Ph.D.; Liat Ries, M.Sc.; Galia Yablonski-Gat, Ph.D.; Iris Vered, M.D.; Uri Givon, M.D.; and Joshua Shemer, M.D., The Suzanne Levy Oncogenetics Unit and the Endocrine Institute, Sheba Medical Center and the Medical Corps, IDF, Israel Several lines of indirect evidence made it plausible (and testable) that in a subset of soldiers with stress fractures, there may be a genetic component. We initially hypothesized that subtle mutations in one of the two genes coding for procollagen type 1 genes that underly Oseteogenesis imperfecta may predispose to stress fractures. Under normal circumstances, these mutations would not result in any phenotype but manifest as stress fractures given the special workload placed on young, training soldiers. Several other genes may also be considered as candidate genes to be involved in the genetic predisposition to stress fractures, primarily those that are involved in pathological bone conditions and osteoporosis: vitamin D receptor, estrogen receptor, and calcium sensing receptor. This candidate gene approach was chosen for the lack of known multigenerational families with a definable pattern of stress fractures inheritance. To test this hypothesis, IDF soldiers with clear evidence of high-grade stress fractures were identified. All pertinent data from these individuals (clinical, epidemiological, biochemical analysis of bone turnover parameters, and bone scan data) were obtained. In addition, constitutional DNA was extracted from peripheral blood leukocytes and analyzed for the existence of mutations within the above mentioned candidate genes by polymerase chain reaction (PCR) amplification, denaturing gradient gel electrophoresis (DGGE), DNA sequencing, and restriction enzyme digest of the relevant PCR products. As controls, two groups of soldiers were used: (1) a symptomatic control group composed of age-matched controls with no objective evidence of stress fractures, and (2) unit and ethnicity-matched controls at the final stages of their basic training who were totally asymptomatic. The latter group completed the detailed questionnaire but underwent no bone scan or DNA testing. Results of the epidemiological and biochemical analyses will be presented in brief. Informed consent was given by 389 individuals, and blood was withdrawn from for DNA extraction. DNA was extracted from 200 soldiers. PCR amplification and DGGE analysis of 11 exons of the COL1A1 (exons 3, 4, 6, 8, 11, 12, 13, 14, 15, 16, 50) and 7 of the COL1A2 (exons 6, 10, 11, 12, 27, 29, 33) were done on all 200 samples. During these analyses, an abnormal migration pattern was detected in six samples in exon 12 of the COL1A2 gene and found to be a base change that could lead to an alternative splicing. Moreover, a mutation within a glycine
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residue was detected in a single patient with osteogenesis imperfecta (OI). In addition, 160 soldiers underwent the vitamin D receptor analysis, and 80 soldiers underwent calcium-sensing receptor and an estrogen receptor novel polymorphism. The results of these genetic analyses, including statistical significance workup, were presented and discussed as a more comprehensive method of evaluating of the genetic component to stress fractures. STRUCTURAL INDICES OF STRESS FRACTURE SUSCEPTIBILITY IN FEMALE MILITARY RECRUITS T.J. Beck,* C.B. Ruff, R.A. Shaffer, K. Betsinger, D.W. Trone, and S. Brodine Introduction Intense military physical training subjects the bones of the lower limb to repetitive bending and torsional stresses that can lead to failure or stress fracture. Because not every military recruit suffers a stress fracture during training suggests that individuals with stress fractures have weaker bones. From an engineering standpoint, this means either that bones of fracture cases bend and twist more easily in training (i.e., producing greater mechanical stresses) or that their bones are made of a weaker material that fails more easily. Studies of Israeli Army (Milgrom et al., 1989) and male U.S. Marine Corps recruits (Beck et al., 1996) support the hypothesis that stress fracture susceptibility is due to geometric factors that determine mechanical stress magnitudes. In this study, dual energy x-ray absorptiometry (DXA) methods were used in male Marine Corps recruits (Beck et al., 1996) to determine whether the same bone geometric factors determine susceptibility in female Marine Corps recruits. Muscle strength may also be a factor in stress fracture susceptibility, since contraction of certain muscle groups tends to resist bending and twisting of long bones protectively. DXA scanners can also measure muscle mass; hence, the method was extended to determine if lower muscle mass was a contributing factor, since weaker muscles may compromise protection by fatiguing more easily. Materials and Methods A total of 671 female U.S. Marine Corps recruits at the Parris Island Recruit Depot were enrolled prior to onset of training. Using a Norland XR26 scanner (Norland Inc., Fort Atkinson, Wis.), DXA scans were obtained at the mid femur and distal third of the lower leg of the right side. A series of anthropometric measurements was also obtained that included height; weight; body mass index; lengths of the thigh and tibia; and girths of the neck, waist, hip, and thigh, as well as breadths of the pelvis, hips, and bicondylar dimensions at the knee. Using programs described previously (Beck et al., 1996), DXA data were used to derive cross-sectional areas (CSA), moments of inertia (CSMI), and section moduli (Z) at scan locations in the femur, tibia, and fibula. In addition, the whole bone strength index (Selker and Carter, 1989) was calculated for each bone as the ratio of Z to bone length. Also, since critical failure may be related to cortical thickness, an estimate of mean cortical thickness * The Johns Hopkins University Outpatient Center, Baltimore, MD 21287-0849
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was computed. The soft tissue of the thigh within the femur scan region was employed to obtain a measurement of relative lean muscle mass using standard Norland software. Recruits were followed through the 12-wk training to ascertain stress fractures using standard diagnostic criteria. Results A total of 36 recruits (5.2 %) suffered stress fractures: 13 in the foot, 10 each in the pelvic girdle and lower leg, and 9 in the femur. Of the main three ethnic groups, equivalent stress fracture rates were observed among Whites and Hispanics (−6.5 %), while the rate among Blacks was only 2.6 percent. Cases were pooled and measurements compared with nonfracture cases using a t-test. No anthropometric variable was significantly different between groups (p > .05). Conventional BMD and estimated cortical thickness were significantly smaller for fracture cases in the femur, tibia, and fibula. Cross-sectional geometry variables measured at the midshaft of the femur and distal third of the tibia were, except for tibial width, significantly smaller in cases than controls (see Table A-1). Interestingly, the cross-sectional properties of the nonweight-bearing fibula were not significantly different from controls (not shown). The DXA-measured fraction of lean muscle mass at the thigh also was significantly smaller in fracture cases, which suggests that subjects with fractures had less muscular thighs, although thigh girths were not significantly smaller. TABLE A-1 Average Values for Femur and Tibia Stress Fracture Cases and Controls and Differences between Groups Femur Tibia Parameter Cases Controls % Diff. Cases Controls % Diff. BMD (g/cm2) 1.296 1.371 -5.5 0.952 1.020 -6.7 CSA (cm2) 2.630 2.860 -8.0 1.669 1.822 -8.4 CSMI (cm4) 0.994 1.138 -12.7 0.472 0.532 -11.3 Width (cm) 2.140 2.200 -2.7 0.185 0.189 -2.1* Section modulus (Z) (cm3) 0.921 1.023 -10.0 0.502 0.556 -9.7 Strength index (Z/bone length) 1.829 2.012 -9.1 1.384 1.493 -7.3 Average cortical thickness (cm) 0.518 0.559 -7.3 0.359 0.392 -8.4 Thigh lean mass fraction 0.736 0.764 -3.7 NOTE: BMD, bone mineral density; CSA, cross-sectional area; CSMI, moment of inertia; Z, section modulus. * All differences are significant (p < 0.05) with the exception of tibia width.
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Discussion and Conclusions Unlike male stress fracture subjects, who were generally smaller in weight and stature (Beck et al., 1996), female fracture subjects were not significantly different in body size. Like males, however, the measured bones of the lower limbs tended to have lower bone mass density (BMD). The mechanical relevance of the lower BMD is evident in the smaller cross-sectional properties of the limb bones of individuals with fractures. Particularly relevant is the smaller bone strength index in female fracture cases, an observation also seen in male Marine Corps recruits. The strength index is based on the observation that the resistance of a bone to bending and twisting is directly dependent on the section modulus and inversely related to bone length (Selker and Carter, 1989) (i.e., long, skinny bones are weaker than short, wide ones). The fact that relative muscle mass in the thigh is significantly reduced in fracture cases supports the hypothesis that muscle strength is, at least in females, a factor in stress fracture susceptibility. These geometric differences support the hypothesis that the bones of test subjects are weaker than controls because they undergo greater stress magnitudes during training. Moreover, the lower relative thigh muscle mass suggests a diminished capability of muscle to resist stresses during training. The anthropometric and DXA methodologies used in this study were designed to measure factors influencing mechanical stress but not material properties. These results, therefore, do not rule out a role for degraded material strength, possibly due to dietary or genetic deficiencies. For example, reduced BMD values may also result from decreased mineralization, which would degrade the material properties, but this cannot easily be determined by current methodologies. A number of additional questions remain. What component of the geometric differences is genetic and what component is environmental? It is known that stress fracture is relatively infrequent in African Americans (Jones et al., 1989); indeed, in the current study, the stress fracture rate in African Americans was less than half of that in Hispanics or Whites. Further work is necessary to answer these questions. Examination of such environmental factors as prior physical activity is also worth study. Certainly, physical activity influences muscle mass and is also believed to improve the geometry of bones in ways that minimize stress (Cowin, 1989). The implication for younger populations is that bone can be strengthened by rigorous training. Unfortunately, information regarding the rates and magnitudes of such change and the factors influencing such change in humans are scanty. Whether pretraining exercise regimens can be designed to precondition bones to minimize stress fractures has yet to be determined. References Beck, T., C. Ruff, F. Mourtada, R. Shaffer, K. Maxwell-Williams, G. Kao, D. Sartoris, and S. Brodine. 1996. DXA derived structural geometry for stress fracture prediction in male U.S. Marine Corps recruits . J. Bone Miner. Res. 11:645–653. Cowin, S., ed. 1989. The mechanical properties of cortical bone tissue. Pp. 97–127 in Bone Mechanics. Boca Raton, Fla.: CRC Press. Jones, B.H., J.M. Harris, T.N. Vinh, and C. Rubin. 1989. Exercise-induced stress fractures and stress reactions of bone: Epidemiology, etiology, and classification. Exerc. Sport Sci. Rev. 17:379–422. Milgrom, C., M. Giladi, A. Simkin, N. Rand, R. Kedem, H. Kashtan, M. Stein, and M. Gomori. 1989. The area moment of inertia of the tibia: A risk factor for stress fractures. J. Biomech. 22:1243–1248.
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A total of 332 soldiers with stress fractures and 448 control soldiers were evaluated in the nested case-control study by initial BUA and BUA at the time of visit 1 for stress fracture care. During the first 45 days of basic training, BUA declined more in stress fracture soldiers than in control soldiers (all p < 0.005) (Figure A-2). However, in soldiers measured during and after day 46 of training, BUA declined in neither group. FIGURE A-2 Percentage change in broadband ultrasound attenuation from initial level. Initial BUA in all three stress fracture groups was lower than in controls (all p < 0.005) (Figure A-3). However, initial BUA of soldiers with femoral or pelvis stress fracture was higher than that in soldiers with foot or tibial stress fracture (p < 0.02). FIGURE A-3 Initial broadband ultrasound attenuation. Both stress fracture during military basic training and osteoporotic fracture are examples of fragility fracture. Evaluating fitness, QUS, and smoking history—three independent determinants of stress fracture risk—gives a complete yet conveniently obtained picture of stress
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fracture risk in young, female soldiers. Although many other traditional risk factors for bone weakness were considered, it is likely that their influence is expressed by the QUS measurement. It remains impossible to predict perfectly stress fracture occurrence in individuals. In conclusion, QUS measurement does not facilitate identification of individuals with a high risk of ''high consequence" (e.g., proximal femur, pelvis) stress fracture. Moreover, QUS declines in stress fracture soldiers during basic training, indicating that those soldiers may respond to the increased physical activity of basic training with increased bone turnover that causes transient downward adjustment in bone strength due to an expansion of the remodeling space. This downward adjustment of bone strength may itself contribute to fracture occurrence. CALCIUM INTAKE AND EXERCISE LEVEL: SYNERGISTIC EFFECTS ON BONE Bonny L. Specker, Ph.D., South Dakota State University, Brookings, SD 57007 Both calcium intake and physical activity are thought to affect bone mass accretion early in life and bone loss later in life. However, results from randomized trials are not consistent. Results from a recently completed trial of gross motor activity during growth indicated that the effect of increased activity on bone mass accretion may be dependent on calcium intake. That is, decreased bone mass accretion was observed in individuals with a low calcium intake who were randomized to gross motor activities compared with those randomized to fine motor activities. In addition, 36 trials evaluating the effects of physical activity on changes in bone density in adults were reviewed. Studies were excluded if they did not include estimates of calcium intake. The studies were conducted in obese individuals; participants were noted to have significant changes in body weight, and/or bone sites not including the spine or radius were measured. A composite of the 16 studies meeting these criteria indicates that calcium intake and physical activity may not act on bone independently of each other. A beneficial effect of physical activity on bone appears to exist only at calcium intake greater than 1,000 mg/d. These results may explain inconsistent findings on the beneficial effects of either calcium intake or physical activity on bone mass. CALCIUM AND IRON: FOOD VERSUS SUPPLEMENTS Connie M. Weaver, Ph.D., Department of Foods and Nutrition, Purdue University, West Lafayette, IN 47907-1264 Mineral Requirements Two nutrients that are most likely to be consumed at inadequate levels by the female military population are calcium and iron. The new calcium requirement for women aged 19 to 50 years, released in August 1997 by the Institute of Medicine is 1,000 mg/d. This recommended intake level is slightly above the ninetieth percentile of calcium intake based on the 1994 CFSII data. Thus, the majority of women are not consuming the recommended levels of calcium intake for optimal bone health. Peak bone mass in the hip is achieved by age 16 years and for the total body by the early 20s. Thus, adequate calcium protects against early bone loss rather than increasing peak bone mass after
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this age except for the spine, which can increase bone mass until age 30. A loss of 1 percent bone results in an increased risk of fracture of approximately 8 percent. The Recommended Dietary Allowance for iron in menstruating women is 15 mg/d. This level of iron is difficult to achieve for most women, especially for those who restrict their intake of lean red meat. Other minerals that may be inadequate in the female military population include magnesium and zinc. Evidence is mounting that adequate intakes of these nutrients are necessary for good bone health. Bioavailability Almost 75 percent of calcium in the American diet comes from dairy products. Other foods have much lower natural levels of calcium. Many plants also contain inhibitors to calcium absorption, notably oxalate and phytate. It would be extremely difficult to consume adequate calcium without consuming liberal quantities of dairy products, fortified foods, or supplements. Bioavailability of most calcium salts is equivalent to milk calcium. Iron content and bioavailability are much greater in animal foods than in plant foods. Iron salts used to fortify foods vary widely in their bioavailability. Nutrient-Nutrient Interactions High sodium intakes and, to a lesser extent, high protein intakes increase urinary calcium losses. Some concern has been raised that high calcium intakes compromise iron, magnesium, and zinc status. Several recent studies have shown no harmful effects on magnesium status of up to 2 g/d of calcium intake. The harmful effects of calcium on iron status are shown in single-meal absorption studies. Chronic high calcium consumption has not been shown to compromise iron status, presumably because of upregulation of iron absorption. Reduced zinc retention has been documented at high calcium intakes in the elderly. Foods Versus Supplements Food patterns of fortification with milk extract have shown greater positive effects on bone than single nutrient interventions. Dairy foods provide a package of nutrients important for bone health. In the American diet, dairy products contribute 75 percent of the calcium, 35 percent of the riboflavin, 34 percent of the potassium, 20 percent of the magnesium, and 17 percent of the vitamin A, as well as being fortified with vitamin D. Because of this contribution and American's poor compliance with supplement regimens, supplements are not the first recommendation. However, supplementation may be indicated for individuals who do not otherwise meet their needs. Fortification with a package of nutrients may be a viable approach.
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DIETARY CALCIUM AND RELATED NUTRIENT INTAKES IN MILITARY MEN AND WOMEN LTC John P. Warber, USA, Ph.D., Nutrition and Biochemistry Division, U.S. Army Research Institute of Environmental Medicine, Natick, MA 01760-5007 Adequacy of nutrient intake by military personnel has been evaluated on a periodic basis since World War II. As new rations have been developed, their acceptability and relationship to performance have been studied. Few military nutrition monitoring studies have been conducted to determine how well military feeding addresses the nutritional requirements of military women. Approximately 14 percent of the active-duty Army population is women. Any investigations on military feeding and women have been done with Army personnel. Low intakes of calcium have been associated with higher risks for osteoporosis, hypertension, and colon cancer. The Healthy People 2000 objective 2.8 calls for 50 percent of female youths less than 24 years of age to consume three or more servings of calcium-rich foods daily (about 900 mg calcium). The calcium standard established for military personnel subsisting under normal operating conditions (AR 40-25, 1985) is 800–1,200 mg. The higher value reflects the greater requirement for 17–18 year olds. The nutritional standard for operational rations (e.g., Meal, Ready-to-Eat; Tray Pack Rations) is 800 mg. The newly released Dietary Reference Intakes for calcium (1997) increase the recommended calcium intake to 1,000 mg/d. Data on food consumption from six military dietary assessment studies between 1980 and 1995 showed female soldiers to have an average calcium intake of 924 mg. The average calcium intake reported by NHANES III and CSFII 1988–1991 for females of like age in the general population is 722 mg. The average calcium intake for males covering eight studies between 1987 and 1996 was 1,428 mg. The mean daily intake of calcium for males exceeded the Military Recommended Dietary Allowances (MRDA) in all dietary studies performed over the past 10 years. Mean daily intakes of phosphorus exceeded the upper level of the MRDA for females across all garrison studies and exceeded general population estimates for similar age categories (1,393 mg vs. 1,092 mg). The intake of phosphorus for males averaged 2,038 mg. The calcium to phosphorus ratios averaged 1:1.5 for females and 1:1.4 for males. Based on data collected from a representative sample of over 3,000 soldiers who participated in the Army Food and Nutrition Survey (1997), female soldiers (n = 467) reported consuming a mean of 2.3 items from the dairy food group per day while male soldiers (n = 2,384) consumed 2.6 items. In general, female soldiers are slightly more aware than the general population of the relationship of dietary calcium intake to health. Seventy-nine percent of female soldiers versus 67 percent of the U.S. female population (USDA, Diet and Health Knowledge Survey, 1988–1991) identified inadequate calcium intake with the potential health outcome of osteoporosis, even though 50 percent of female soldiers believed their diets were too low in calcium. This underscores the need for research about what causes people to change their dietary behavior. References AR 40-25, 1985. Army Food and Nutrition Survey, 1997.
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Institute of Medicine (IOM). 1997. Dietary Reference Intakes. Healthy People 2000. USDA, Diet and Health Knowledge Survey, 1988-91. EFFECTS OF PROLONGED INACTIVITY ON THE MUSCULOSKELETAL SYSTEM WITH EVALUATION OF COUNTERMEASURES Steven R. Smith, M.D., Pennington Biomedical Research Center, Baton Rouge, LA 70808 Introduction Prolonged inactivity is a component of many military missions and civilian occupations or circumstances. It is well recognized that these situations result in increased bone turnover, bone loss, nitrogen loss, and skeletal muscle deconditioning. A series of studies were conducted in vivo that focused on improving an existing catabolic model of inactivity and testing countermeasures to prevent nitrogen, muscle, and bone loss within the enhanced catabolic model. Review Prolonged bedrest, with the head lower than the feet, is an accepted model of spaceflight. These studies require 4 to 8 weeks to reach maximal calcium loss and typically are 17 weeks in duration. An improved model of bedrest catabolism was developed by combining 4 weeks of bedrest with low doses of thyroid hormone (triiodothyronine, T3). The model was compared with bedrest without T3 and shown to be superior to 4 weeks of bedrest alone. Specifically, bone turnover was increased, and calcium balance was more negative, with T3 treatment. Additional data on dynamic changes in markers of bone turnover complement calcium balance data and show that increases in bone resorption are a critical component of both bedrest and mild hyperthyroidism. Increases in bone formation occur late and are blunted relative to the increase in bone resorption. These studies then prompted a shift in focus to testing several countermeasures to prevent muscle and bone loss in the enhanced model. First, alendronate, a bisphosphonate inhibitor of bone resorption, was used concurrent with bedrest to prevent the increase in bone turnover and calciuria seen in the placebo-treated individuals. Testosterone was used as an anabolic agent to attenuate total-body nitrogen loss and losses of lean mass as measured by dual energy x-ray absorptiometry (DXA). Muscle strength was not preserved, however. Studies have recently been completed in 12 men to test a novel nutritional supplement, beta-hydroxy beta-methyl butyrate (HMB) to prevent loss of skeletal muscle. Previous studies have demonstrated a decrease in exercise-induced skeletal muscle turnover in healthy individuals treated with HMB, which suggests utility in states of increased protein flux. Conclusions Prolonged inactivity results in bone and muscle loss. Increased bone resorption is the primary mechanism by which calciuria occurs. The molecular mechanism(s) that increases bone resorption is unknown. Similarly, skeletal muscle protein turnover is increased via an unknown mechanism. These
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results demonstrate that skeletal muscle should be studied in a functional context, as preservation of mass does not necessarily preserve strength. The studies provide information useful for situations such as military missions requiring prolonged inactivity, postsurgical and/or postinjury immobility, inactivity associated with aging, and microgravity/spaceflight. EFFECT OF MODULATORS OF BONE TURNOVER ON CHANGES IN MARKERS OF BONE TURNOVER Michael Kleerekoper, M.D., Ph.D., Endocrine Division, Wayne State University, Detroit, MI 48201 Bone is subject to continuous turnover throughout life. This consists of removal of older bone (resorption) and replacement with new bone at the same site (formation). During growth and development, turnover is high, particularly in the first 2 years of life and again during the pubertal growth spurt. In these periods, formation exceeds resorption with accretion of bone and positive skeletal balance. Turnover, also termed remodeling, reaches a nadir as peak bone mass is attained during the third and fourth decades and the skeleton is in zero balance. After age 40, for unknown reasons, remodeling remains low, but resorption exceeds formation, and there is negative skeletal balance. With the menopause, there is an acceleration of remodeling with a greater excess of resorption over formation and more rapid negative balance. Turnover appears to slow down about 10 years postmenopause, only to accelerate again during the eighth decade and beyond. There are several markers of the resorption process, all of which are based on breakdown products of the bone matrix (type 1 collagen). These include the major amino acid hydroxyproline and the pyridinium cross-links of collagen. There are several markers of these cross-links: pyridinoline and deoxypyridinoline, and the amino- and carboxy-terminal telopeptides of these cross-links. Markers of bone formation are gene products of the osteoblast, bone-specific alkaline phosphatase, osteocalcin, and procollagen extension peptides. Many medical conditions and therapies modulate turnover, and these effects can be monitored by changes in the above markers. Generally, these reflect systemic events, including estrogen deficiency from any cause, thyroid, parathyroid, and cortisol excess from endogenous and exogenous sources; malnutrition and malabsorption; and immobilization. In military-age women, there is cause for concern about hypothalamic amenorrhea (excess exercise, anorexia, bulimia), and therapy with gonadotrophin-releasing hormone (GnRH) agonists that may be used in endometriosis. Contraceptives (oral and depot) may have adverse effects on the skeleton, but this is not yet firmly established, and the potential mechanisms are unclear. Local events such as traumatic, fragility, and stress fractures increase remodeling locally, but the assays may not be sensitive enough to detect this in all cases. It has been postulated that remodeling may be initiated at some sites as a means of repairing microdamage. If microdamage accumulates at a rate faster than can be corrected by remodeling processes, the damage propagates to demonstrable fracture. This may be the case with stress fractures, although it remains speculative. High rates of remodeling, as are seen during puberty, do not appear to result in stress fractures, so it is unlikely that systemically induced increases in remodeling will result in stress fractures. In contrast, local abnormalities in remodeling, particularly when formation is markedly increased, may be associated with local stress fractures. Examples include osteomalacia, Paget's disease of bone, and therapy with sodium fluoride. Because fluoride therapy only results in lower-extremity stress fractures, an apparent link exists between local factors
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predisposing to stress fractures and mechanical load bearing. The relevance of this link to stress fractures in the military remains speculative. IGF-1, MUSCLE MASS, AND BONE DENSITY* C.J. Rosen, W G. Beamer, L.R. Donahue, D.J. Baylink, J. Rogers, C.H. Turner, and J.P. Bilezikian, St. Joseph Hospital and Maine Center for Osteoporosis Research and Education, Bangor, ME 04401 and The Jackson Laboratory, J. L. Pettis VA Hospital, Southwest Foundation for Biomedical Research, Indian University Medical Center, and Columbia University, New York,NY. Insulin-like growth factor (IGF)-I, a ubiquitous polypeptide induced by growth hormone, has traditionally been considered a critical element for the linear growth of mammals. However, IGF-1 is abundant in the adult skeleton and plays a central role in remodeling as well as in acquisition of peak bone mass. Moreover, growth hormone and IGF-1 are major determinants of lean body mass. In the circulation as well as in muscle and bone, tissue-specific IGF regulatory elements (e.g., IGF binding proteins [BPs], IGF receptors, IGFBP proteases) control the bioavailability of this potent growth factor. However, discerning the true functional relationship between IGF-1 and peak bone mass has been difficult for several reasons: (1) the lack of a good in vivo model, (2) the use of serum IGF-1 as a surrogate marker for tissue IGF-1, and (3) a host of confounding variables that affect IGF-1. Two model systems were used to examine the role of IGF-1 in musculoskeletal acquisition and maintenance: (1) males with the syndrome of idiopathic osteoporosis (IOM), and (2) healthy, inbred strains of mice. These authors recently described a cohort of men with osteoporosis who have low serum IGF-1 and reduced bone formation. In the syndrome of IOM, homozygosity in a simple sequence-length polymorphism (SSLP) of a nontranscribing region in the IGF-1 gene is considerably more frequent (65%) than in a healthy control group (32%) (O.R. 3.9, p < 0.003) and is associated with low serum IGF-1 levels in a random population of men, women, and pubertal girls. Coincident with these human studies, these authors recently reported large differences in both skeletal and serum IGF-1 in two inbred strains of mice. Using intercrosses to produce F2 progeny, it was noted that IGF-1 phenotype cosegregates with bone mass and accounts for 40 percent of the variance in bone mineral density (BMD). Currently, genes are being mapped that control serum IGF-1 through quantitative trait localization (QTL). Moreover, in functional experiments with a GH-deficient mouse (little), these authors have found that introducing a portion of the genome from a high-density, high IGF-1 mouse can markedly increase both BMD and serum IGF-1, despite the absence of growth hormone. These studies suggest that IGF-1 plays a major role in the acquisition and maintenance of bone mass. Through the use of congenic strains and QTLs for BMD, these laboratories are currently determining whether the genetic loci that regulate bone density are similar or identical to those that control tissue and serum IGF-1 expression. In addition to that genomic search, an examination has also begun of the difference in body composition between these two inbred strains. High IGF-1 (high-density) strains have greater muscle mass, uterine weights, and less fat than do the low IGF-1 (low-density) strain. Similarly, femur strength, toughness, failure load, and moment of inertia are markedly greater (p < * Supported by grants from the U.S. Public Health Service and Department of Defense Health Research Program grants.
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0.0001) in the high-density, high IGF-1 strain than in the low-density mice. These data suggest that IGF-1 may play a key role in determining peak bone mass, and the heritable differences in the expression of IGF-1 could determine musculoskeletal strength and balance. These findings, produced through the unique collaborative efforts of several investigators, may have future implications for screening recruits who are at high risk for subsequent stress fractures. DIETARY ENERGY REQUIREMENTS IN PHYSICALLY ACTIVE MEN AND WOMEN: THRESHOLD EFFECTS ON REPRODUCTIVE FUNCTION Anne B. Loucks, Ph.D., Department of Biological Sciences, Ohio University, Athens, OH 45701 The prevalence of amenorrhea is elevated in civilian and military women who restrict their diets and who are intensely physically active. The endocrine mechanism of this amenorrhea involves the disruption of luteinizing hormone (LH) pulsatility. This presentation shows preliminary results from a randomized, prospective experiment that is testing the following hypothesis: LH pulsatility and energy metabolism are disrupted at an energy availability of 10 kcal/kg lean body mass (LBM)/d in women but not in men, and LH pulsatility is disrupted abruptly at a particular threshold of energy availability in women. In repeated trials, young, regularly menstruating, untrained women and young untrained men performed controlled exercise and consumed controlled diets differing in energy content for 5 days. The women were tested in the follicular phase of their menstrual cycles. In subjects who had energy availabilities of 45 and 10 kcal/kg LBM/d, low energy availability (defined as dietary energy intake minus exercise energy expenditure) appears to have (1) reduced plasma glucose while increasing serum b-hydroxybutyrate; (2) reduced serum T3, serum insulin-like growth factor (IGF)-I, and serum insulin while increasing serum cortisol, serum growth hormone (GH), and serum insulin-like growth factor binding protein (IGFBP)-1; (3) increased fat oxidation while reducing carbohydrate oxidation during rest and exercise; and (4) reduced 24-h LH pulse frequency while increasing LH pulse amplitude in women but not in men. These preliminary results suggest that low energy availability has similar metabolic effects in men and women as their bodies conserve glucose for the brain where LH pulsatility is controlled, but that women require more energy availability than men to maintain normal LH pulsatility, and therefore to maintain their reproductive and skeletal health. EFFECT OF PREGNANCY ON THE FITNESS AND HEALTH OF POSTPARTUM SOLDIERS COL Joseph R. Dettori, LTC (R) Kathleen Westphal, CPT Tony Pusateri, LTC (R) Alana Cline, COL (R) Paul Smith, Troy Patience, Rex Hoyt, Madigan Army Medical Center, Ft. Lewis, Tacoma, WA 98431-5000 Pregnant and postpartum soldiers have unique needs that require adjustments in the demands placed on them in a military environment. This prospective cohort study was undertaken to examine
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the effects of pregnancy on the fitness and health of postpartum soldiers. A total of 468 females were studied in three different groups: a nonpregnant, active-duty group (NPAD, n = 215); a postpartum, family member group (PPFM, n = 126); and a postpartum, active-duty group (PPAD, n = 127). Soldier fitness was assessed using scores from soldiers' semi-annual Army physical fitness test (APFT) immediately prior to study entry and 6 to 9 months later. Subjects underwent blood draws to assess calcium status, anthropometric measurements to determine body composition, and dual energy x-ray absorptiometry to measure bone mineral density (BMD) immediately postpartum and at 6 months follow-up. Medical records of active-duty subjects were reviewed between January 1, 1992, and study termination to calculate injury and illness rates at baseline (preconception) and during the various phases of postpartum recovery. Results Forty-eight percent of postpartum soldiers failed to return to their prepregnancy fitness level 6 to 9 months postpartum and were nearly four times more likely to fail their postpartum APFT compared with nonpregnant soldiers (relative risk (RR) = 3.89, 95% confidence interval (CI) = 1.61, 9.39). BMD decreased in the postpartum period by 2.1 percent and 1.9 percent in the lumbar spine and femoral neck, respectively, for nonnursing PPAD mothers (p = 0.001 and 0.0086, compared with NPAD) and 5.0 and 5.4 percent for nursing PPAD mothers (p = < 0.0001 and < 0.0001 compared with NPAD). For women not in basic combat training or advanced individual training, crude injury and illness rates were 7.3 and 18.0 per 100 soldiers per month, respectively. These rates increased in the postpartum period (RR = 1.37 for injury, 95% CI = 1.08, 1.75; RR = 1.24 for illness, 95% CI = 1.06, 1.46). Conclusions Postpartum soldiers in the year following delivery demonstrated significantly reduced fitness, increased injury and illness rates, and reduced trabecular BMD. More research is needed to determine the reasonableness of the U.S. Army's requirement to return postpartum soldiers to body fat standards and minimum military fitness 135 days postpartum. In addition, strategies that optimize the postpartum soldier's restoration to prepregnancy fitness should be explored. THE ART AND SCIENCE OF LONGITUDINAL STUDIES OF HEALTHY YOUNG PEOPLE Tom Lloyd, Ph.D., The Pennsylvania State University College of Medicine, Hershey Medical Center, Hershey, PA 17033 Osteoporosis prevention is currently at a delicate stage. Recognition that calcium intake by young American women declined markedly in the past three decades while the prevalence of osteoporosis among older women increased dramatically has led to widespread investigations of the relationships between calcium intake and bone health. However, current understanding of age-related calcium requirements and bone health—both of which are multifactorial—is inadequate at present.
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Women gain approximately 50 percent of the bone mineral content of their skeleton during adolescence. However, adolescent women in the United States consume on average only 65 percent of the Recommended Dietary Allowance of 1,200 mg/d calcium. The advent of dual energy x-ray absorptiometry provided the key to designing a study to quantify bone changes in children and young women and to assess the effect of calcium supplementation. The Pennsylvania State Young Women's Health Study is an ongoing National Institutes of Health-funded longitudinal study of cardiovascular, bone, and endocrine development in healthy, young women. The study was initiated in 1990 when the subjects were, on average, 12 years of age; they are now, on average, 19 years of age. A major initial objective of this study was to determine the effect of calcium supplementation on bone gain during adolescence. During the 4 years of active intervention as the participants grew from age 12 to age 16, the calcium-supplemented group made significantly greater bone gains than did the placebo group. However, within 1 year after treatment, by age 17, the differences had largely disappeared as the placebo treated group "caught up." The definitive assessment of the effect of the calcium supplementation program on peak bone mass cannot be made until the study cohort has reached skeletal maturity. Because Caucasian women acquire at least 95 percent of their bone mass by age 20, the volunteers must be studied again at and after this age. With regard to bone health, attention has been focused on fracture reduction, bone density has been used to reflect fracture risk. In fact, fractures occur when bone strength and bone microarchitecture diminish past some as yet poorly understood set of circumstances. Bone strength and architecture are much more than bone mass and bone density. Studies from sports and the workplace report that bone geometry and bone strength are affected by physical activity. bone geometry and the nonmineral bone components must now be studied for their roles in establishing bone strength. Three important research design issues that are specific to longitudinal studies with healthy participants include: (1) the research design requires planning that often spans several years, (2) research teams are often large and multidisciplinary to take advantage of multiple measurements, and these teams must remain functional for many years, and (3) the success of longitudinal studies depends entirely on continuing retention of a large proportion of the study subjects. Longitudinal studies offer several advantages over cross-sectional investigations. These advantages include: (1) Measurement of the endpoint variables is generally better than with cross-sectional studies because the measurement technology is repeated, and often the same personnel are making the repeated measurements. (2) both intersubject and intrasubject variability can be estimated. Thus, longitudinal studies have greater statistical power to detect differences in group assignment or due to treatment effects. (3) Only longitudinal studies offer the opportunity to follow-up unexpected observations made during the collection of the primary outcome variables. Strategies to retain healthy volunteers in longitudinal studies must be addressed in planning and conducting the study. For studies involving healthy young people, six strategic areas relate to participant retention: (1) incentives, (2) personal health knowledge and benefits, (3) social responsibility, (4) study ownership, (5) continuity of study personnel and continuity of study measurements, and (6) the use of noninvasive procedures. Careful attention to each of these areas has allowed this laboratory to retain 72 percent of the starting participants in the Pennsylvania State Young Women's Health Study.
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Representative terms from entire chapter: