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the desired degree of physical fitness during basic training without incurring an excessive injury rate with its associated loss of training time seems appropriate at this time.
2. What is the relationship of genetics and body composition to bone density and the incidence of stress fractures in women?
Genetics is a determinant of peak bone mass, but it is not known what genes are important, nor is it known how prominent they are in the risk assessment profile for stress fractures. Body mass and composition per se influence bone density. Greater body mass is associated with higher levels of bone mineral mass and density. Hence, individuals who are heavier and have a larger and denser skeleton are at lower risk of bone fracture, particularly osteoporosis-related fractures.
Stress fractures are associated not only with reduced skeletal muscle mass and its concomitant increased fatigability and lower fitness levels but also with an excessive skeletal muscle mass and its enhanced strength. Reduced skeletal muscle mass with associated lower strength and greater fatigability may limit the ability of muscle to decrease bone stress. This relationship between small muscle mass and increased bone fragility may account for the observed association between lower calf circumference and stress fracture risk in women. Bone stress created by excessive or rapid incremental skeletal muscle contraction and loading forces can cause fractures at specific anatomic sites.
In sum, body mass and composition play a substantial role in establishing an individual's skeletal properties including ability to stimulate bone growth and turnover and to withstand physical demands that predispose to stress fracture.
3. What are the effects of diet, physical activity, contraceptive use, and other lifestyle factors (smoking and alcohol) on the accrual of peak bone mineral content, incidence of stress fractures, and development of osteoporosis in military women?
Energy intake by military women should be adequate (2,000–2,800 kcal/d) to maintain weight during moderate and intensive physical fitness training. A diet adequate in calcium, phosphorus, magnesium, and vitamin D (as defined by IOM, 1997) and moderate in sodium and protein (NRC, 1989) should optimize bone health in the short term and theoretically should reduce the long-term risk of developing osteoporosis.
A healthy, active lifestyle should reduce further the long-term risk for osteoporosis. Physical activity of a weight-bearing nature should be introduced in a gradual and progressive manner to minimize risk of stress fractures. The use of oral contraceptives that contain estrogen with or without progestational agents are not considered to have long-term detrimental effects on women's bone health. Some studies show, in fact, that the use of such agents might have a positive impact on the developing young female skeleton. In contrast, the use of long-acting, depot preparations of progestational agents, like medroxyprogestrerone (Depo-Provera) has been associated with relative estrogen deficiency and reduced bone mass.
Gonadotropin-releasing hormone (GnRH) agonists used to treat endometriosis (Lupron) interfere with the hypothalamic-pituitary-ovarian axis, inducing a state of estrogen deficiency. Long-term use of such GnRH agonists is of concern because (1) it mimics the menopausal state of estrogen deficiency, and (2) it has been associated with bone loss.
Agents used to treat the pain and inflammation of injury can include steroids and nonsteroidal anti-inflammatory agents (NSAIDs). Because of the potential of steroids to induce bone loss by the