Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 362
TABLE 1 Dietary Reference Intakes for Phosphorus by Life Stage Group DRI values (mg /day) EARa RDAb AIc ULd males females males females Life stage group NDe 0 through 6 mo 100 7 through 12 mo 275 ND 1 through 3 y 380 380 460 460 3,000 4 through 8 y 405 405 500 500 3,000 9 through 13 y 1,055 1,055 1,250 1,250 4,000 14 through 18 y 1,055 1,055 1,250 1,250 4,000 19 through 30 y 580 580 700 700 4,000 31 through 50 y 580 580 700 700 4,000 51 through 70 y 580 580 700 700 4,000 > 70 y 580 580 700 700 3,000 Pregnancy £ 18 y 1,055 1,250 3,500 19 through 50 y 580 700 3,500 Lactation £ 18 y 1,055 1,250 4,000 19 through 50 y 580 700 4,000 a EAR = Estimated Average Requirement. b RDA = Recommended Dietary Allowance. c AI = Adequate Intake. d UL = Tolerable Upper Intake Level. Unless otherwise specified, the UL represents total intake from food, water, and supplements. e ND = Not determinable. This value is not determinable due to the lack of data of adverse effects in this age group and concern regarding the lack of ability to handle excess amounts. Source of intake should only be from food to prevent high levels of intake.
OCR for page 363
PART III: PHOSPHORUS 363 PHOSPHORUS T he element phosphorus is found in nature (e.g., foods, water, and living tissues) primarily as phosphate (PO4). It is a major component of bones and teeth. In fact, 85 percent of total body phosphorus is found in bone. Phosphorus helps maintain a normal pH in the body and is involved in meta- bolic processes. The adult requirements for phosphorus are based on studies of serum inor- ganic phosphate concentration in adults. The Tolerable Upper Intake Level (UL) was derived using data on the normal adult range for serum inorganic phos- phate concentration. DRI values are listed by life stage group in Table 1. Nearly all foods contain phosphorus, and it is also common in food addi- tives. Phosphorus deficiency is generally not a problem; the average adult diet contains about 62 mg phosphorus per 100 kcal. Excess phosphorus intake is expressed as hyperphosphatemia, and essentially all adverse effects of phos- phorus excess are due to the elevated inorganic phosphorus in the extracellular fluid (ECF). PHOSPHORUS AND THE BODY Function Phosphorus is a major component of bones and teeth. Its main functions are to maintain a normal pH (by buffering excesses of acid or alkali), temporarily store and transfer energy derived from metabolic fuels, and activate catalytic proteins via phosphorylation. Structurally, phosphorus occurs in the body as phospholipids (a major component of biological membranes) and as nucle- otides and nucleic acids. Dietary phosphorus supports tissue growth and re- places phosphorus stores that are lost through excretion and the shedding of skin cells. Absorption, Metabolism, Storage, and Excretion Phosphorus found in foods is a mixture of organic and inorganic forms, and most phosphorus absorption occurs as inorganic phosphate. Approximately 55–70 percent of dietary phosphorus is absorbed in adults and about 65–90 percent in infants and children. The majority of phosphorus absorption occurs through passive concentration-dependent processes.
OCR for page 364
DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 364 The amount of phosphorus ingested does not appear to affect absorption efficiency, which suggests that this efficiency does not improve with low intakes (unlike calcium absorption). By the same token, when serum phosphorus is abnormally high, even dangerously so, phosphorus continues to be absorbed from the diet at a rate only slightly lower than normal. Phosphorus absorption is reduced by aluminum-containing antacids and pharmacological doses of cal- cium carbonate. However, when consumed at intakes in the typical adult range, calcium does not significantly interfere with phosphorus absorption. In adults, 85 percent of phosphorus is found in bone, with the remaining 15 percent distributed through the soft tissues. Excretion is achieved mainly through the kidneys. In healthy adults, the amount of phosphorus excreted in the urine is essentially equal to the amount absorbed through diet, less small amounts lost in the shedding of skin cells and intestinal mucosa. DETERMINING DRIS Determining Requirements The adult requirements for phosphorus are based on studies of serum inor- ganic phosphate concentration.The EAR, and hence the RDA, for healthy ado- lescents aged 9 through 18 years is based on a factorial approach and is higher than the adult value. This is because this age range brackets a period of intense growth, with growth rate, absorption efficiency, and normal values of inorganic phosphorus in the extracellular fluid changing during this time. Criteria for Determining Phosphorus Requirements, by Life Stage Group Life stage group Criterion 0 through 12 mo Human milk content 1 through 18 y Factorial approach 19 through 50 y Serum Pi (serum inorganic phosphate concentration) 51 through > 70 y Extrapolation of serum Pi from 19 through 50 years Pregnancy £ 18 y Factorial approach 19 through 50 y Serum Pi Lactation £ 18 y Factorial approach 19 through 50 y Serum Pi
OCR for page 365
PART III: PHOSPHORUS 365 The UL The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse effects for almost all people. Members of the general population should not routinely exceed the UL. The UL value for phosphorus was derived using data on the normal adult range for serum inorganic phosphate concentration and represents total intake from food, water, and supplements. Phosphorus exposure data, based on data from the Continuing Survey of Food Intakes by Individuals (CSFII, 1994–1996) and the 1986 National Health Interview Survey (NHIS), indicated that only a small percentage of the popula- tion was likely to routinely exceed the UL for phosphorus; however, because food composition data do not always indicate phosphorus from food additives, the full extent of phosphorus intake is not known. DIETARY SOURCES Foods Phosphorus is found naturally in many foods in the form of phosphate (PO4) and as a food additive in the form of various phosphate salts, which are used for nonnutrient functions during food processing, such as moisture retention, smoothness, and binding. According to data from the National Health Survey (1976–1980), the phos- phorus content of the average adult diet for both men and women is about 62 mg/100 kcal. Dietary intake of phosphorus appears to be affected more by total food intake and less by differences in food composition. People with a high intake of dairy products will have diets with higher phosphorus density values because the phosphorus density of cow milk is higher than for most other foods. People who consume several servings per day of colas or a few other soft drinks that contain phosphoric acid also tend to have high phosphorus intake. A 12- ounce serving of such beverages contains about 50 mg, which is only 5 percent of the typical intake by an adult woman. However, when consumed in a quan- tity of 5 or more servings per day, such beverages may contribute substantially to total phosphate intake. Dietary Supplements Phosphorus supplements are not widely used in the United States. Based on the 1986 NHIS study, about 10 percent of adults and 6 percent of children aged 2 to 6 years took supplements containing phosphorus. Supplement usage and
OCR for page 366
DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 366 dosage was similar for men and women, with a median intake from supple- ments of 120 mg/day. Bioavailability Most foods exhibit good phosphorus bioavailability. However, foods derived from plant seeds (e.g., beans, peas, cereals, and nuts) contain phytic acid (also called phytate), a stored form of phosphorus that is not directly available to humans. Absorption of this form requires the presence of phytase, an enzyme found in some foods and in some colonic bacteria. Because yeasts can hydro- lyze phytate, whole grains that are incorporated into leavened bread products have higher phosphorus bioavailability than do grains used in unleavened bread or breakfast cereals. Also, unabsorbed calcium in the digestive tract combines with phytic acid and interferes with its digestion and absorption. This may partly explain why calcium interferes with phosphorus absorption. In infants, phosphorus bioavailability is highest from human milk (85–90 percent), intermediate from cow milk (72 percent), and lowest from soy formu- las, which contain phytic acid (59 percent). However, the higher amounts of phosphorus contained in cow milk and soy formulas offset this decreased bioavailability. Dietary Interactions There is evidence that phosphorus may interact with certain nutrients and di- etary substances (see Table 2). INADEQUATE INTAKE AND DEFICIENCY Phosphorus deficiency is generally not a problem. This is because phosphorus is so ubiquitous in the diet that near total starvation is required to produce dietary phosphorus deficiency. However, if inadequate phosphorus intake does occur, such as in individuals recovering from alcoholic bouts, from diabetic ketoacidosis, and from refeeding with calorie-rich sources without paying at- tention to phosphorus needs, it is realized as hypophosphatemia. The effects of hypophosphatemia include the following: • Anorexia • Anemia • Muscle weakness • Bone pain • Rickets (in children) and osteomalacia (in adults) • General debility
OCR for page 367
PART III: PHOSPHORUS 367 TABLE 2 Potential Interactions with Other Dietary Substances Substance Potential Interaction Notes SUBSTANCES THAT AFFECT PHOSPHORUS Calcium Pharmacological doses of Calcium in the normal adult intake range is not likely calcium carbonate may to pose a problem for phosphorus absorption. interfere with phosphorus absorption. Aluminum When taken in large doses, antacids that contain aluminum may interfere with phosphorus absorption. PHOSPHORUS AFFECTING OTHER SUBSTANCES Calcium Excess intake of phosphorus This is less likely to pose a problem if calcium intake is may interfere with calcium adequate. absorption. • Increased susceptibility to infection • Paresthesias • Ataxia • Confusion • Possible death Special Considerations Antacids: Antacids that contain aluminum can bind with dietary phosphorus and, when consumed in large doses, produce hypophosphatemia. Treating malnutrition: The refeeding of energy-depleted individuals, either orally or parenterally, must supply adequate inorganic phosphate. Otherwise, severe and perhaps fatal hypophosphatemia may occur. EXCESS INTAKE Excess phosphorus intake from any source can result in hyperphosphatemia, the adverse effects of which are due to an elevated concentration of inorganic phosphate in the extracellular fluid. Hyperphosphatemia from dietary causes becomes a problem mainly in individuals with end-stage renal disease or in such conditions as vitamin D intoxication. The potential effects of hyperphosphatemia include the following:
OCR for page 368
DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 368 • Reduced calcium absorption (less problematic with adequate calcium intake) • Calcification of nonskeletal tissues, particularly the kidneys Concern about high phosphorus intake has been raised because of a probable population-level increase in phosphorus intake through colas and a few other soft drinks that contain phosphoric acid and processed foods containing phos- phate additives. High intakes of polyphosphates found in additives may inter- fere with the absorption of iron, copper, and zinc. However, further research is necessary in this area.
OCR for page 369
PART III: PHOSPHORUS 369 KEY POINTS FOR PHOSPHORUS The element phosphorus is found in nature primarily as 3 phosphate (PO4). It is a major component of bones and teeth. In fact, 85 percent of total body phosphorus is found in bone. Phosphorus helps maintain a normal pH in the body and is 3 involved in metabolic processes. Dietary phosphorus supports tissue growth and replaces phosphorus stores that are lost through excretion and the shedding of skin cells. The adult requirements for phosphorus are based on studies of 3 serum inorganic phosphate concentration. The UL was derived using data on the normal adult range for serum inorganic phosphate concentration. Nearly all foods contain phosphorus; dairy products are a 3 particularly rich source. Foods derived from plant seeds (e.g., beans, peas, cereals, 3 and nuts) contain phytic acid (also called phytate), a stored form of phosphorus that is poorly absorbed in humans. Phosphorus deficiency is generally not a problem; the average 3 adult diet contains about 62 mg phosphorus per 100 kcal. Excess phosphorus intake from any source can result in 3 hyperphosphatemia, the adverse effects of which are due to an elevated concentration of inorganic phosphate in the extracellular fluid. Hyperphosphatemia from dietary causes becomes a problem mainly in individuals with end-stage renal disease or in such conditions as vitamin D intoxication. There is concern about the population-level increase in 3 phosphorus intake through colas and a few other soft drinks that contain phosphoric acid and processed foods containing phosphates. High intakes of polyphosphates found in additives may interfere with the absorption of iron, copper, and zinc. However, further research is necessary in this area.