PHOSPHORUS AND CALCIUM

PHOSPHORUS AND CALCIUM. Phosphorus and calcium are essential macronutrients in the human diet. Most of the phosphorus in the human body is found in association with calcium (as the mineral complex hydroxyapatite) in bones and teeth, where it is essential to structure. This form of phosphorus, along with phosphorylated sugars and proteins, is termed inorganic phosphorus. Organic phosphorus occurs primarily in the form of phospholipids.

Phosphorus absorption in the human body is proportional to the dietary intake, unlike calcium, where absorption is inversely related to the logarithm of intake. Absorption of phosphorus is facilitated by vitamin D. Phosphorus levels in urine are also directly related to dietary intake.

Important regulatory functions of phosphorus in the human body include maintaining the pH of body fluids,

the cellular osmotic pressure, and the energy transfer system in cells. Phosphorus is also an important component of DNA and RNA. Phosphorus deficiencies are rare and have been linked to anorexia, rickets, and skeletal and muscular abnormalities.

The amount of calcium consumed is not always related to the amount retained by the body. Availability of calcium for absorption is referred to as bioavailability, and is improved by factors such as the presence of phosphorus, vitamin D, and the extent to which the calcium mineral salt is ionized to form the divalent Ca²⁺ ion. Cellulose and hemicellulose fibers decrease absorption of calcium in the intestine. Sodium and caffeine both increase urinary calcium. Caffeine also causes an increase of calcium secretion into the human gut. Unabsorbed fats interfere with calcium absorption by forming a fatty acid–calcium soap complex, which is subsequently excreted. The bioavailability of calcium ranges from 5 to 70 percent for most foods, depending upon the type of mineral from which it is derived, the food product, and the presence of inhibitory substances. Oxalate and phytate both inhibit calcium absorption. A diet deficient in calcium may increase the risk of rickets, hypertension, osteoporosis, and scurvy. Vitamin D stimulates the absorption of calcium and phosphorus into bones. Osteoporosis is one consequence of low calcium intake during the younger years of bone mineralization. There are conflicting opinions on the efficacy of increasing the intake of calcium for older people who are already suffering from osteoporosis.

About 1.5 percent of the weight of an average person is made up of bones, which contain 99 percent of the body's calcium. Approximately 0.05 percent of the calcium in bones is exchanged daily by a process of solubilization and precipitation. In addition to its structural function, calcium in its ionized form allows blood clotting to take place. Calcium also regulates muscle contraction and relaxation.

Calcium is often found in association with phosphorus, forming the mineral calcium phosphate. This mineral is less soluble at higher temperatures (unlike most other minerals), resulting in "boiler-scale," a white precipitate on surfaces that come into contact with hot mineralized water. Table 1 presents a selection of foods containing phosphorus and calcium and gives the ratio of calcium to phosphorus in each; this is one important indicator of bioavailability. A low calcium to phosphorus ratio is associated with bone loss over time.

Milk is a very good source of dietary calcium. Calcium in milk is found primarily in the mineral calcium phosphate, which is approximately 30 percent bioavailable. It is possible that the high level of insoluble calcium phosphate in milk could cause calcification of the mammary gland, but casein milk proteins solubilize the mineral calcium phosphate. Calcium phosphate and proteins are responsible for much of the high buffering capacity of milk, which is a resistance to change in pH after the addition of acid or base. This allows milk to absorb a large amount of acid, such as that found in the human gut, providing some relief from acid reflux. Milk also contains a large amount of lactose. This precludes the use of milk as a source of dietary calcium and phosphorus for people suffering from lactose intolerance, an inability to metabolize lactose. For these people, other dairy products such as matured cheese and yogurt can be used to provide dietary calcium. Both of these dairy products are low in lactose. More information on calcium phosphate and lactose in milk can be found in Table 1 under Dairy Products.

Calcium in used in food to increase the gel formation of polysaccharides such as alginate and pectin, which can be used to increase the firmness of canned vegetables. Calcium also increases the firmness of milk gels and the rate of milk clotting, such as what occurs in cheese manufacture, by promoting aggregation of casein milk proteins. Phosphorus is present in emulsifiers used to produce the smooth texture required for processed cheese. It is also used to increase moisture retention in comminuted meats, as a leavening agent, as a pH buffering agent, and to acidify beverages. Phosphorus-containing agents such as phytate, oxalate, and phosphate act as chelating agents by binding calcium strongly.

The recommended dietary allowance for calcium is 210 mg/day (from birth to six months), 270 mg/day (aged seven to twelve months), 500 mg/day (aged one to three years), 800 mg/day (aged four to eight years), 1,300 mg/day (aged nine to eighteen years) and 1000 mg/day above eighteen years of age. The recommended dietary allowance for phosphorus is 100 mg/day (from birth to six months), 275 mg/day (aged seven to twelve months), 460 mg/day (aged one to three years), 500 mg/day (aged four to eight years), 1,250 mg/day (aged nine to eighteen years) and 700 mg/day above eighteen years of age.

See also Body Composition ; Composition of Food ; Dairy Products ; Dietary Assessment ; Dietary Guidelines ; Digestion ; Enteral and Parenteral Nutrition ; Lactation ; Milk, Human ; Nutrient Bioavailability ; Nutrients .

BIBLIOGRAPHY

Fennema, O. R., ed. Food Chemistry. New York: M. Dekker, 1966.

Hunt, Sara M., and James L. Groff. Advanced Nutrition and Human Metabolism. St. Paul, Minn.: West, 1990.

David W. Everett