Dietary requirements
The dietary requirements of calcium and phosphorus for children and adults have been investigated extensively. There is not, however, universal agreement among the experts on the interpretation of the findings.
There are many factors that affect the amount of calcium absorbed and retained by the body. A person is said to be in equilibrium with respect to any nutrient if the intake approximately equals the output. The assumption that end products of metabolism appear in the urine and unabsorbed material in the feces does not hold for calcium and phosphorus. Some metabolized (endogenous) calcium and phosphorus may be excreted through the intestinal tract. Moreover, the evidence that the humans can maintain calcium balance over a wide range cium intakes (the amount required to maintain this balance is largely determined by past dietary history) has made it impossible to determine the minimum requirement of this mineral. However, when balance studies have been conducted with groups of people accustomed to intakes of calcium, their daily calcium losses through the various routes have been found to fall within relatively narrow ranges.
The calcium and phosphorus requirements for growth have been investigated in children of different observing the level of intake at which maximum of calcium and phosphorus is attained. Growth of bone requires the storage of new calcium and phosphorus as well as replacement. The growth requirement with age, being highest in relation to weight in the infant, lower and fairly constant after the first year and ty, when there is a rise again during the period of rapid growth.
Except during infancy the calcium and phosphorus requirements are the same for all ages, although it is recognized that the intake of phosphorus is almost invariably higher than that of calcium. A 2:1 ratio of calcium to phosphorus (as found in human milk) has been shown to result in maximum calcium absorption and retention in animals, but the evidence for humans is less definitive. However, it appears that the high phosphorus intake from cow's milk (Ca to P ratio 1.2:1) may be involved in the development of hypocalcemic tetany in some infants during the first week of life, 10 Because a higher Ca:P ratio may be beneficial and certainly is not harmful as is evident in breast-fed infants, a 1.5: 1 ratio is recommended for the first year of life.
In breast-fed infants the calcium and phosphorus needs are amply met from milk, and maintenance of the recommended Ca:P ratio is possible even when substantial amounts of other foods are added to the diet toward the end of the first year. However, because of the lower Ca:P ratio of cow's milk and most commercial infant formulas, which vary from 1.2 to 1.4:1, intakes with lower than the recommended Ca to P ratios are likely for a large number of infants.
For older children and adults, the requirements for calcium and phosphorus are most easily met by including 2 to 3 cups of milk a day or its equivalent in milk products. Avoiding excessive consumption of high-phosphorus low calcium foods and beverages is essential if the recommended 1: 1 ratio of these two minerals is to be maintained. Although a wide rnnge of Ca to P ratios appears to be tolerated by adults as measured by calcium balance, this is an area in which more research is needed.
During all periods of life, vitamin D, or sunshine, is essential for the most efficient absorption and utilization of these two minerals.
The relationship between calcium intake and osteoporosis needs special consideration. It is now recognized that a gradual reduction in bone mass occurs with age (aging bone loss) in 11 great majority of the population, usually beginning during the fifth decade and advancing faster in women than in men. There is evidence that the condition of the bones in later life is related to the amount of bone present in early adult life and not primarily related to the calcium intake during adult life.21 Both a decreased rate of bone formation and an accelerated rate of bone resorption probably contribute to the osteoporotic changes. Although it is not known why some people develop the disease, whereas other appear to be protected, it is becoming increasingly clear that multiple factors are involved, including nonnutritional factors, such as genetics, changes in the levels of hormones (estrogen, parathyroid hormone, calcitonin), and lack of physical activity. Deftos and co-workers recently reported that the level of circulating calcitonin decreased with age in both sexes. Release of calcitonin in response to infusion of calcium also diminished with age, more frequently in women than in men.
The role of nutritional factors in both the prevention and treatment of osteoporosis remains unclear and controversial. Beneficial effects of calcium supplementation on calcium balance and in the prevention of bone loss in postmenopausal women have been reported, but the long-term significance of such supplementation remains obscure. Such factors as the increase in urinary calcium loss due to a high intake of protein, the decreased absorption of calcium wi th advancing age (which may nega tively affect a person's adaptability to low calcium intakes), and the bone loss that results in animals on a high intake of phosphorus would suggest that dietary modifications in susceptible people appear prudent. Because white women of small stature have the highest risk of developing osteoporosis, it has been suggested that they should consume liberal amounts of calcium and avoid excessive intake of phosphorus from the fifth decade on.
Because of the apparent ability to adapt to a wide range of calcium intake, the existence of calcium deficiency due to inadequate dietary intake has been questioned in the past. 26 Recent studies, however, of school age children in South Africa have revealed hypocalcemia, hypocalciuria, elevated alkaline phosphatase concentrations, and in some cases, radiologically detectable bone defects, in association with low calcium intakes. These changes are characteristic of vitamin D-deficiency rickets in younger children, but no evidence of vitamin D inadequacy has been found. Supplementation with calcium has resulted in rapid restoration of serum calcium to normal levels and in a gradual fall in the concentration of alkaline phosphatase. Other recent reports have suggested calcium deficiency as a possible cause of rickets. Hypophosphatemia due to inadequate phosphorus intake is unlikely but can develop from prolonged ingestion of nonabsorbable antacids.
