Phosphorus calcium absorption

Dietary phosphorus also affects calcium metabolism. Polyphosphate decreases calcium absorption in young men while orthophosphate supplement does not (26J. However, in the rat all forms of phosphate decrease calcium absorption about equally (31). However, widely divergent dietary calcium phosphorus ratios do not seem to affect calcium utilization by people as long as there is adequate phosphorus intake (32). In general phosphorus stimulates calcium retention in man (3277... 

Based on limited epidemiologic evidence, fluoride supplements, with or without calcium, estrogen and vitamin D, are used by clinicians for the treatment of osteoporosis. However, knowledge of the effects of fluoride on calcium and phosphorus metabolism in normal animals is limited although Spencer et al. (32) reported that ingestion of fluoride by three osteoporotic men did not affect calcium absorption but caused a decrease in urinary excretion. Moreover, there is a need to determine the long-term effects of fluoride treatment on bone strength and on soft tissues ( ). 

Milk is one of the best dietary sources of calcium, not only because of the significant quantity of the mineral present but also because of (1) its calcium-to-phosphorus ratio (1.3 1), which is conducive to optimal skeletal growth, and (2) the presence of nutrients such as lactose and vitamin D in vitamin D-fortified milk, which promote calcium absorption (Ziegler and Fomon 1983 Schaafsma 1983 Renner 1983). 

There is some evidence, mostly from animal studies, to suggest that high dietary levels of phosphorus, especially if dietary levels of calcium are low, may adversely affect bone mass and calcium metabolism (Greger and Krystofiak 1982). However, in humans there is little direct evidence to indicate that large variations in dietary phosphorus or in the Ca P ratio have any significant influence on calcium utilization or balance (Heaney et al. 1982). Some preliminary findings, however, suggest that the form of phosphorus may influence calcium absorption (Zemel et al. 1982). Hexametaphosphate, as compared with orthophos-... 

The next stage involves the synthesis of specific calcium-binding proteins, typified by the intestinal CaBP253 discussed in Section 62.1.3.4.5, which probably stimulates the transport of calcium. The role of the protein in vitamin D-dependent absorption of calcium is supported by the good correlation between the concentration of CaBP and the rate of calcium absorption. Under conditions of low calcium or phosphorus diets, chicks and other animals produce more intestinal CaBP to increase the efficiency of uptake of calcium. In general, adaptation to a low calcium diet involves increased synthesis of l,25-(OH)2D3 and the intestinal CaBP. Lowered requirements for calcium in old age are manifested by lower levels of both factors.449,450... 

High phosphorus content may interfere with calcium absorption. 

Fig. 5. The depressive effect of phosphate on calcium absorption. All samples contained 1 mg % calcium. Maximum depression is reached at 0.4 mg % phosphate (concentration as phosphorus) (from reference (Z3) by permission).

Berkelhammer C, Wood RJ, Sitrin MD. Inorganic phosphorus reduces hypercalciuria during total parenteral nutrition by enhancing renal tubular calcium absorption. J Parenter Enteral Nutr 1998 22(3) 142-6. 

Calcitriol is the main active metabolite of vitamin D, and synergizes with parathormone in mobilizing bone calcium and Increasing calcium absorption from the intestine. Vitamin D occurs in a number of sterol forms. These include vitamin D3 (cholecaldferol - the form in foods and made in the skin by the action of UV) vitamin Dj (ergocalciferol -also from plants). These forms are 25-hydroxylated in the kidney, and then la-hydroxylated in the kidney (under the control of parathormone), to make the most active form. This is available as calcitriol. Vitamin D facilitates the absorption of calcium and to a lesser extent, phosphorus, from the intestine and promotes deposition into the bones. A deficiency of vitamin D therefore results in bone deficiency disorders, e.g. rickets in children. Therapeutic replacement of vitamin D in cases of severe deficiency requires quantities of the vitamin best provided by one of the synthetic vitamin D analogues (e.g. alfacalcidol and dihydrotachysterol). 

Calcium carbonate is the salt of choice because it contains the highest amount of elemental calcium and is the least expensive (see Table 88-5). The fraction of calcium absorbed is dose-limited, so maximum single doses of 600 mg or less of elemental calcium are recommended. Calcium carbonate tablets should be taken with meals to enhance absorption. Calcium citrate absorption is acid-independent and need not be administered with meals. Although tricalcium phosphate contains 39% calcium, nonabsorbable calcium-phosphorus complexes may limit overall calcium absorption compared to other products. This product may be required for up to 10% of seniors with hypophosphatemia that cannot be resolved with increased dietary intake. Disintegration and dissolution rates vary significantly between products and lots. Products with good disintegration and dissolution rates and lead contents of less than 1 mcg/day should be recommended. 

Vitamin D is a fat-soluble vitamin that has a major role in regulating calcium and phosphorus metabolism and is needed for calcium absorption from the intestines. 

Heaney, R.P. (2000) Dietary Protein and Phosphorus Do Not Affect Calcium Absorption, Am. J. Clin. Nutr. 72,758-761. 

Chemical reactions between two or more species from the analytical sample also can occur. The calcium-phosphorus system has been one of those most extensively studied. Calcium, in the presence of phosphate, produces a stable calcium-phosphorus compound that removes large numbers of calcium atoms from the flame. The result is a severe depression of the calcium absorption signal. The same effect is observed with strontium in the presence of phosphate. 

The beneficial effect of vitamin D upon calcium absorption has already been noted and is usually stressed as the major action of this vitamin upon calcium and phosphorus metabolism. There is considerable evidence, however, of further effect upon this metabolism within the body, but its precise nature is still a subject for speculation. 

Phosphate level—Adequate vitamin D enhances the levels of phosphates in the body, because of (1) improved absorption of phosphorus through the intestinal wall, independent of calcium absorption and (2) increased resorption of phosphates from the kidney tubules. When sufficient vitamin D is not available, urinary excretion of phosphate increases and the blood level drops. 

Dietary fat does not affect calcium absorption except in individuals with diseases that impair fat malabsorption (e.g., short bowel syndrome, ceUac disease, and pancreatitis). In these conditions, the calcium forms an insoluble and unabsorbable soap with the unabsorbed fat in the alkaline lumen of the small intestine, potentially resulting in impaired bone mineralization. In addition, the luminal calcium is not available to precipitate the oxalates, meaning that the free oxalates will be hyperabsorbed leading to increased risk for renal oxalate stones. Neither dietary phosphorus nor a wide range of phosphorus-to-calcium ratios affect intestinal calcium absorption in very low-birth-weight infants and adults. 

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