Factors affecting absorption and retention
Not all of the calcium and phosphorus in foods is available to the body. Approximately 20% to 40% of the calcium and 70% of the phosphorus consumed by a person becomes available when it is absorbed from the intestinal tract into the bloodstream. The amounts absorbed, however, may be greatly increased during periods of rapid growth when mineral needs are high. People with limited intakes of calcium are also more efficient in their utilization of it than those with a more liberal intake
Various factors, in addition to need and previous intake, influence the efficiency of calcium and phosphorus absorption. Adequate amounts of vitamin D, an acid pH in the upper part of the intestinal tract, and a normal motility of the gastrointestinal tract enhance the absorption of these minerals. Other dietary factors that appear to favor calcium absorption include some amino acids and lactose. On the other hand, large amounts of phytates (phosphorus compounds found in cereals) or oxalates, which can form insoluble compounds with calcium, may interfere with intestinal absorption. Even though these factors are not considered of practical significance when the intake of calcium is liberal, they may need attention in some populations that obtain an important fraction of their total calcium intake from greens high in oxalates. Steatorrhea due to impaired absorption offat is associated with high loss of calcium in feces, believed to result from the formation of insoluble calcium salts \vith free fatty acids which are present in excessive amounts in the intestine.
Increasing intake of phosphorus in relation to calcium has possible adverse effects that excess intake of phosphorus may have on metabolism of calcium. Recent research suggests that, contrary to previous assumption, increase in the intake of phosphorus within a range that is likely in human diets does not alter the intestinal availability of calcium significantly.8-10 Of major concern, however, is the possibility of increased mobilization of calcium by bone resorption and its subsequent loss as endogenous fecal calcium, as observed in rats. Induction of secondary hyperparathyroidism by a slight fall in serum calcium, which follows high phosphorus intake, appears to be involved. The evidence in humans is only suggestive, and there are differences in interpretation of experimental results.
The high protein intake may alter the retention of calcium. Even though protein may slightly enhance the intestinal absorption uf calcium because of the specific effects of somp amino acids, this appears to be offset by the increaspo urinary excretion of calcium, which follows a high intake of protein. A reduced rate of renal tubular reabsorption of calcium has been detected within a few hours after protein ingestion and during high protein intake continued for months. Also, more calcium appears to be filtered due to an increase in the glomerular filtration rate.
Still far from clear is the role of protein intake from mixed diets in determining the loss of body calcium. In a recent study in which varying in takes of meat were used to alter protein intake, slight calciuria was observed during the first few days on high meat diets, but the level of urine calcium soon returned to the level observed on the lower intakes of meat. The authors suggested that the high intake of phosphorus from the high-meat diet actually may have protected against urinary loss of calcium. There is also disagreement whether the total loss of body calcium is affected by high protein intake, \vith negative balances being observed in some studies,13 but not in others,2.17 and whether adaptation to high protein intake occurs with time.
Recent studies have demonstrated that decreasing the intake of protein reduces hypercalciuria in patients with urinary calculi more effectively than decreasing the intake of calcium,18 which is part of the usual treatment.
Because of the observed reduction in calcium absorption with age and at least partial loss of the capacity to adapt to changes in calcium intake, the possible roles that protein and phosphorus intakes may play in the varying degrees of bone loss associated with aging is of major concern and requires additional investigation.
In cases of parathyroid disturbances variations in calcium absorption and retention are even greater than under normal conditions. Hypercalcemia (high serum calcium) may occur in hyperparathyroidism, and hypocalcemia (serum calcium below normal) may result after removal of the parathyroid glands.
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