Reduction of Residual Calcium in the Chyme
An inevitable consequence of reduced calcium intake is a reduction of calcium in the food residue delivered to the ileum, cecum, and colon. This is both because less calcium is ingested and because fractional absorption rises as intake drops. The quantitative features of the relationship between diet and fecal calcium are illustrated, which is based on over 500 calcium balance studies performed by the author in middle-aged women. With the primitive diet, net calcium absorption would have been approx 10%, and fecal calcium would thus have been nearly as high as dietary intake.
The upshot of low intakes is that there is less calcium available to form complexes with other food residue substances that may be harmful in some individuals. The two conditions for which this effect is best established are colon cancer and renolithiasis. The potentially harmful chyme substances are free fatty acids and bile acids for colon cancer, and oxalic acid for renolithiasis. There is no good chemical theory for a mixture as complex as the chyme at any location through the gastrointestinal tract, let alone in its distal segments. Nevertheless, a few observations concerning the quantities of reactants required to Complex possibly noxious substances in the chyme may be useful. Fat absorption is highly efficient; nevertheless from 1 - 5% of ingested fat normally escapes absorption in the small bowel and is delivered to the cecum as free fatty acids.
For a typical North American diet with 35% of calories from fat, that translates to 3 - 1.5 mmol of unabsorbed fatty acid, plus a smaller quantity of unreabsorbed bile acid. If there were no other ligands for the calcium in the chyme, this quantity of fatty acid would require, for its full neutralization, the presence in the mixture of 1.5 - 15 mmol (60 - 600 mg) of calcium [depending on the mixture of Ca(FA)+ and Ca(FA)2]' As the relationship, it would require a diet containing up to 680 mg calcium to achieve that level. This is slightly above the median for U.S. women. But fatty acids are not the only ligand competing for calcium. Phosphate, oxalate, phytate, and various organic acids produced by bacterial fermentation of colonic contents are also present in varying abundance.
Unabsorbed phosphate from a typical diet alone amounts to 10 - 15 mmol, and would be expected to bind with an equivalent quantity of calcium (forming CaHPO 4)' Thus, it can be estimated that chyme calcium levels of more than 30 mmolld are required to neutralize the potentially noxious products of digestion left in the food residue. As the relationship of indicates, that level in the chyme requires an intake of at least 34 mmol (1360 mg) Ca/d. This value should be taken as a lower estimate, as the efficiency of ligand binding and the respective binding constants in a medium such as chyme are unknown.
