I hope that this summary may save one of my readers from a trip to the morgue. Vitamin D is the major regulator of calcium absorption in the intestine. The entire US RDA of 400 IU of Vitamin D is added to every US quart of processed milk sold. D3 levels are markedly influenced by sunlight, which means that for most people sunlight is their major source of vitamin D. Higher rates of bone fractures among people in temperate climates reflects this fact. I take a multi-mineral supplement containing 200 IU of D3 plus Life Extension Mix, which contains 300 IU of D3 per daily dose.
Food sources have a significant effect on calcium absorption.
Phytate, Oxalate, Fiber and Uronic Acid can significantly decrease calcium absorption. In infants, lactose can nearly double the rate of calcium absorption. A typical diet has enough uronic acid to bind about 150 mg of calcium, but in a vegetarian diet this could reach 360 mg. Phytate in wheat bran can also reduce calcium absorption, and this effect is compounded by the fiber. Aside from the issue of dietary calcium absorption, there is the issue of calcium excretion.