The recommended calcium intake for an adult (including non-pregnant, pre-menopausal females) is estimated at 800 mg/day [MODERN NUTRITION IN HEALTH AND DISEASE by Shils, Olsen & Shike (1994)]. I find this same figure in many sources, and it allows for that fact that typical absorption is in the range of 25% to 50% of what is consumed. For example, milk and a variety of calcium supplement salts have shown an average calcium absorption of 32% [NEW ENGLAND JOURNAL OF MEDICINE 317:532-536 (1987)]. It is estimated that in the United States about 75% of calcium intake is from dairy products [THE AMERICAN JOURNAL OF CLINICAL NUTRITION 35:783-808 (April 1982)]. Typically, an ounce of cheese (28 grams) will contain 150-250 mg of calcium. An 8-ounce (245 gram) glass of skim milk will contain about 300 mg of calcium.

Vitamin D is the major regulator of calcium absorption in the intestine. Rickets is a disease of bone demineralization due to insufficient calcium due to insufficient Vitamin D. 84% of serum Vitamin D comes from D3, even though the major dietary source is D2. 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.

Both calcium and Vitamin D affect absorption of lead from the intestine, with low levels of these nutrients associated with higher levels of lead absorption. Calcium and lead compete for binding sites on intestinal mucosal proteins [NEW ENGLAND JOURNAL OF MEDICINE 302(20):1128-1131 (1980)].

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. Although 30% to 40% of the calcium in milk is absorbed, only 3% to 10% of the calcium in spinach is absorbed, due to the high levels of oxalate [AMERICAN JOURNAL OF CLINICAL NUTRITION 50:830-832 (1989)]. Broccoli, however, is high in calcium (about a half milligram of calcium per gram of broccoli) and low in oxalic acid. Uronic acid constitutes 10% of the non-cellulose fraction of cereal fiber — and 40% in fruits & vegetables. 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. In one study, replacing white flour products with whole wheat/bran products (fiber increase from 22 gm/day to 53gm/day) increased dietary calcium from 960 mg/day to 1302 mg/day, but reduced calcium balance from +32 mg/day to −77 mg/day. In another study, adding fruits & vegetables to a normal diet increased calcium intake from 1070 mg/day to 1166 mg/day, but reduced calcium balance from +77 mg/day to −122 mg/day.

Aside from the issue of dietary calcium absorption, there is the issue of calcium excretion. At protein intakes below 200 gm/day, 1.2 mg of calcium is excreted in the urine for every gram of protein consumed [JOURNAL OF NUTRITION 120:134-136 (1990)]. Protein increases the rate of calcium oxalate stone formation in the kidney. One study showed a decrease in kidney stones when calcium was taken in the diet, but an increase in kidney stones when calcium was taken as supplements [ANNALS OF INTERNAL MEDICINE; Curhan,GC; 126(7):497-504 (1997)]. But the standard therapy for kidney stones due to excessive intestinal oxalic acid is high supplemental calcium intake [ANNALS OF INTERNAL MEDICINE; Heaney,RP; 127(9):846 (1997)]. High calcium from either diet or supplements would be expected to combine with oxalate in the intestine and prevent oxalate absorption, so there must have been a component in the supplements used in the study that somehow resulted in increased oxalate absorption.

Calcium signalling dysregulation has been implicated in cognitive decline associated with aging. Aging neurons release calcium from intracellular stores, resulting in a sustained increase in free intracellular calcium [BIOFACTORS; Oliveria,AMM; 37(3):168-174 (2011)].

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