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)].
Calcium supplementation is recommended for elderly people (particularly post-menopausal women) to protect bone health. The majority of men and women over age 50 take a calcium supplement. Calcium intake above 700 mg (milligrams) daily does not further reduce the risk of osteoporosis [BMJ; Warensjo,E; 342:d1473 (2011)]. But excessive calcium intake can increase the risk of fatal heart disease according to several large epidemiological studies. A study of over sixty thousand women found that calcium intake above 1,400 mg per day more than doubled all-cause mortality, and increased the risk of heart disease in particular [BMJ; Michaelsson,K; 346:f228 (2013)]. Another study involving more than twenty thousand people found increased heart disease mortality for calcium intake above 1,130 mg daily, but (unlike the previous study) this study did not adjust for smoking — those taking calcium supplements were more likely to be smokers [HEART; Kuanrong,L; 98:920-925 (2012)]. A study of nearly four hundred thousand people found that taking calcium supplements increased heart disease death in men, but not in women [JAMA INTERNAL MEDICINE; Xiao,Q; 173(3):639-646 (2013)].
A review of the role of micronutrients in osteoporosis suggests that micronutrients in combination with calcium are important in keeping calcium in bone and off of blood vessels. Calcium supplements should be taken with food because calcium is better absorbed with food. Calcium carbonate tablets are typically 40% calcium, whereas calcium citrate tablets are only about 23% calcium [AMERICAN JOURNAL OF CLINICAL NUTRITION; Nieves,JW; 81(suppl):1232S-1239S (2005)]. Oversupplementation with calcium can lead to relative zinc deficiency, osteochondrosis, and retard bone remodeling [ TOPICS IN COMPANION ANIMAL MEDICINE; Cline,J; 27(4):159-164 (2012)].
Vitamin D synthesis in the skin declines
with age, so 800-1000 IU per day may be required for bone health in people
over age 65. Adults with Vitamin D deficiency have muscle weakness,
and are thus more likely to fall and fracture bones. Intake of Vitamin A
and phosphorous should be neither inadequate or excessive for optimal bone
mineralization. Milk products are the only food group that is high in both
calcium and phosphorous. Five to ten servings of fruits and vegetables per
day are recommended for adequate potassium, magnesium, Vitamin C, and
Vitamin K to prevent
osteoporosis [AMERICAN JOURNAL OF CLINICAL NUTRITION; Neives,JW; 81(Suppl):1232S-1239S
(2005)]. A daily multi-vitamin/mineral supplement with means can be an
insurance policy against inadequate nutrition for those who might eat less
than 5 to 10 servings of fruits and vegetables per day.