Researchers at the United States Department of Agriculture conducted an experiment to determine the bioavailability (absorption) of three common sources of calcium. Dietary surveys carried out in the US have consistently shown that the average daily intake of calcium is far below that required for prevention of osteoporosis; this is especially true for elderly people. The recommended daily intake for adults over the age of 51 years is now 1200 mg. This amount is difficult to obtain from the diet especially among older people who may have impaired calcium absorption due to a lack of stomach acid (hypochlorhydria), intestinal resistance to vitamin D or, in the case of postmenopausal women, an estrogen deficiency. The researchers believe that increased milk consumption or supplementation is required in order to ensure an adequate daily calcium intake. Their experiment to determine the absorption of calcium included skim milk, calcium-fortified orange juice, and calcium carbonate tablets. Twelve volunteers participated in the 6-week crossover study. The supplement protocol consisted of a 15 oz glass of skim milk at breakfast and dinner, an 11 oz glass of orange juice fortified with calcium citrate maleate at breakfast and dinner, or a 500 mg calcium carbonate tablet with breakfast and dinner. The researchers monitored the blood concentration of parathyroid hormone (PTH suppression test), calcium serum and urinary levels, and urinary collagen type I N-telopeptide cross-links (NTX) – a measure of bone resorption. The evaluation of each calcium supplement was preceded by a week on a low calcium diet and no supplementation. The researchers conclude that calcium from skin milk, fortified orange juice, and calcium carbonate are equally well-absorbed, i.e. all three are good, bioavailable sources of calcium. They did note that the phosphorous intake, when on the skim milk supplementation program, was significantly higher (by a factor of 4 to 5) than when supplementing with orange juice or calcium carbonate.
Martini, Ligia and Wood, Richard J. Relative bioavailability of calcium-rich dietary sources in the elderly. American Journal of Clinical Nutrition, Vol. 76, December 2002, pp. 1345-50