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Abstract
This update focuses on the bioavailability of dietary calcium for humans. Fundamentals of calcium metabolism, intestinal absorption, urinary excretion and balance are recalled. Dietary factors, especially lactose and other milk components, influencing calcium bioavailability at intestinal and renal levels are reviewed. A critical examination of all the methods used for evaluating calcium bioavailability is made. This includes in vitro assays, classical and isotopic balances, urinary excretion, isotope labeling in the urine, plasma and bones, long term evaluation of bone mineralization and the use of biological bone markers. Importance and advantages of animal models are discussed. The state of the art in the comparative bioavailability of calcium in foods is detailed including a comparison of sources of calcium (dairy products and calcium salts) in human studies and in some animal studies, casein phosphopeptides, proteins, lactose and lactase and their relation with calcium bioavailability (in humans and rats). An update on the consumption of dairy products and bone mass is presented. Emphasis on peculiarities and advantages of calcium in milk and dairy products is given.
Key teaching points:
• Milk provides large amounts of calcium and phosphorus and components such as lactose and casein phosphopeptides which may enhance calcium absorption and mineral retention.
• Using a variety of methods, no one has shown that the calcium in milk is more efficiently absorbed that most calcium salts.
• Intestinal absorption does not necessarily reflect the bioavailability of calcium to the whole organism because calcium must be retained and used in bone formation and mineralization.
• Three sources of calcium, milk, calcium carbonate and calcium citromalate, have been extensively studied. They all ensure the efficient absorption of calcium and also ensure, over the long term, that calcium is retained and used for bone mineralization.
• There is, as yet, no evidence showing that the calcium from mineral water is as effective.
• Many direct or indirect methods may be used to evaluate calcium bioavailability. The values obtained depend on the method; thus, conclusions or comparisons must be drawn with care.
Key teaching points:
• Milk provides large amounts of calcium and phosphorus and components such as lactose and casein phosphopeptides which may enhance calcium absorption and mineral retention.
• Using a variety of methods, no one has shown that the calcium in milk is more efficiently absorbed that most calcium salts.
• Intestinal absorption does not necessarily reflect the bioavailability of calcium to the whole organism because calcium must be retained and used in bone formation and mineralization.
• Three sources of calcium, milk, calcium carbonate and calcium citromalate, have been extensively studied. They all ensure the efficient absorption of calcium and also ensure, over the long term, that calcium is retained and used for bone mineralization.
• There is, as yet, no evidence showing that the calcium from mineral water is as effective.
• Many direct or indirect methods may be used to evaluate calcium bioavailability. The values obtained depend on the method; thus, conclusions or comparisons must be drawn with care.
The authors are indebted to ARILAIT-Recherches (Paris) for constructive discussion and financial support. We also thank Owen Parkes, Colette Colin and Marie-Claire Kopka for their help in editing the text.