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.

Nutrition is important to bone health, and a number of minerals and vitamins have been identified as playing a potential role in the prevention of bone diseases, particularly osteoporosis. Despite this, there is currently no consensus on maximum levels to allow in food or as dietary supplements. The benefits of supplementation of populations at risk of osteoporosis with Ca and vitamin D are well established. Prolonged supplementation of Ca and vitamin D in elderly has been shown to prevent bone loss, and in some intervention studies to prevent fragility fractures. Although P is essential to bone health, the average intake is considered to be more than sufficient and supplementation could raise intake to adverse levels. The role of vitamin K in bone health is less well defined, though it may enhance the actions of Ca and vitamin D. Sr administered in pharmacological doses as the ranelate salt was shown to prevent fragility fractures in postmenopausal osteoporosis. However, there is no hard evidence that supplementation with Sr salts would be beneficial in the general population. Mg is a nutrient implicated in bone quality, but the benefit of supplementation via foodstuffs remains to be established. A consensus on dietary supplementation for bone health should balance the risks, for example, exposure of vulnerable populations to values close to maximal tolerated doses, against evidence for benefits from randomised clinical trials, such as those for Ca and vitamin D. Feedback from community studies should direct further investigations and help formulate a consensus on dietary supplementation for bone health.

Reply by Brandolini et al.M. Arnaud does not appear to accept any criticism about the large sulfate content of some Ca-rich mineral waters. He raises some important points again, but does not answer the main errors of interpretation 1 that we developed in our first letter 2 . We maintain our earlier point of view 2 . The 20 mg/d urinary Ca difference that we observed in our study 3 was statistically significant and the number of subjects and the crossover design of our study led to our conclusion. We maintain that the metabolic behaviour of inorganic sulfate is not different from that of sulfate derived from the catabolism of sulfur amino acids and that both induce an acidification of urine. We are well aware of the essential role of many sulfur-containing compounds in the body and of the urinary excretion of sulfurconjugated or -bound organic compounds, but the net requirement of sulfur is low compared with the dietary intake and most of the absorbed sulfate is excreted in the urine in an inorganic form. We still do not agree with the conclusions drawn from the study by Aptel et al.4 on the effect of some mineral waters on bone. Concerning the potential deleterious effect of an excess of dietary sulfate on colonic epithelium, we referred to the analysis and conclusions of Florin et al. 5. Considering that several studies have shown that bicarbonate mineral waters are more beneficial for bone than sulfate-rich mineral waters, the only way to be done with this on-going controversy would be to carry out a similar study to ours, but with a more complete design (full metabolic balance), comparing a bicarbonate water with a sulfate water providing the same amounts of Ca and other nutrients (that is possible with another water but not with milk).

It is well known that the intestinal availability of Ca from Ca-rich mineral waters is equivalent to that of milk Ca. However, the effect of associated anions on Ca urinary loss needs to be addressed. The aim of the current study was to compare, under ordinary conditions of consumption, milk and a SO 4 -rich mineral water as the Ca provider in a large number of subjects consuming the same quantity of Ca from the two sources in a crossover study lasting for an extended period. Thirty-seven healthy women completed a 12-week protocol, divided into four periods of 3 weeks (W). In the first (W1-3) and third (W6 -9) periods, dietary Ca intake was restricted to 600 mg/d. In the second (W4 -6) and final (W10-12) periods, either 400 ml/d medium-fat milk or 1 litre of a Ca-and SO 4 -rich mineral water, each providing about 480 mg Ca/d, was added to the diet in a random manner. Dietary evaluation, blood and urinary measures were performed during the last week (W6 and W12) of each Ca supplementation period. The urinary excretion of Ca was higher (0·5 mmol/d more) with water than with milk (P, 0·001). An examination of all the dietary factors known to influence calciuria suggested that the acidogenic action of SO 4 was responsible for this increased calciuria. Thus, despite an equal Ca intake and assuming an unchanged intestinal absorption, these results suggest that Ca balance is better with milk consumption than with CaSO 4 -rich water.

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