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.

In contrast to corn, wheat and triticale exhibit high phytase activities. This enzyme enhances phytic phosphorus availability, as demonstrated in pigs given wheat diets. To study the utilization of triticale phosphorus in pigs, the importance of dietary phytase content and the mineral and bone disorders related to high phytate feeding, a nutritional experiment was carried out in 12 growing pigs fed either a corn- or a triticale-based diet for 6 wk. The diets were almost identical except for the cereal component; their phosphorus contents were low (0.4%) and mainly phytic. The following parameters were measured: calcium and phosphorus balances, bone and plasma contents of calcium and phosphorus, plasma vitamin D metabolites and parathyroid hormone (PTH), bone bending moments and intestinal phosphatase activities. Both diets provoked a phosphorus deficiency, but hypophosphatemia occurred less rapidly, hypercalciuria and hypophosphaturia were less marked and phosphorus availability was greater when the triticale diet was fed. This was attributed to the high phytase content of triticale because intestinal phytase and alkaline phosphatase activities were similar in pigs fed either diet. Calcium absorption was not modified by calcium retention was greater for pigs fed triticale and led to higher bone scores. In conclusion, the higher the phytase activity of the diet, the greater the phytate P availability and the lower the bone-mineral disorders.

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Phytoestrogens are plant-derived compounds with estrogen-like activity. Phytoestrogen-rich diets may prevent postmenopausal osteoporosis and these molecules maintain bone mass in ovariectomized animals. We compared the effects of the isoflavone daidzein, which has no action on tyrosine kinases, and 17␤-estradiol on the development and activity of osteoclasts in vitro. Nonadherent porcine bone marrow cells were cultured on dentine slices or on culture slides in the presence of 10

The role of estrogens and estrogen-like molecules, including isoflavones, in regulating bone cell activities is essential in understanding the etiology and treatment of post-menopausal osteoporosis. Although estrogen replacement (HRT) has been the main therapy used to prevent and treat osteoporosis, there are concerns about its safety. Isoflavones have attracted attention to their potential roles in osteoporosis prevention and treatment. We have compared the effects of the isoflavone daidzein (1 nM), which has no effect on tyrosine kinases, and 17beta-estradiol (1 nM) on the development and function of cultured osteoblasts isolated from long bones of young female piglets. Daidzein increased ALP activity, osteocalcin secretion, and mineralization, while E2 increased only ALP activity. The content of ERbeta and osteoprotegerin secretion by control cells gradually increased during osteoblast differentiation, whereas the ERalpha and RANK-L content decreased. Daidzein enhanced only the nuclear ERbeta whereas estradiol increased both ERalpha and ERbeta. Daidzein and estradiol increased osteoprotegerin and RANK-L secretion. Daidzein had a more pronounced effect than did estradiol. Daidzein and estradiol increased the membrane content of RANK-L and the nuclear content of runx2/Cbfa1. Daidzein enhanced the nuclear content of progesterone and vitamin D receptors but not as much as did estradiol. All the effects of daidzein were blocked by ICI 182,780. We conclude that a low concentration of daidzein may exert its anti-resorptive action by increasing the activity of porcine mature osteoblasts via ERbeta, by regulating runx2/Cbfa1 production, and by stimulating the secretion of key proteins involved in osteoclastogenesis, such as osteoprotegerin and RANK-ligand.