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A study by researchers from Purdue University and published in the Journal of Bone and Mineral Research has found that rats fed a diet of non-fat dry milk solids (NFDM) for 10 weeks had improved indicators of bone health during growth and that the NFDM protected bones against calcium depletion compared to those animals reared on supplements like calcium carbonate(CaCO3) .
During early childhood, intake of calcium is usually sufficient but once a child reaches early puberty this intake decreases as the trend is to replace milk and milk products with soft drinks. Public health policies acknowledge that supplementation therefore maybe necessary to achieve recommended calcium intakes. The aims of this study, funded by the US National Dairy Council, aimed to compare NFDM with CaCO3 for bone acquisition and investigate the effect of early diet on bone health during lack of dietary calcium.
The study by Weaver et al fed 4 week old rats with an adequate calcium diet (0.4% calcium) with calcium provided either by NFDM (not fortified with vitamin D or A) or CaCO3 for 10 weeks. At 10 weeks the researchers measured bones for density, strength, length and weight on 50 rats taken from each of the two groups. Following these measurements, some rats continued on the same diet whilst other were assigned to a calcium deficient diet (0.2% calcium provided by CaCO3) for another 10 weeks in order to look at the effect of consuming NFDM before periods of inadequate calcium intake. In the NFDM diet, 57.5% of dietary protein was provided by NFDM with the rest provided by casein. Protein in the other diet was provided by egg white which also contributed nearly 4.5% of the calcium. The NFDM diet contained 17% lactose.
Weaver et al discovered that the NFDM fed rats gained more weight than those on CaCO3 over the 10 week period. There was no difference between the two groups for soft tissue or fat or lean mass but higher weight was associated with higher whole body bone mineral density and bone area. Body weight differences in rats reassigned to the low calcium diet for the second period disappeared after approximately 2 weeks.
The study showed that bone density and calcium concentration was greatest in the rats fed NFDM for 20 weeks. They were also higher in rats fed NFDM for 10 weeks prior to switching to a low calcium diet than those on CaCO3 for 10 weeks and also switched. The researchers noted that bone width and strength showed no difference after 20 weeks feeding on the sufficient calcium diets.
In summary the researchers state that those rats fed NFDM at levels approximately equivalent to 3 cups of milk a day had larger, stronger bones than when CaCO3 was the source. Even when the rats were fed a diet containing inadequate calcium, following the early feeding of NFDM, these had improved bone measures compared with the rats who had been maintained on sufficient levels of calcium (as CaCO3) for 20 weeks.
It was concluded that the advantage of feeding NFDM during early growth periods over a diet using CaCO3 to give the same levels of calcium intake, suggests that the nutritional value of milk is greater than just its calcium content. This suggestion supports the US dietary guidelines for 2005 which indicated that milk and milk products provide more than 10% of the daily requirement for a range of vitamins and minerals. Weaver et al note however that the presence of these micronutrients in NFDM are unlikely to explain the advantage to bone. As the NFDM used was not fortified with vitamin D levels of this were not increased and while NFDM contained lactose, calcium absorption was not significantly different between the two sources used in this study. The researchers did not identify the cause of the improvements in bone health attributed to NFDM, although they suggested that the aromatic amino acids present in NFDM may influence calcium utilisation via a mechanism other than absorption.
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