Not only does the calcium in milk help to curb bone loss, but it may also impart some protection against atherosclerosis (hardening of the arteries) and clogging of blood vessels with plaque, according to an animal study in the February JOURNAL OF NUTRITION.
However, that same study shows that when a calcium-rich diet also contains a substantial excess of vitamin D, the opposite occurs — animals develop an elevated susceptibility to atherosclerosis. Explains Donald Beitz, one of the researchers at Iowa State University in Ames, “Vitamin D plays a role in calcium deposition.” Since plaque hardens as it matures, there was some suspicion that an excess of vitamin D might aid in this calcification. Working with groups of six male kids, the researchers fed each group of goats a diet with a different milk ration: plain goat milk, milk with 2 to 2.5 times the recommended calcium level, milk with 100 times the recommended vitamin D, and milk with double the calcium and 100 times the vitamin D. Kids receiving the calcium-only supplement had the healthiest arteries of all; those with the calcium/vitamin D combo fared worst, with serious calcification and atherosclerosis. Beitz says they chose a 100-fold excess of vitamin D to accelerate the onset of any related effect during their 20-week study. If a comparable effect occurs in humans, he speculates, it might take only a 5- or 10-fold excess of the vitamin — over a 50-year period — to achieve a similar effect.
As to why calcium alone might be protective, Beitz says one theory is that it may form a calcium salt of the bile acid, thereby causing a drain on cholesterol, from which these bile acids are synthesized. The Iowa State researchers plan to study the vitamin D/calcium interaction next in pigs — an animal that better models what one might expect to occur in humans.