Adequacy of Cow Milk as a Source of Magnesium for Rats

Duncan, Huffman and Robinson 1 reported that calves restricted to whole milk diets alone, or supplemented with iron, copper and manganese, eventually manifest tetany which is indistinguishable from tetany in rats on a magnesium-low diet. 2 The tetany was prevented and serum magnesium was restored to normal by the administration of magnesium carbonate or magnesium oxide. 3 Since the content of magnesium in milk appears to be rather low, in relation to the calcium and phosphorus, it is possible that prolonged restriction to milk diets of young of other species might eventuate in magnesium deficiency, as suggested by Schmidt and Greenberg. 4 According to data cited by Cox and Mueller, 5 milk of cows and humans may contain about 0.013 and 0.005% magnesium respectively. Analyses of rat milk gave 0.031% magnesium. 5

But in several investigations rats have been reared to adulthood on cow milk supplemented with iron and copper and in no instance have symptoms been reported which indicate the occurrence of magnesium deficiency. 6 This suggests that rats are either (a) better able than calves to utilize the magnesium of cow milk or (b) their requirement for the element is less than that of calves. Duncan and associates believed that “…. there is a failure in the magnesium metabolism which prevents the animal (calf) from utilizing the available magnesium.” Whatever might be the explanation of these observations, the findings suggest that direct investigation should be made of the adequacy of cow milk as a source of magnesium for rats.

Young rats weighing 40 to 45 g were used. They were kept in individual cages and fed exclusively commercial grade A pasteurized cow milk ad libitum, supplemented with 0.25 mg thiamin per liter.