Ever since research on the vitamines began, lard has almost invariably been used in "synthetic diets" as a source of fat free from vitamine A. To explain the absence of this vitamine in lard has always been somewhat of a puzzle, since animal fats are as a rule quite rich in vitamine A. Drs. Drummond and Zilva have come forward with an explanation. They point out that storage of vitamine A occurs in the body fat of the pig, provided the animal receives a diet containing considerable amounts of this essential (when, for example, the animals are grass-fed). When, however, the diet consists almost entirely of topping and whey, then the body fat shows vitamine A deficiency. During lard manufacture most of the vitamine, if any, is destroyed; the destruction is probably the result of oxidation (see preceding section). The low nutritive value of lard, then, is due to two causes: (1) to a diet offered to pigs which is usually poor in vitamine A, and (2) to the processes of lard manufacture. That the kind of diet of the animal should affect the vitamine content of the fat of the animal is a discovery that has since been repeatedly verified, and one to which we shall refer again.