-- Ray PeatBetween the first and second world wars, cod liver oil was recommended as a vitamin supplement, at first as a source of vitamin A, and later as a source of vitamins A and D. But in the late 1940s, experimenters used it as the main fat in dogs' diet, and found that they all died from cancer, while the dogs on a standard diet had only a 5% cancer mortality. That sort of information, and the availability of synthetic vitamins, led to the decreased use of cod liver oil.