These have been studied to determine their value as a source of fat-soluble A. This substance, it will be remembered, is especially abundant in butter fat, egg yolk fat and in the leaves of certain plants. McCollum and Davis (42) and also Osborne and Mendel (43) found that lard had no appreciable growth-promoting properties when added to a diet complete except for fat-soluble A. Osborne and Mendel showed that cod liver oil is an excellent source of fat-soluble A. They examined the more liquid portion of beef fat and found that it contained in demonstrable amount the dietary factor in question, whereas the unfractionated fat was very poor in it (44). Halliburton and Drummond (45) obtained similar results in their study of the nutritive properties of various kinds of margarines.

Recently Daniels and Loughlin (46) have presented data which indicate that some samples of lard contain demonstrable amounts of fat-soluble A. For periods of five months they were able to secure growth in young rats with diets containing 28 per cent of lard as a source of this dietary essential. In other experiments where but 21 per cent was used the animals were not able to grow after two months. The findings in our own laboratory indicate that there was probably a certain amount of fat-soluble A in their ration derived from the extract of wheat germ added for the purpose of supplying water-soluble B.

These results are not to be interpreted as indicating that lard is in any sense comparable as a source of this dietary essential to the fats of milk, egg yolk or the leafy vegetables. It is doubtless true that the body fats of all animals which have been fed liberal amounts of a forage plant will contain sufficient amounts of fat-soluble A to make possible a short positive demonstration of its presence. It should be kept in mind that when questions relating to the adequacy of human dietaries are to be judged from the results of animal experiments, the content of the material fed, and the life history of the animals must be taken into consideration. The dietary history of the animals from which came the fat used in feeding should be considered, in order to make it clear whether the samples examined were typical. It seems that the products used by Daniels and Loughlin were not representative of lards in general.