Steenbock and Gross (28) have studied the more important fleshy roots used as food in the United States for man or animals, especially with respect to their content of fat-soluble A. They adopted the standard method of McCollum and Davis, feeding a purified food mixture supplemented with varying amounts of a natural food as a source of the dietary essential in question. Their results are of unusual interest for they showed that carrots and sweet potato, both of which contain much yellow pigment, are far better sources of fat-soluble A than are any of the roots which do not have a yellow color. Rutabega, dasheen, red beet, parsnip, potato, mangle and sugar beet appear from their results to contain practically none of this substance. They were inclined to the belief that this dietary essential is associated in foods with the yellow pigment.

This idea rested on insufficient evidence, as will be shown in Chapter XII (Chemical Studies Of The Dietary Essential, Fat-Soluble A. 286. Best Sources Of Fat-Soluble A), where the literature relating to this subject will be discussed. The roots so far as they have been studied, appear to be effective sources of water-soluble C.