Variations in vitamin A content of fish-liver oils, with particular reference to seasonal fluctuations in the potency of halibut-liver oil

Abstract

The A potency of liver oils from adult fish (23 species) covers a range of at least 2500:1. Good average cod liver oil contains approx. 0.04%. No consistent parallelism can be traced between A and D potencies. In certain species the A-potency increases with age and/or size of fish, the total A reserve increasing more rapidly than the oil potency. Probably 80% of the total carotene annually made available by the diatoms for conversion into A by marine animal life is initially synthesized during a few weeks in the spring or early summer. Halibut-liver oil, the richest known natural source, varies over a wider range than any other source. Oils containing from 0.17 to 10% of A have been examined, and these apparently do not represent extreme limits. Halibut-liver oils also show well marked seasonal fluctuations, which cannot be attributed to spawning, but which are curiously similar to the fluctuations in the abundance of the plankton. In the northern hemi sphere the richest oils are most likely to be obtained from large halibut caught in arctic waters in late spring or early summer, and in the autumn. Very rich oils at other times are exceptional.