Vitamin A plasma kinetics in northern fur seals (Callorhinus ursinus), using vitamin A2 as a tracer

Vitamin A2 (3,4-didehydroretinol), a natural analog of retinol, was used to determine the plasma kinetics of vitamin A in two northern fur seals (Callorhinus ursinus) receiving two levels of vitamin A supplements. After a single oral dose, the time required to reach maximum plasma vitamin A2 concentration for both subjects was approximately 2 h, which is much less than the 8–12 h observed with humans. The level of vitamin A supplementation did not affect base-line plasma retinol concentrations or plasma kinetics. The mean sojourn time, the average time spent by a molecule of vitamin A in the body before being lost, was 40 days for one animal and 73 days for the other. The daily vitamin A requirement estimated from the disposal rate was 0.32–0.64 μmol per day for the two animals. The presumed upper safe level was calculated at 1.3–6.4 μmol vitamin A per day. It would appear that the fish diet commonly provided to these animals can meet their vitamin A requirements. Supplementation may only be warranted to offset vitamin loss during storage and handling of the food. However, a conservative approach to supplementation should be used, as chronic vitamin A toxicity is a danger.