Effect of exercise on the mobilization of retinol and retinyl esters in plasma of sled dogs

Fasting dogs do transport vitamin A (VA) in plasma not only as retinol but predominantly as retinyl esters. Contrary to retinol, nothing is known concerning the effects of athletic performance on plasma retinyl ester concentrations. The aim of this study was therefore to examine whether physical stress because of exercise and modification of the oxidative stress by supplementation of alpha-tocopherol influences the concentrations of retinol and retinyl esters in plasma of sled dogs. The study was carried out on 41 trained adult sled dogs, which were randomly assigned into two groups. One group (19 dogs) was daily substituted with 50 mg dl-alpha-tocopheryl acetate per kilogram body weight and the control group (22 dogs) was maintained on a basal diet during 3 months prior to exercise. The plasma concentrations of retinol, retinyl esters, alpha-tocopherol and triglycerides were measured immediately before, directly after and 24 h after exercise. The supplementation of alpha-tocopheryl acetate had no effect on plasma retinol and retinyl ester concentrations at any measurement time point. However, retinyl ester levels doubled in the non-supplemented group immediately after the race (p < 0.001), whereas in the supplemented group similar high levels were observed not until 24 h post-racing (p < 0.001). The high levels of retinyl esters were paralleled to some extent by an increase in plasma triglyceride concentrations, which were significantly higher 24 h post-racing than immediately before (p < 0.001) and after exercise (p < 0.001) in both groups. The increase in retinyl ester concentrations might be indicative of their mobilization from liver and adipose tissue. Whether plasma retinyl esters can be used as an indicator for the extent of nutrient mobilization during and post-exercise in sled dogs remains to be elucidated.