Effect of glycerol feeding on endurance and metabolism during prolonged exercise in man

This study evaluated the effectiveness of pre-exercise glycerol feeding in protecting against development of hypoglycemia and sparing muscle glycogen during prolonged, intense exercise. Thirty minutes after ingesting either glycerol (1 gm X kg-1 body weight) or a placebo, 10 cyclists performed as much exercise on a cycle ergometer as they were able in 150 min. The average exercise intensity was 72% of VO2max during both trials. Glycerol ingestion increased blood glycerol concentration 100-fold, but did not alter the respiratory exchange ratio (R), plasma levels of insulin and free-fatty acids, or blood lactate and beta-hydroxybutyrate. The only significant effect of glycerol feeding was to postpone the decline in blood glucose by about 30 min. This suggests that glycerol served, to a limited extent, as a gluconeogenic substrate; however, glycerol ingestion did not spare muscle glycogen during 90 min of treadmill exercise at 71% VO2max. It appears that man cannot utilize glycerol as gluconeogenic substrate rapidly enough to serve as a major energy source during strenuous exercise.