The effect of prolonged intermittent exercise, combined with food deprivation, on plasma metabolite concentration.

To examine the effect of continuous fasting combined with prolonged, intermittent exercise on glucose homeostasis, 16 endurance-trained subjects, ranging in age from 18-21 years, were completely deprived of food during 81 h of field maneuvers. Water was supplied to avoid dehydration. Participants marched 10 h each night at an estimated intensity of 35-45% of mean VO2 max covering a total distance of 105 km, and had a relative rest during day time. Blood was sampled prior to the beginning of the march, at the end of 81 h, and after 24 h of recovery. Samples were analyzed for plasma glucose, insulin, alanine, free fatty acids (FFA), and 3-hydroxybutyrate (3-HB). Body weight decreased from a mean (+/- S.E.M.) of 73.0 +/- 0.6 kg at pre-march to 66.5 +/- 0.6 kg at 81 h of fasting (p less than 0.05), and remained unchanged at 24 h after march termination. Glucose, insulin, and alanine decreased, whereas FFA and 3-HB increased significantly at 81 h fast (p less than 0.05). Within 24 h of recovery all parameters changed significantly (p less than 0.05), approaching baseline values. The results indicate that in trained individuals under extreme survival conditions, as in the present study, blood glucose is maintained above hypoglycemic levels at the expense of fat and fat-derived substrates that become the main energy sources utilized.