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Influence of Fasting on Carbohydrate and Fat Metabolism during Rest and Exercise in Men,

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Metabolic effects of an overnight fast or a 3.5-day fast were compared in 8 healthy young men at rest and during exercise to exhaustion at 45 VO2 max, Glucose rate of appearance Ra and disappearance Rd were calculated from plasma glucose enrichment during a primed, continuous infusion of glucose. Serum substrates and insulin levels were also measured. Glycogen content of the m. vastus lateralis was determined in biopsies taken before and after exercise. At rest, glucose flux and whole body carbohydrate oxidation determined from the respiratory exchange ration were lower in F than PA but muscle glycogen levels were similar. During exercise, glucose flux, whole body carbohydrate oxidation and the rate muscle of glycogen utilization were significantly lower during the fast. In the PA state, glucose Ra and Rd increased together throughout exercise. However, in the F state Ra exceeded Rd during the first hour of exercise, causing an increase in plasma glucose to levels similar to those of the PA state. The increase in glucose flux was markedly less throughout F exercise. Lower carbohydrate utilization in the F state at rest and during exercise was consistent with higher circulating fatty acids and ketone bodies, lower levels of plasma insulin and the maintenance of physical performance as reflected by similar time to exhaustion. Keywords Starvation Lactate Alanine Free fatty acids Glycerol.

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  • Anatomy and Physiology

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