Human Forearm Muscle Metabolism during Exercise VI. Substrate Utilization in Prolonged Fasting

Abstract

Five patients undergoing therapeutic starvation were studied after 21–26 days of fasting. Brachial artery blood flow and substrate concentrations in arterial and deep venous blood were determined at rest and during rhythmic forearm exercise. Forearm glucose uptake was markedly attenuated both at rest and during exercise in comparison with results for the postabsorptive state. The forearm RQ was low (0.80 ± 0.07) and FFA uptake high in comparison with the postabsorptive state. Even though levels of 3-hydroxybutyrate and acetoacetate were high, these substrates were not utilized at rest or during exercise. At rest there was conversion of acetoacetate to 3-hydroxybutyrate in the forearm muscle. It is concluded that alterations in muscle substrate utilization contribute importantly to whole body metabolic adaptation in prolonged fasting.

Key Words:

• Exercise
• forearm
• free fatty acids
• glucose
• ketone bodies
• lactate
• muscle substrate utilization
• starvation