Feeding, fasting and starvation: factors affecting fuel utilization

Abstract

The ability of an organism to withstand periods of inadequate nutrition depends on the presence of appropriate nutrient stores together with the necessary adaptive responses. This is particularly true when considering energy metabolism during underfeeding and starvation, where the adaptations to undernutrition are vital for ensuring that the nutrient stores are utilized optimally. Professor Jtquier’s (1995) paper in the present symposium considers the utilization of substrates during the postprandial period; the present paper considers energy metabolism and substrate use in undernutrition, and the responses to food ingestion in the undernourished state. The major focus will be on studies in humans, mainly in experimental undernutrition. The terminology used in describing undernutrition is a significant source of confusion. For example, the most important single contribution in this area, The Biology of Human Starvation by Keys et al. (1950), did not investigate starvation but rather studied prolonged underfeeding (approximately 6 MJ/d for 6 months). In the human studies described in the present paper, a distinction will be made between fasting for up to 24 h, starvation for several (2-4) days and undernutrition-underfeeding for 7 d or more. In addition, chronic severe undernutrition will be referred to as chronic energy deficiency.