Starvation response

Starvation response in animals (including humans) is a set of adaptive biochemical and physiological changes, triggered by lack of food or extreme weight loss, in which the body seeks to conserve energy by reducing the amount of calories it burns. Equivalent or closely related terms include famine response, starvation mode, famine mode, starvation resistance, starvation tolerance, adapted starvation, adaptive thermogenesis, fat adaptation, and metabolic adaptation. Ordinarily, the body responds to reduced energy intake by burning fat reserves and consuming muscle and other tissues. Specifically, the body burns fat after first exhausting the contents of the digestive tract along with glycogen reserves stored in liver cells and after significant protein loss. After prolonged periods of starvation, the body uses the proteins within muscle tissue as a fuel source, which results in muscle mass loss. The magnitude and composition of the starvation response (i.e. metabolic adaptation) was estimated in a study of 8 individuals living in isolation in Biosphere 2 for two years. During their isolation, they gradually lost an average of 15% (range: 9–24%) of their body weight due to harsh conditions. On emerging from isolation, the eight isolated individuals were compared with a 152-person control group that initially had had similar physical characteristics. On average, the starvation response of the individuals after isolation was a 180 kCal reduction in daily total energy expenditure. 60 kCal of the starvation response was explained by a reduction in fat-free mass and fat mass. An additional 65 kCal was explained by a reduction in fidgeting. The remaining 55 kCal was statistically insignificant. The energetic requirements of a body are composed of the basal metabolic rate (BMR) and the physical activity level (ERAT, exercise-related activity thermogenesis). This caloric requirement can be met with protein, fat, carbohydrates, or a mixture of those. Glucose is the general metabolic fuel, and can be metabolized by any cell.