Starvation response

Starvation response in animals is a set of adaptive biochemical and physiological changes that reduce metabolism in response to a lack of food. Starvation response in animals is a set of adaptive biochemical and physiological changes that reduce metabolism in response to a lack of food. 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. Starvation contributes to tolerance during infection, as nutrients become limited when they are sequestered by host defenses and consumed by proliferating bacteria. One of the most important causes of starvation induced tolerance in vivo is biofilm growth, which occurs in many chronic infections. Starvation in biofilms is due to nutrient consumption by cells located on the periphery of biofilm clusters and by reduced diffusion of substrates through the biofilm. Biofilm bacteria shows extreme tolerance to almost all antibiotic classes, and supplying limiting substrates can restore sensitivity. 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. 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 and the physical activity level. 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. Fructose and some other nutrients can only be metabolized in the liver, where their metabolites transform into either glucose stored as glycogen in the liver and in muscles, or into fatty acids stored in adipose tissue. Because of the blood–brain barrier, getting nutrients to the human brain is especially dependent on molecules that can pass this barrier. The brain itself consumes about 18% of the basal metabolic rate: on a total intake of 1800 kcal/day, this equates to 324 kcal, or about 80 g of glucose. About 25% of total body glucose consumption occurs in the brain. Glucose can be obtained directly from dietary sugars and by the breakdown of other carbohydrates. In the absence of dietary sugars and carbohydrates, glucose is obtained from the breakdown of stored glycogen. Glycogen is a readily-accessible storage form of glucose, stored in notable quantities in the liver and skeletal muscle. When the glycogen reserve is depleted, glucose can be obtained from the breakdown of fats from adipose tissue. Fats are broken down into glycerol and free fatty acids, with the glycerol being turned into glucose in the liver via the gluconeogenesis pathway.