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.

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.

Starvation mode is a state in which the body is responding to prolonged periods of low energy intake levels. During short periods of energy abstinence, the human body will burn primarily free fatty acids from body fat stores, along with small amounts of muscle tissue to provide required glucose for the brain. After prolonged periods of starvation the body has depleted its body fat and begins to burn primarily lean tissue and muscle as a fuel source.

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. After prolonged periods of starvation, the body will utilize the proteins within muscle tissue as a fuel source. People who practice fasting on a regular basis, such as those adhering to energy restricted diets, can prime their bodies to abstain from food while reducing the amount of muscle burned.

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.

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