Starvation mode

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

Ordinarily, the body responds to reduced caloric intake by burning fat reserves first, and only consumes muscle and other tissues when those reserves are exhausted. Specifically, the body burns fat after first exhausting the contents of the digestive tract along with glycogen reserves stored in muscle and 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 caloric restricted diets, can prime their bodies to abstain from food without burning lean tissue.. Resistance training (such as weight lifting) can also prevent the loss of muscle mass while a person is caloric restricted.

Process

The body uses glucose as its main metabolic fuel if it is available. About 20% of the total energy consumption occurs in the brain. The rest of the glucose consumption fuels muscle tissue and red blood cells.

Glucose can be obtained directly from dietary sugars and carbohydrates. In the absence of dietary sugars and carbohydrates, it is obtained from the breakdown of glycogen. Glycogen is a readily-accessible storage form of glucose, stored in small quantities in the liver and muscles. The body's glycogen reserve can provide glucose for about 6 hours.

After the glycogen reserve is used up, glucose can be obtained from the breakdown of fats. Fats from adipose tissue are broken down into glycerol and free fatty acids. Glycerol can then be used by the liver as a substrate for gluconeogenesis, to produce glucose.

Fatty acids can be used directly as an energy source by most tissues in the body, except the brain, since fatty acids are unable to cross the blood-brain barrier. After the exhaustion of the glycogen reserve, and for the next 2-3 days, fatty acids are the principal metabolic fuel. At first, the brain continues to use glucose, because, if a non-brain tissue is using fatty acids as its metabolic fuel, the use of glucose in the same tissue is switched off. Thus, when fatty acids are being broken down for energy, all of the remaining glucose is made available for use by the brain.

However, the brain requires about 120 g of glucose per day (equivalent to the sugar in 3 cans of soda), and at this rate the brain will quickly use up the body's remaining carbohydrate stores. However, the body has a "backup plan," which involves molecules known as ketone bodies. Ketone bodies are short-chain derivatives of fatty acids. These shorter molecules can cross the blood-brain barrier and can be used by the brain as an emergency metabolic fuel.

After 2 or 3 days of fasting, the liver begins to synthesize ketone bodies from precursors obtained from fatty acid breakdown. The brain uses these ketone bodies as fuel, thus cutting its requirement for glucose. After fasting for 3 days, the brain gets 30% of its energy from ketone bodies. After 4 days, this goes up to 70%.

Thus, the production of ketone bodies cuts the brain's glucose requirement from 120 g per day to about 30 g per day. Of the remaining 30 g requirement, 20 g per day can be produced by the liver from glycerol (itself a product of fat breakdown). But this still leaves a deficit of about 10 g of glucose per day that must be supplied from some other source. This other source will be the body's own proteins.

After several days of fasting, all cells in the body begin to break down protein. This releases amino acids into the bloodstream, which can be converted into glucose by the liver. Since much of our muscle mass is protein, this phenomenon is responsible for the wasting away of muscle mass seen in starvation.

However, the body is able to selectively decide which cells will break down protein and which will not. About 2–3 g of protein has to be broken down to synthesise 1 g of glucose; about 20–30 g of protein is broken down each day to make 10 g of glucose to keep the brain alive. However, this number may decrease the longer the fasting period is continued in order to conserve protein.

Starvation ensues when the fat reserves are completely exhausted and protein is the only fuel source available to the body. Thus, after periods of starvation, the loss of body protein affects the function of important organs, and death results, even if there are still fat reserves left unused. (In a leaner person, the fat reserves are depleted earlier, the protein depletion occurs sooner, and therefore death occurs sooner.)

The ultimate cause of death is, in general, cardiac arrhythmia or cardiac arrest brought on by tissue degradation and electrolyte imbalances.

References

• Proc Nutr Soc. 1995 Mar;54(1):267-74. Feeding, fasting and starvation: factors affecting fuel utilization. MacDonald IA, Webber J. Department of Physiology and Pharmacology, Queen's Medical Centre, University of Nottingham Medical School. PMID: 7568259
• Clin Nutr. 2000 Dec;19(6):379-86. Hunger disease. Elia M. Addenbrooke's Hospital, Cambridge, UK. PMID: 11104587