Starvation metabolism

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Hunger metabolism is the change in metabolism when there is a lack of food , e.g. B. fasting (with the extreme form of zero diet ) or anorexia . The change in metabolism causes a decrease in calorie consumption. This phenomenon is often referred to as “metabolism that has fallen asleep”.

biochemistry

Due to the lack of food, the metabolism switches to catabolism over the course of several days . The basal metabolic rate is lowered and the metabolism slows down. In the case of food deprivation, the body must obtain the energy it needs to maintain important body functions from its energy stores. To meet the energy requirement, energy supplies in the form of carbohydrates (e.g. glycogen ), proteins (e.g. muscles) and later also fats (e.g. subcutaneous adipose tissue ) are used. The blood sugar level drops to around 80 mg / dl, the glucagon levels rise, the insulin level falls. Depending on the composition of the food, with reduced food intake, a deficiency in essential amino acids , lipids , vitamins and trace elements can occur.

A starving male adult with no ingestion of food consumes around 1,800 kilocalories per day. Without food, around 75 grams of muscle and around 160 grams of neutral fats are broken down. The lower the remaining amount of food intake, the more pronounced the effects of hunger metabolism.

Some cell types in the body -  erythrocytes , as well as some cell types in the brain and the adrenal medulla  - only use glucose and certain other substances such as B. ketone bodies . Gluconeogenesis , with which glucose is generated in the body, is available to supply these cells . a. Glycerine released during fat breakdown , as well as glucogenic amino acids which are released when muscle proteins break down. The ammonia and uric acid levels in the serum rise due to the breakdown of protein . On the other hand, after a certain period of starvation, ketogenesis is available for energy supply, which breaks down fatty acids in the course of fat metabolism and produces ketone bodies via beta oxidation . This is expressed u. a. as ketosis - an increased number of ketone bodies in serum and urine - as well as partly via exhaled acetone as diet halitosis .

During reduced food intake, there is a certain adjustment to the lack of nutrients. This process is called hunger adaptation . The metabolic rate is reduced with a reduced food intake. Heart rate, blood pressure and body temperature decrease; an extreme example is hibernation in animals. The adrenal gland releases the stress hormone adrenaline. The brain's glucose consumption is reduced to 30 percent of the initial value, i.e. H. from 140 grams per day to around 40 grams per day. The remaining energy requirements of the brain are covered by ketone bodies . In the long term, the lack of food can lead to deficiency diseases and ultimately death. The consequence of prolonged lack of food is emaciation or inanition . It can lead to a complete loss of strength, which is also called cachexia .

Breakdown of carbohydrates

First, the short-term available energy reserves of humans are used. This includes the glycogen of the liver, kidneys and muscles, which is broken down into glucose molecules. These quickly available energy reserves are around 6,700 kilojoules (1,600 kilocalories) and are used up within a day. The body excretes more water through the kidneys. The body weight is reduced a lot at the beginning, later less so.

Breakdown of proteins

After mobilizing the quickly available energy reserves, there is a breakdown of protein of up to 75 grams per day. Muscle proteins in particular are broken down, but also proteins in other cells. The degraded proteins are either metabolized directly in the sense of catabolism or used for gluconeogenesis from amino acids .

When fasting for longer periods of time, the breakdown of protein is throttled to protect the organs. The metabolism changes after about two weeks. The subsequent reduced protein breakdown (in the range of 20 to 25 grams per day) leads to a reduced urea excretion (urea is a protein breakdown product) in the urine. A reduction in the protein concentration in the blood ( hypoproteinemia ) leads to the development of so-called hunger edema due to the accumulation of water in the tissue and to kwashiorkor . The loss of protein has a negative effect on the immune system . Infections occur more frequently during fasting, and existing infections can worsen or become manifest. The loss of muscle mass, which also affects the heart muscle, is about 25 percent of the total weight loss. The half-life of plasma proteins is about two weeks, the half-life of framework and connective tissue proteins is about 160 days. After very long starvation, for example when a third to half of the total body proteins have been broken down, death occurs through starvation.

Marasmus

The term marasmus describes the most severe degree of malnutrition with atrophy due to a lack of calories . Severe diarrhea appears before death . Many Starving die from the consequences of their by protein deficiency -related infections . There have also been reports of sudden cardiac death in starving people (suspected cardiac arrhythmias due to potassium deficiency ). When the blood sugar level falls below 10 mg / 100 ml, a coma occurs . From values ​​below about 30 mg / 100 ml, the brain performance decreases significantly, confusion , anxiety and depression occur. Very low glucose levels can lead to spasms and uncontrolled movements .

Breakdown of lipids

Fats - here mainly the triglycerides  - are increasingly broken down after the first week after the start of the fast, and the breakdown of fat ( lipolysis ) and the formation of the ketone bodies acetoacetate and beta-hydroxybutyric acid also increase in the first week .

Women and children

Due to hormonal changes, there are changes in menstruation in women to the point of complete absence. It has been statistically proven that the majority of pregnant Muslim women fast between sunrise and sunset during Ramadan , although the Islamic law of fasting does not apply to pregnant women and children . This reduces the birth weight and the length of the pregnancy, and physical and especially mental disabilities are more likely.

Theories on how long to survive without food

Without water, a healthy person will die of thirst after about three to four days at normal ambient temperatures . However, this time period is strongly dependent on temperature. There are different statements about starvation in the literature. A distinction is made here as to whether it is a matter of leaving out energy sources alone or whether vitamins and minerals are also not available. The zero diet, for example, lacks chemical energy sources in the diet such as carbohydrates, proteins and fats, while vitamins and minerals do not have to be dispensed with. Healthy people can survive without food for anywhere from 30 days to more than a year if enough water is available.

Jerrold M. Olefsky, an endocrinologist at UC San Diego , estimates the survival time in a person of normal weight to be around 60 days, Walter Siegenthaler gives around 50 to 80 days of survival time with a complete fast, provided water and vitamin intake. Overweight people have survived up to 382 days under these conditions.

Every kilogram of body fat has a physiological calorific value of around 30  megajoules or 7,000 kilocalories of energy. He assumes that people of normal weight are around ten kilograms of body fat. Bobby Sands , an IRA - hunger striker (see also 1981 Irish hunger strike ), lived without food 66 days and Holger Meins 57 days in 1974 (although he occasionally fed artificially was) before they eventually died as a result of the hunger strike.

Individual evidence

  1. ↑ a b c d e f Jeremy M. Berg, John L. Tymoczko, Lubert Stryer : Biochemistry. 5th edition. Freeman, New York 2002, ISBN 0-7167-4684-0 , Section 30.3 on the NCBI Bookshelf.
  2. ^ A b c John R. Butterly, Jack Shepherd: Hunger: The Biology and Politics of Starvation . UPNE, 2010, ISBN 978-1-58465-950-1 , pp. 94ff ..
  3. ↑ Y. Yamada, RJ Colman, JW Kemnitz, ST Baum, RM Anderson, R. Weindruch, DA Schoeller: Long-term calorie restriction decreases metabolic cost of movement and prevents decrease of physical activity during aging in rhesus monkeys. In: Experimental Gerontology . Volume 48, number 11, November 2013, pp. 1226-1235, doi: 10.1016 / j.exger.2013.08.002 , PMID 23954367 , PMC 3882119 (free full text), ISSN  1873-6815 .
  4. ^ Joseph B. Martin. In: Clinical Neuroendocrinology , 1977.
  5. ^ A b Walter Siegenthaler (Ed.): Clinical Pathophysiology . 6th edition. Thieme, Stuttgart / New York 1987, ISBN 3-13-449606-2 .
  6. ↑ Douglas Almond, Bhashkar Mazumder: Health Capital and the Prenatal Environment: The Effect of Material Fasting During Pregnancy. Working Paper 14428, National Bureau of Economic Research , 2009.
  7. ↑ a b W. K. Stewart, LA Fleming: Features of a successful therapeutic fast of 382 days' duration . In: Postgraduate Medical Journal . tape 49 , March 1973, p. 203-209 , PMC 2495396 (free full text).
  8. ↑ Jean Donald Wilson (Ed.): Harrison's principles of internal medicine . 12th edition. McGraw-Hill, New York 1991, ISBN 0-07-070890-8 , pp. 411 .