Appetite is the desire to eat food, sometimes due to hunger. Appealing foods can stimulate appetite even when hunger is absent. Appetite exists in all higher life-forms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the brain. Appetite has a relationship with every individual's behavior. Appetitive and consummatory behaviours are the only processes that involve energy intake, whereas all other behaviours affect the release of energy. When stressed, appetite levels may increase and result in an increase of food intake. Decreased desire to eat is termed anorexia, while polyphagia (or "hyperphagia") is increased eating. Dysregulation of appetite contributes to anorexia nervosa, bulimia nervosa, cachexia, overeating, and binge eating disorder.
Physiological factors
Cannon and Washburn (1912) proposed that eating begins when we have an empty stomach. They suggested that the walls of an empty stomach rub against each other to produce what are commonly called "hunger pangs". Some skeptics called Cannon's explanation of hunger "the rumble theory". However, observations of surgical patients indicated that there was more to the onset of eating than hunger pangs. Removal of the stomach did not abolish hunger pangs, and these patients reported the same feelings of hunger and satiety that they had experienced before surgery (Inglefinger, 1944). (The patients had had their stomachs removed because of cancer or large ulcers, and their esophagi had been attached directly to their small intestines). Although the patients ate small, frequent meals because they had no stomachs to hold food, their reports of feelings of hunger and their total food intake were essentially normal.
Depletion of the body's store of nutrients is a more likely cause of hunger. The primary fuels for the cells of our body are glucose (a simple sugar) and fatty acids (compounds produced by the breakdown of fats). If the digestive system contains food, these nutrients are absorbed in the blood and nourish our cells. But the digestive tract is sometimes empty; in fact, it is empty when we wake up every morning. There must be a reservoir that stores nutrients to keep the cells of the body nourished when the gut is empty. Indeed, there are two reservoirs: a short-term reservoir and a long-term reservoir. The short-term reservoir stores carbohydrates, and the long-term reservoir stores fat.
A number of variables have been found to relate to appetite sensation in individuals. The most influential of these is gender and age, with females experiencing greater appetite satisfaction than males and a decrease in appetite with age. Although BMI was not found to influence appetite, smokers and women ovulating experienced a lower appetite than their counterparts.
Regulation
The regulation of appetite (the appestat) has been the subject of much research in the last decade. Breakthroughs included the discovery, in 1994, of leptin, a hormone produced by the adipose tissue that appeared to provide negative feedback. Leptin is a peptide hormone that affects homeostasis and immune responses. Lowering food intake can lower leptin levels in the body, while increasing the intake of food can raise leptin levels. Later studies showed that appetite regulation is an immensely complex process involving the gastrointestinal tract, many hormones, and both the central and autonomic nervous systems. The circulating gut hormones that regulate many pathways in the body result in appetite stimulation.
Effector
The hypothalamus, a part of the brain, is the main regulatory organ for the human appetite. The neurons that regulate appetite appear to be mainly serotonergic, although neuropeptide Y (NPY) and Agouti-related peptide (AGRP) also play a vital role. Hypothalamocortical and hypothalamolimbic projections contribute to the awareness of hunger, and the somatic processes controlled by the hypothalamus include vagal tone (the activity of the parasympathetic autonomic nervous system), stimulation of the thyroid (thyroxine regulates the metabolic rate), the hypothalamic-pituitary-adrenal axis and a large number of other mechanisms. Opioid receptor-related processes in the nucleus accumbens and ventral pallidum affect the palatability of foods.
The nucleus accumbens (NAc) is the area of the brain that coordinates neurotransmitter, opioid and endocannabinoid signals to control feeding behaviour. The few important signalling molecules inside the NAc shell modulate the motivation to eat and the affective reactions for food. These molecules include the DA, Ach, opioids and cannabinoids and their action receptors inside the brain, DA, muscarinic and MOR and CB1 receptors respectively.
Sensor
The hypothalamus senses external stimuli mainly through a number of hormones such as leptin, ghrelin, PYY 3-36, orexin and cholecystokinin; all modify the hypothalamic response. They are produced by the digestive tract and by adipose tissue (leptin). Systemic mediators, such as tumor necrosis factor-alpha (TNFα), interleukins 1 and 6 and corticotropin-releasing hormone (CRH) influence appetite negatively; this mechanism explains why ill people often eat less.
In addition, the biological clock (which is regulated by the hypothalamus) stimulates hunger. Processes from other cerebral loci, such as from the limbic system and the cerebral cortex, project on the hypothalamus and modify appetite. This explains why in clinical depression and stress, energy intake can change quite drastically.
Role in disease
A limited or excessive appetite is not necessarily pathological. Abnormal appetite could be defined as eating habits causing malnutrition and related conditions such as obesity and its related problems.
Both genetic and environmental factors may regulate appetite, and abnormalities in either may lead to abnormal appetite. Poor appetite (anorexia) can have numerous causes, but may be a result of physical (infectious, autoimmune or malignant disease) or psychological (stress, mental disorders) factors. Likewise, hyperphagia (excessive eating) may be a result of hormonal imbalances, mental disorders (e.g. depression) and others. Dyspepsia, also known as indigestion, can also affect appetite as one of its symptoms is feeling "overly full" soon after beginning a meal.
Abnormal appetite may also be linked to genetics on a chromosomal scale. In the 1950s, the discovery of the Prader Willi Syndrome, a type of obesity, displayed a causation at a gene locus. Additionally, anorexia nervosa and bulimia nervosa are more commonly found in females than males â€" thus hinting at a possibility of a linkage to the X-chromosome.
Dysregulation of appetite lies at the root of anorexia nervosa, bulimia nervosa and binge eating disorder. Anorexia nervosa is an eating condition categorized by a penetrating fear of being fat and severe limiting of food consumption. Furthermore, anorexics might do excessive exercise. Individuals that have anorexia have high levels of ghrelin, a hormone that stimulates appetite, so the body is trying to cause hunger, but it is being suppressed by the person. Binge eating disorder (commonly referred to as BED) is described as eating excessively (or uncontrollably) between periodic time intervals. The risk for BED can be present in children and most commonly manifests during adulthood. Studies suggest that the heritability of BED in adults is approximately 50%. Similarly to bulimia some people may be involved in purging and binging. They might puke after food intake or take purgatives. However, the person may still believe they are overweight.
Various hereditary forms of obesity have been traced to defects in hypothalamic signaling (such as the leptin receptor and the MC-4 receptor) or are still awaiting characterization â€" Prader-Willi syndrome â€" in addition, decreased response to satiety may promote development of obesity.
Other than genetically-stimulated appetite abnormalities, there are physiological ones that do not require genes for activation. For example, ghrelin and leptin are released from the stomach and pancreas, respectively, into the blood stream at the signal of the hypothalamus. Ghrelin stimulates feelings of hunger, whereas leptin stimulates feelings of satisfaction from food. Any changes in normal production levels of these two hormones can lead to obesity. Looking at leptin, the more cells present in a body, the more adipose tissues there are, and thus, the more leptin would be produced. This overproduction of leptin will cause the hypothalamus to become resistant to leptin and so, although the pancreas is producing leptin, the body will not understand that it should stop eating. This will produce a perpetual cycle for those that are obese.
Pharmacology
Mechanisms controlling appetite are a potential target for weight loss drugs. Appetite control mechanisms seem to strongly counteract undereating, whereas they appear weak to control overeating. Early anorectics were fenfluramine and phentermine. A more recent addition is sibutramine which increases serotonin and noradrenaline levels in the central nervous system, but had to be withdrawn from the market when it was shown to have an adverse cardiovascular risk profile. Similarly, the appetite suppressant rimonabant (a cannabinoid receptor antagonist) had to be withdrawn when it was linked with worsening depression and increased risk of suicide. Recent reports on recombinant PYY 3-36 suggest that this agent may contribute to weight loss by suppressing appetite.
Given the epidemic proportions of obesity in the Western world, and the fact that it is increasing rapidly in some poorer countries, observers expect developments in this area to snowball in the near future. Dieting alone is ineffective in most obese adults â€" and even obese adults who successfully lose weight through dieting overwhelmingly put weight back on afterwards.
See also
- Specific appetite
- Orexigenic
References
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