Behind the Scenes of Hunger

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What makes us want to eat? We typically base our eating on the physical sensations of hunger and fullness – we eat in response to a grumbling stomach, and we stop when we feel full. Behind the scenes, however, the process is regulated by several hormones.

Ghrelin

One of the hormones controlling our appetite is the hormone ghrelin, or, as it is often called ‘the hunger hormone.’ Ghrelin is secreted by the stomach, and the levels of ghrelin circulating in the blood correlate with hunger. Ghrelin levels peak before meals, and dip after we eat in proportion to the size of the meal. Ghrelin levels also depend on the macronutrient composition of the meal. A carbohydrate-dense meal suppresses ghrelin levels to a greater extent than meals dense in fats and proteins. Scientists have found that artificially injecting ghrelin in rodents and humans increases energy intake.

Cholecystokinin

Cholecystokinin is a hormone secreted by the cells of the duodenum. The duodenum is the first part of the intestine, located immediately beyond the stomach. The secretion of CCK has a number of effects: it stimulates the secretion of gastric acid, stimulates pancreatic secretion, and suppresses energy intake. CCK is secreted in response to the intake of food and decreases hunger, with concentrations rising within 15 minutes of eating. CCK levels subsequently fall in 3-5 hours. However, while the role of CCK is known to increase the sensation of fullness during a meal, it’s role in appetite regulation between meals is not as well studied.

Appetite Diagram

Leptin

Leptin is a hormone secreted by adipose (fat) tissue. Leptin indicates the amount of energy the body has stored in the form of fat, and when it binds to receptors in the hypothalamus, it tells the body to reduce food intake.

Insulin

Insulin is a hormone secreted by the pancreas. Like leptin, the amount of insulin released is proportional to the amount of fat stored in the body. Insulin decreases appetite, and studies have shown that injecting insulin in rodents reduces both their food intake and their weight. Insulin also stimulates the production of leptin and binds to some of the same targets as leptin in the hypothalamus.

Peptide YY and Oxyntomodulin

Both Peptide YY (PYY) and oxyntomodulin are hormones that are secreted by a special type of cells in the intestine called L-cells. Like CCK, PYY is secreted in response to the intake of food. Studies have showed that a protein and fat heavy diet can lead to greater and longer lasting increases in PYY, leading to reduced hunger for longer durations of time.

Oxyntomodulin is also released in response to the ingestion of food. Studies have showed that oxyntomodulin inhibits the secretion of ghrelin after meals are consumed, contributing to the feeling of fullness. In addition, oxyntomodulin also lowers the rate at which at which the stomach is emptied, further reducing hunger.

These hormones are only part of the story. The brain plays a vital role in the process. Aside from integrating the signals from the hormones, reward signals – such as dopamine when one takes a bite into a favorite dessert – are also integrated in the brain and can also influence appetite and food consumption patterns. The gut microbiome also plays an important but still not entirely understood role in appetite regulation. With increasing rates of obesity, metabolic disorders such as diabetes, and increasing research on the role of such disorders in diseases like Alzheimers or cancer, scientists have begun to study processes like appetite regulation a lot more, and it has shown that the process is much more complex than initially suspected.

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