HUNGER AND APPETITE

Hunger is vital to our survival, and it ensures we eat enough for our bodies to function. But a lot of the time we eat not because we are hungry but because we enjoy food—this is down to our appetite.

Hunger and satiety

Hunger is controlled by a complex interconnected system including our brain, digestive system, and fat stores. The desire to eat can be triggered by internal factors, such as low blood sugar or an empty stomach, or external triggers, such as the sight and smell of food. After we have eaten, satiety, or “fullness” signals are produced, which tell us we have had enough.

Hunger vs. appetite

Appetite is different from hunger, but the two are linked. Hunger is the physiological need for food, driven by internal cues such as low blood sugar or an empty stomach. Appetite is the desire to eat, driven by seeing or smelling food or something we link with it. Memory for how much we have eaten is also important in appetite, and people with short-term memory loss may eat again soon after eating. Stress can also increase the desire to eat. Some substances can help control appetite by specific actions on the body.

Appetite and obesity

People with a tendency to obesity may respond differently to external hunger cues. They may also be less sensitive to the fullness hormone, leptin. Unfortunately, taking leptin as a drug doesn’t help obesity. The body quickly adapts to be even more insensitive to leptin, even at high doses.

Cravings

Cravings are a dramatic and specific desire for a certain type of food, and most of us have experienced them. Occasionally, they are caused by specific nutrient deficiencies, and may be the body’s way of telling you about the problem. But mostly they are purely psychological, driven by stress or boredom. Normally, craved foods are high in fat or sugar (or high in both), which trigger a rush of pleasurable chemicals in the brain when eaten. It may be this feeling that we crave rather than the actual food.

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1. Hunger triggers

Seeing food can trigger a desire to eat whether or not we are hungry. (The same response is triggered by anticipation of a mealtime). The food passes to the stomach via the esophagus.

2. Empty stomach

When the stomach has been empty for around 2 hours, the gut muscles contract, clearing out any last debris. Low blood sugar levels exacerbate the feelings of hunger. Levels of a hunger hormone called ghrelin also rise.

3. Stomach stretches

As the stomach fills, stretch receptors detect expansion, causing hunger reducing chemicals to be released. (Liquids, including water, stretch the stomach temporarily, but are quickly absorbed, so hunger returns.)

4. Pancreas releases insulin

The stretching stomach and the rise in glucose in the bloodstream, triggers the release of insulin. This allows the conversion of glucose to glycogen (in the liver) and then to fat. Insulin may also make the brain more sensitive to satiety signals.

5. Leptin travels to brain

Fat cells release a hunger-inhibiting hormone called leptin. After eating, more leptin is secreted and we feel full. (Conversely, leptin levels decrease with fasting, making us feel hungry.)

6. Brain receives “full” signals

The vagus nerve sends signals straight to the hypothalamus, telling the brain that food has been consumed and reducing the hunger drive.

WHY DOES MY STOMACH RUMBLE WHEN I’M HUNGRY?

After eating, your stomach muscles contract to push food through to the intestines. With an empty stomach, this still happens, but with nothing to dampen the sound, you hear the growls!

In "How Food Works", Dorling Kindersley (DK), USA, 2017, Editors Lili Bryant, Wendy Horobin, Janet Mohun, Martyn Page, Francesco Piscitelli and Margaret Parrish, excerpts pp. 12-14. Digitized, adapted and illustrated to be posted by Leopoldo Costa.