Hunger — more than a feeling
Hunger and satiety are the central regulators that control our food intake and ultimately our body weight. The regulation of hunger and satiety is extremely complex and based on the interplay of physiological and psychological mechanisms. Since real hunger — which is characterized by a genuine, persistent lack of food — has become rather rare in our affluent society, it is rather the expected pleasure that tempts us to eat nowadays. In the abundance of supply, most people eat differently than they should.
Is hunger really hunger?
The eating behavior of our society today is shaped from birth on. Just as in animals, humans have physiological, metabolic and central regulatory mechanisms that cause states of hunger and satiety and thus regulate appetite and food intake. By strictly following the sensations of hunger and satiety, an adult is usually able to maintain a constant body weight for decades. This is an amazing feat when you consider that even a small calorie surplus every day — for example, a small glass of beer or 20 grams of peanuts (equivalent to an energy content of about 100 kcal) — can add up to more than four kilograms of weight gain over the course of a year. Such a small calorie surplus comes quickly, especially if you eat without being hungry. Fatally, the human body seems to adapt to the once increased body weight and to defend its reserves in hunger phases with all means, which makes — especially short-term — slimming attempts very difficult in most cases.
What happens in the body when we are hungry?
Several brain areas are involved in the control and processing of hunger and satiety signals, above all the hypothalamus, a part of the diencephalon; it is a vital area that regulates, for example, body temperature and water balance. Hunger therefore originates in the brain, not in the stomach. The hypothalamus provides the drive to eat by generating feelings of hunger and sets the hunger metabolism in motion. Unfortunately, evolutionarily, the mechanisms that protect us from starvation are much better developed than those that protect us from gaining weight. In order to regulate body weight, the brain must, on the one hand, measure the body’s energy status — in particular the amount of stored energy — and, on the other hand, regulate and adjust hunger and satiety. In this process, the hypothalamus plays a dominant role: it acts as an “interface” to the periphery.
Obviously, there are different anatomical regions in the hypothalamus to which different functions of the hunger or satiety mechanism can be assigned. Already more than 50 years ago it was shown in rats that the destruction and consequently a failure of different regions of the hypothalamus have exactly opposite effects on food intake. Destruction of the satiety center leads to binge eating and resulting obesity, while destruction of the hunger center can cause food refusal and even starvation.
So how does the feeling of hunger occur? The hypothalamus continuously measures the level of sugar in the blood. A rise in blood glucose inhibits activity in the “hunger center” and thus leads to increased activity in the satiety center. Conversely, the falling blood glucose level is the main trigger of the feeling of hunger. But what is the detector? At the cellular level, sugar sensors have been discovered in the hypothalamus, which are also found in the liver and are closely interconnected. Their presence in the liver is extremely useful from a physiological point of view, as they can most rapidly register the fluctuations in blood glucose associated with food intake.
Are hunger and satiety controlled by hormones?
In recent years, a wealth of signal substances has been identified that have an influence on food intake, or hunger and satiety. The most recent example is ghrelin, a peptide produced in the stomach and also known as the hunger hormone. When ghrelin is injected into the bloodstream or brain of mice and rats, it stimulates food intake and leads to significant weight gain by promoting hunger. Ghrelin levels usually drop again 20–30 minutes after eating, which is why satiety is delayed after eating. With frequent dieting, researchers suspect that persistent starvation leads to permanently high ghrelin levels, which is why diets are often abandoned. Low calorie intake further exacerbates the persistent feeling of hunger.
Its counterpart — leptin — is a hormone from adipose tissue and a kind of “energy meter.” It reports to the hypothalamus the filling of fat depots in the body. The more fat tissue, the higher the leptin release. Researchers derive from this the explanation that people with a very low body fat percentage feel permanently hungry because the leptin levels are permanently low. In normal-weight people, the amount of leptin in the blood is usually low only before the immediate intake of food and increases during the meal. Therefore, leptin is often referred to as the satiety hormone. It is now also known that many overweight and obese people have what is known as leptin resistance, because in them the concentration of leptin is permanently elevated, but the satiety signal from the brain fails to appear despite food intake.
The best diet is to listen to yourself
Many people have simply forgotten to listen to their true biological hunger. Eat only what the body needs and stop when the need is met. Those who no longer know the feeling basically only have to do one thing: wait and exhaust the “real” hunger. The body has stored and learned over the years which food provides it with which nutrients and what is good for it. If you satisfy your hunger with exactly the food you really feel like eating, you will feel full and good. Short-term diets with a reduced energy intake lead to nothing but an increased feeling of hunger. More sustainable is a slow change of diet, with the goal of a long-term, small calorie deficit. In this way, the metabolism is not switched to a hunger metabolism and the hormone balance is not upset.
Text Sources:
(1) Taschenatlas Ernährung (Biesalski HK, Grimm P, Nowitzki-Grimm S; Thieme Verlag 2015)
(2) Ernährungsumschau: Hunger und Sättigung (Langhans, W; Ernährungsumschau 2010)
(3) PODINGBAUER A, EKMEKCIOGLU C: Regulation der Nahrungsaufnahme: Physiologische Mechanismen und klinische Relevanz, Journal für Ernährungsmedizin 2005; 7 (1) (Ausgabe für Österreich), 22–29
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