We all need food. It’s what gives us the energy to live our daily lives, keeps us healthy, and brings us pleasure. But what is it exactly that tells us when to eat and, just as importantly, when to stop? These are questions which scientists around the world are working to answer.
And it’s important they do. Because obesity is an increasing problem across the globe. There are all sorts of reasons why people over-eat. But that’s only half the story.
Our weight is regulated by two principal factors:
- energy in – in the form of the food and drink we consume; and
- energy out – the extent to which we burn off that energy through exercise and daily living, and to maintain our body temperature and keep our organs functioning.
‘Energy homeostasis’ is the balance between the energy we take on board (calories, fat, protein, etc) and the energy we expend through physical activity and to maintain the basic metabolic rate we all need to survive. Get it right and we maintain our correct body weight for optimal physical and mental functioning. But if we don’t achieve energy homeostasis, it can make us underweight in the case of insufficient energy intake or, more commonly, overweight, when ‘energy in’ exceeds ‘energy out’.
So far, so good – it’s basic common sense. But understanding the science behind the simple facts is far more complicated. Our appetite and feelings of satiety result from complex interactions of signals which constantly pass between stomach and brain, regulating food intake. Our understanding of how the body works has increased dramatically over recent decades, especially with the arrival of genetics. And one of the major breakthroughs as far as understanding hunger is concerned is the discovery of the ‘hunger hormone’ ghrelin.
Ghrelin is a peptide produced by certain cells in the stomach, which sends messages to the brain via the peripheral nervous system and blood stream. As well as controlling hunger, it also plays a significant role in regulating the distribution and rate at which we use energy and store fat, and is thus an important factor governing body weight and energy balance.
Ghrelin is continuously secreted, but when the stomach is empty, its secretion increases. When the stomach is stretched (ie full of food), secretion is reduced. It acts on hypothalamic brain cells to signal increased hunger and the secretion of gastric acid to prepare our bodies to take in food.
There’s a specific receptor in the brain to receive this ‘hunger message’. This ghrelin receptor is also found on some of the cells that express the receptor for leptin, a ‘satiety hormone’ which has the opposite effect to ghrelin. So these brain cells effectively receive and process the messages produced by the body which tell it when we need to eat, and when we should stop, showing just how complicated the system of regulating our energy homeostasis is.
This communication between stomach and brain involves many different signalling pathways, not all of which are well understood yet. But what we do know is that, in addition to regulating the hunger we feel, ghrelin also plays an important role in controlling the pleasure we derive from eating and drinking, with ghrelin levels peaking just before a meal, and at their lowest immediately after.
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