Hormones and eating

Hormones act as chemical messengers to help control how much food we eat. The hormones ghrelin and leptin are used to signal to our brain when we need to feel hungry, and when we need to stop eating. The feeling of ‘being full’ that makes us want to stop eating is called satiety. When these hormone signals don’t work properly people may eat more or less than their bodies need, leading to problems with their weight and health. 

Hunger and satiety

Hunger is a powerful feeling which affects our behaviour. However motivated we may be to control our eating, it can be very difficult to eat when we are not hungry, or to avoid eating when we are. But where does hunger come from? How does the brain decide it’s time to eat something? Feelings of hunger and satiety are created by the brain in response to a combination of signals, including from two key hormones. The amount of these hormones in our blood rises and falls, signalling to the brain how much hunger is required at any particular time.

• Ghrelin has short-term effects: it makes us hungry for our next meal. 
• Leptin has longer-term effects: it helps us maintain our normal body weight by controlling our overall levels of hunger and satiety.

Like all human hormones, ghrelin and leptin travel in the blood. From the bloodstream they are able to reach many tissues and organs, including the brain. In the brain, their messages affect a region called the hypothalamus, which is the control centre for hunger and satiety. Like many hormones, ghrelin and leptin have a lot of different effects, with different cell types responding differently to their message. Their release is also affected by a number of different factors, including other hormones. It’s complicated! But this article will focus on their roles in the control of eating. 

Ghrelin: The hunger hormone

The hormone Ghrelin is sometimes called “The Hunger Hormone” because of its key role in hunger. It is used to signal to the brain how long we have gone without food. Ghrelin is made by stomach cells. The longer it has been since our last meal, the more ghrelin the stomach releases. High levels of ghrelin in the blood are a signal that we have gone too long without food. When ghrelin reaches the brain, the brain responds by increasing our appetite. Ghrelin also affects brain regions linked to feelings of reward from eating. People given extra ghrelin feel hungrier and eat more food.   When we eat, our stomach stops releasing so much ghrelin. The level of ghrelin in our blood falls, and our brain stops telling us we are hungry.

Ghrelin and bodyweight

Obesity is a condition where the body has stored an unhealthy amount of fat. There are many factors that can cause obesity. You might think that obese people would have more ghrelin, but usually they have less. An exception is in the genetic disease Prader-Willi syndrome, which causes very high levels of ghrelin. People with this disease have extreme hunger and severe obesity. The hunger is thought to be caused by their high levels of ghrelin. Other complications of the disorder include learning difficulties. Ghrelin levels increase when people lose weight through dieting. This may be one of the reasons people often struggle to avoid putting the weight back on. In contrast, ghrelin levels are low after gastric bypass (weight-loss) surgery, which might partly explain why this treatment is so successful in the long term. High levels of ghrelin are also found in people with the eating disorder anorexia nervosa and the wasting disease cachexia. As with dieting, this is thought to be a response to weight loss. You can find out more about obesity and weight-loss surgery in our article about obesity . 

Ghrelin and energy regulation

Ghrelin has many other roles related to food beyond making us hungry when it’s time to eat. It is especially important in the regulation of energy-rich molecules in the body. This includes having some control over the storage and release of fats and carbohydrates.

The pancreas (an organ near the stomach, view article) responds to high levels of ghrelin by making less insulin and more glucagon. This increases blood sugar by breaking down stored carbohydrate. This is necessary to keep blood sugar levels from falling too low between meals. You can read more about these hormones in our Sugar Control in the Body article.

 In fatty tissues, ghrelin encourages fat storage. This is thought to be related to long-term management of energy in the body. In the brain, the pituitary gland responds to high ghrelin levels by releasing growth hormone. Growth hormone leads to the breakdown of fat tissue (the opposite effect to ghrelin!) and the build-up of muscle. These effects are all important when thinking about ghrelin’s role in obesity.

Leptin: The fat controller

The hormone Leptin is used to control eating over the longer term. High levels of leptin gives us feelings of satiety (of having eaten enough). Leptin is made by fat cells. The more fat there is in storage, the more leptin is released. This means that leptin levels can be used as a measure of how much fat there is in the body. Fat storage is important, especially to help us survive during periods of starvation. Leptin’s main function is in helping to stop the amount of body fat from falling too much. Leptin signals are used by the brain to decide whether it’s necessary to get us to eat more food. If leptin levels are going down, that means the body is breaking down its fat stores, which means we are taking in less energy than we need, and so the brain increases hunger. This creates a problem that is familiar to many people who have lost weight through dieting. Losing weight lowers leptin levels, which increases hunger: people who have lost weight through dieting will find their brains telling them to eat more. This is one of the reasons why it is very difficult to keep weight off. Unfortunately, leptin works to help us maintain our current levels of fat, rather than helping us change them to a healthy ideal.

Leptin: What happens when the system fails?

Some people with obesity have a condition called leptin resistance. This means that their brains do not respond properly to leptin. These people have very high levels of leptin in their blood, but they still feel like they need to eat more. There’s a breakdown in communication – their brains are not listening to their stomachs.

There is also an extremely rare genetic disease that gives the opposite effect. A few families in the UK are affected by congenital leptin deficiency, which means their bodies can’t produce leptin. Untreated, this makes their bodies think they have no fat at all – and so their brains make them extremely hungry and driven to eat. This can lead to severe obesity. The good news is that this condition can be treated very well with injections of leptin.