This week’s expert is Joe Mclean who holds a master’s degree in clinical research from Imperial College London specialising in human nutrition. Part #1 of our hunger and appetite posts is all about signals that drive us to eat. Hungry to learn?
Key Question: What hormones make me feel hungry?
We’ve all been there… a rumbling in our stomach and the thought of our next meal coming to mind. But what is causing us to feel hungry and to reach for a bit more food? Well, let’s unwrap this (if you’ll excuse the pun)!! Eating is partly driven by an increase in our appetite, basically our desire to eat. There is a refined system in our bodies that controls our appetite. This should come as no surprise, considering our bodies consume around ONE MILLION calories every year. If you don’t change your weight over a year, your body is performing an amazing calorie balancing act, where your energy expenditure and energy intake are pretty perfectly matched, often without you consciously knowing it (for more info on energy balance click here). Part of this balancing act is the body controlling our desire to eat.
When understanding the factors that regulate appetite we have to look at the body as a whole. We know the brain is important in dictating whether we feel hungry or not. Deep within the brain is the hypothalamus, which has the role of keeping our body in a balanced state, and one example of that is helping control our appetite. There are two sections in the hypothalamus that we need to know about, the ventromedial hypothalamus, which is responsible for satiety signals (these make us feel full) and the lateral hypothalamus, which is responsible for signals that make us feel hungry. These parts work together to drive us to eat, but also then to tell us to stop eating afterwards.
So who is sending the messages to the hypothalamus? Well, this is where a group of cells throughout the GI tract, the stomach and the pancreas come into play. They are responsible for releasing hormones which act as messengers to the brain, which can make a decision about whether to suppress appetite or not. We have now managed to isolate several gut hormones that help make us feel full. Both GLP-1 and PYY are hormones released in the GI tract, and these have gained a lot of interest with scientists. I’m sure you won’t be surprised, but there is also a lot of pharmaceutical interest in finding a hormone that can suppress appetite!! In blog #25 (click here), we will talk more about hormones which make us feel full.
To date, there has been just one hunger hormone identified in research. This is called ghrelin. Ghrelin, unlike most other appetite hormones, is released from the stomach. It is increased first thing in the morning and decreases after you eat a meal. If you artificially increase ghrelin in people, they eat more. In fact, ghrelin has been successfully used to stimulate appetite in people with cancer, depression and anorexia (click here).
Can we manipulate our ghrelin levels?
It has been shown many times that endurance-type (especially moderate-high intensity) exercise reduces circulating ghrelin levels. This means after your 5 mile run you’re not only getting the physical benefits (e.g. for the immune system as discussed here), but you’re decreasing appetite signals afterwards too. This makes exercise a nice strategy to suppress appetite, at least in the short -term. Having a short amount of sleep has also been linked to higher ghrelin levels as highlighted in a recent review, and nutritional strategies, such as eating protein, can help to supress ghrelin levels, which can reduce appetite and decrease the drive to eat (click here). Although I focused on gut hormones today, there are also many other signals that can influence our appetite, one of those is a protein called leptin which is released from the fat tissue. Leptin is a key player in energy homeostasis acting via hunger and satiety signals. However, we think leptin could actually increase appetite and help-regulate changes in body weight in the longer-term, especially in response to weight loss and an energy deficit (for more information on how your body might try to protect against weight loss click here). I am sure you also know that many non-biological factors, such as being around food, social pressures and norms can also alter the amount you eat, even if you are not feeling hungry. That’s a story for another day though!
Understanding how the body controls our appetite has the potential to have huge benefits for health across the world. Can we use our knowledge of different appetite hormones to treat obesity and related disease? These are questions that scientists are exploring!
The take-home message: There is a highly refined system in your body helping to control your desire for food. As scientists we are trying to find ways to manipulate levels of these hormones to better understand (and treat) obesity and other diseases. For ghrelin, getting good sleep, eating protein, and doing exercise may all help to manage this hunger signal.
Thanks Joe! Don’t forget to sign up to the blog and click here to learn about the hormones that make us feel full.