The feeling is all too familiar: a growling in the pit of your stomach that usually starts around late morning when breakfast is just a memory and lunchtime is still a tiny speck on the horizon. It’s hunger – a feeling that begins with the hormone known as ghrelin. Once your body has finished digesting and using up the energy from your last meal, your blood sugar and insulin levels drop. In response to this, ghrelin is produced in the gut and travels to the brain, letting it know that sustenance is needed. The brain then commands the release of a second hormone called neuropeptide Y, which stimulates appetite.
Once you have answered the call and filled up on a good meal, your stomach gets to work on digestion. Nerves in your stomach sense stretching that lets your brain know you’re full up. Three other hormones also secreted by your digestive system take messages to the brain: cholecystokinin (CCK), GLP-1 and PYY. CCK helps to improve digestion by slowing down the rate at which food is emptied from the stomach into the small intestine, as well as stimulating the production of molecules that help to break down food.
GLP-1 tells the pancreas to release more insulin and also reduces appetite. The hormone PYY is secreted into the bloodstream by the small intestine after eating. It binds to receptors in the brain to make you feel full up. Once all of the food is digested, the blood sugar and insulin levels drop and ghrelin is produced once more, so the hunger cycle continues.
HUNGER STRIKES – The gut produces ghrelin to let your brain know that you’re hungry.
FEELING FULL – Once you’re full, fat cells secrete a hormone called leptin that actually inhibit your appetite so you don’t keep eating.
ENERGY STORAGE – Insulin moves glucose from the blood into your body’s cells, so it can be used during exercise, for example.
AFTER EATING – Once you’ve eaten, your body digests the food and energy is extracted.
ROLE OF THE LIVER – The liver keeps the level of blood glucose and insulin within a healthy range and stops excessive fluctuations.
BLOOD CHEMISTRY – Hormones stimulate your pancreas in order to release more insulin into your bloodstream.
INSULIN CONTROL – This hormone works to speed up the rate at which cells in the body take up glucose.
When our bodies tell us we are hungry, it’s an innate reaction – the hormones in our systems let us know we need sustenance. But when our minds get involved, it’s a whole different story. There’s not much nutritional value in a bacon sandwich or a doughnut, for example, so it’s not a ‘need’ for a treat, it’s a ‘want’.
This is because > the first time you experienced a doughnut, the mesolimbic centre of your brain (the region that processes pleasure) lit up, as the fatty, sugary goodness of the treat released chemicals known as opioids that bind with receptors in the brain. This triggers the release of dopamine, the hormone that makes us happy. Your brain remembers this response, and encourages you to repeat the pleasurable feeling by eating more.