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[Michaelangelica]

Hormones in the intestines coordinate the use of food for energy use and storage — sending messages to receptors informing the brain there is enough energy present.

"There are receptors for different nutrients," he says. "There is a sugar receptor and there are fat receptors. If you eat sugar without any fat, the stomach will empty very quickly and you will get a big spike of sugar in your blood.

"If you add fat to the sugar, the stomach empties more slowly and that spike of sugar is flattened out."

Wittert says within this basic system of hunger and satiety there is a hormonal mechanism that monitors fat levels.

"Your fat makes a hormone called leptin, which tells the brain how much fat there is in the body," he says. So, theoretically, if we inject obese people with more leptin, we can trick the brain into thinking they have had enough food.

Unfortunately the body works in the opposite way as well, because leptin is actually more important to defend against a decrease in fat mass.

"If you decrease energy, leptin falls and suddenly all these mechanisms in the brain and body act to say: 'Get food, emergency, we are at risk, hunt'."

How much of hunger is hormonal? - Ask an Expert (ABC Science)