Food and Behaviour Research

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20 December 2016 - The Conversation - Christmas stuffing: here's what happens to your body when you overeat

Adam Taylor

As the festive season approaches, our attentions turn to Christmas dinner. But it is also the season of overeating and feeling stuffed. So, before you reach for just one more roast potato, let’s consider what really goes on in our digestive system when we consume large amounts of food.

Our digestive system runs from the entry point (the mouth) to the exit point (the anus) but for the average person, what goes on between these two points is a mystery. The function of the digestive system is to turn food into useful currency for the body to burn, grow or store for future use. This process begins in the mouth where the teeth, tongue and salivary glands facilitate mechanical and chemical digestion to form a bolus (a ball of food) that fits down the oesophagus and into the stomach. Imagine trying to get a tortilla chip from your mouth to your stomach in the form that it comes out of the bag; it’s going to hurt.

The stomach is a muscular bag that sits inside the abdomen and it is usually no bigger than your fist when empty. However, it has the capacity to expand and accommodate a much larger volume. The specialised muscle is folded to allow expansion as the stomach fills, and it produces acid to help break food down as well as churning to mechanically smash up food. The empty stomach contains about 40ml of liquid, but the average “full” volume of the stomach is about 800-1,000ml – and this is increased in people who are obese and binge eat.

After food has passed through the stomach, it goes into the small intestine where digestion continues and the (now broken down nutrients) are absorbed into the blood stream. The small intestine, which is about 20ft long, connects to the large intestine, which is about 6ft long. The large intestine sees most of the water absorbed into the bloodstream and the remaining waste matter made into faeces.

Another wafer-thin mint?

We often feel very full after eating a large amount because there is a delay for signals from the stretching stomach to reach the brain.

You may wonder why you go from feeling hungry to feeling full to bursting without any in-between feeling. Our body has a very complex way of telling us when we are hungry and full; it requires a number of hormones that are produced in response to the presence or absence of food in the digestive system. If we get the amount of food we consume right, we have the feeling of satiety – fullness that suppresses the urge to eat.

Two of the most important hormones are ghrelin and leptin. If we consider these hormones simplistically, ghrelin increases appetite and leptin decreases appetite. They are produced predominantly in the stomach and fat cells respectively. Grehlin is usually at a high level before you eat and reduces afterwards. Leptin tells the brain that we are full. So you would assume that people with more fat cells would produce more leptin and therefore be more likely to want to eat less. However, obese people build up a resistance to leptin, which means they have to produce more and more leptin for it to have an effect and reduce their appetite.

So here are a few things to bear in mind before you have that last wafer-thin mint.