Couldn’t Eat Another Bite - But Why?

The reason we feel full while eating

The Bagpiper 1624
The Bagpiper, 1624 Image: Hendrik ter Brugghen / Wikimedia Commons

Amanda has a great post asking people what they'd choose for their last meal. I think I could only answer this if my death was to be a surprise - anything else is too depressing to contemplate. Although if I were being executed, I'd probably go with the fantastically poisonous fugu fish if only to beat my captors to the punch.

There is that other possibility - death by eating rather than death after eating. This is a fate that after this weekend I think we can all agree is not nearly as appealing as it sounds. Still, it got me thinking: what exactly is going on when we feel full anyway?

Perhaps a quick anatomy review is in order. At the other end of the esophagus from your fork lies the elastic, muscular pouch called your stomach. It's roughly J-shaped, something I always chalked up to having to fit in around various spleens and livers and things. But it turns out that the shape is ingenious. It allows food to settle in the bottom of the J, where it steeps in our famously acidic gastric juices plus a cocktail of digestive enzymes. As your stomach muscles contract in rhythm, it slops the digested slurry of dinner toward the pylorus, or far end, of your stomach and into your small intestine.

It's not unlike a set of bagpipes: you fill the pouch (with air or food, depending), and by applying pressure on the walls of the pouch you force the filling out the far end. Generally, the less said about the noises produced by either device the better, but we do have a great word for stomach-rumbling: borborygmus.

The stomach is incredibly stretchy, capable of expanding from about a quarter-cup to a half-gallon-carton-of-ice-cream size and back several times per day. We begin to realize we're full - a condition the experts call satiation - as food fills that scooped part of the J in our stomach. And we continue to feel full (this lingering sensation is called satiety by vocab-happy food scientists) until digestion has liquefied the meal and muscular contractions have slopped enough of it up over the tip of the J and into the intestine. This is when we wander back over to the turkey and start nibbling again.

These motions, and the shapes of each of our stomachs, affect why why some people go on eating forever while others start to groan after precisely 11 French fries. Weakly J-shaped tummies fill up (and empty) quickly, while stomachs that are closer to a U in shape take more stuffing. In those latter cases, the high placement of the pyloric valve makes it difficult for the stomach to empty, which can lead to indigestion. (Amazingly, people were studying this back in 1916. Using X-rays.)

The dieting industry has known about this a lot longer than I have, and all sorts of products attempt to use stomach geometry to make you feel full. Proposed offerings start with bulky diet shakes and progress to things like pH-sensitive algae that form gels when they contact stomach acid. More conventional liquids can also help. Recent work (involving real-time stomach movies!) has shown that a cup of tomato soup keeps an egg sandwich in people's stomachs a full 30 minutes longer than the sandwich alone. Though personally, the mere thought of tomato soup and egg salad has a depressing effect on my appetite. I might just order the fugu.

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