Why You’ll Still Have Room for Pie After Turkey and Stuffing

It’s a gastronomic phenomenon that some call the “dessert shelf”: the remarkable ability of many a Thanksgiving eater to feel completely full after the main course, yet still have room for dessert. Of course, the ability to eat sweets on a full stomach isn’t limited to Thanksgiving, but it’s especially apparent after the holiday feast.

What makes this possible? Scientists have long known that a hormone called ghrelin, which is produced by cells lining the stomach, plays a role in inducing appetite. A counterpart hormone called leptin, which produced in fat cells and other types of tissue, suppresses appetite. When levels of ghrelin in the bloodstream are high, we feel hungry; after eating, ghrelin levels drop off and leptin levels increase, signaling to our brain that we’re full. That, anyway, is how it’s supposed to work.

However, a study involving ghrelin-deficient rats published this past summer by researchers from Carleton University in Canada suggests that something else is going on when we are confronted with sweets. Ghrelin could be leading us to eat high-calorie, high-fat foods like pumpkin pie even after our stomachs are full.

In the experiment, the researchers studied 10 normal rats and 10 rats from a special strain that lacked the gene that codes for the brain’s ghrelin receptors. For this group of knockout rats, no matter how much ghrelin their stomachs produced, the brain had no way of registering the hormone and registering that the rat was hungry.

For four days in a row, the researchers gave all of the rats access to standard-grade rat food from 10 a.m. to 2 p.m. Both groups of rats ate roughly the same amount of food, which provided enough calories to give them sufficient energy to go about their day.

On the fifth day, though, after the rats ate their standard meal, the researchers gave them an unexpected treat: a 30-gram ball of cookie dough. Since the average lab rat is roughly 700 grams at maturity, that’s the equivalent of giving a 125-pound person a ball of cookie dough over 5 pounds in weight.

As you might expect, both groups of rats ate some cookie dough. But at least among this small sample, the normal rats—those whose brains could be affected by ghrelin—ate a fair amount more: 8 grams of cookie dough, on average, compared with the 6 grams the knockout rats ate. This 2-gram difference might not seem like much, but in terms of the rats’ size, it’s significant—roughly the difference between a person eating 1.5 pounds of cookie dough, instead of just a little over a pound.

Ghrelin doesn’t completely explain why we’re able to eat dessert after feeling full—but it seems to play a role. “This result supports the idea that ghrelin is involved in reward-based feeding and delays the termination of a meal,” Veronique St-Onge, a Ph.D. candidate at Carleton University and the lead author the paper, said in a statement. It was the persistent influence of ghrelin, she and coauthor Alfonso Abizaid speculate, that led the already-full rats to eat even more cookie dough.

Other research has looked at the role of ghrelin in stimulating stress-based eating. In one study, normal rats exposed to a stressful situation gravitated towards high-calorie, high-fat food, while the special rats without ghrelin receptors did not, suggesting that the hormone could act as something like an antidepressant, enabling the brain to use food as a reward after a period of anxiety. Another study has even implicated the hormone in alcoholism and the excessive consumption of other drugs as well.

So on Thanksgiving, when the main course is over and you find yourself with room for dessert, blame (or maybe give thanks for) ghrelin.

Joseph Stromberg | | READ MORE

Joseph Stromberg was previously a digital reporter for Smithsonian.