We’ve made countless jokes about our “Thanksgiving pants“ and planned belt unbuckling as we prepare to indulge in a big meal on Thursday. And, in case you missed it, we’ve also done our best to calculate the number of calories we might consume if we don’t rein it in a little bit. But what actually happens to your system when you overeat during the holidays?
We asked Dr. Jay Kuemmerle, a gastroenterologist and professor of medicine at Virginia Commonwealth University and Dr. Daniel Hurley, an endocrinologist and consultant in the Division of Endocrinology, Diabetes, Metabolism and Nutrition at the Mayo Clinic, Rochester to walk us through how our bodies really handle the feast before us.
“The main difference that relates to Thanksgiving is the volume and constituents of the meal,” says Kuemmerle. “In large part the high fat can lead to feeling very full and slower digestion. This can cause the stomach to expand to a greater degree, which can be uncomfortable.”
Uncomfortable how? Well, as the stomach gets more distended from overeating, the growing pressure is relieved by releasing gas — that means some people will experience acid reflux and the urge to belch. Kuemmerle suggests thinking of the stomach as a balloon: It has some elasticity, but eventually reaches a breaking point and must relieve pressure.
Our bodies have a natural stopping point, but the brain is capable of overriding the stomach’s wishes to stop eating. That’s particularly true during a holiday meal, where variety and abundance are prized.
“There’s some suggestion that a wide variety of food, like at the Thanksgiving meal, tends to increase food intake,” says Hurley. This is often referred to as the “smorgasbord effect,” according to the Columbia University Press.
Thanksgiving differs from other meals mostly in ritual: The holiday prizes tradition over digestive mindfulness, hence the problems with variety and satiety. But in all other ways, the meal looks about the same to your digestive tract. (Which may be a comment on our abundant year-round food supply and not this holiday of abundance).
Below, how digestion works — on Thanksgiving and on all other days:
It turns out the expression “feast your eyes” is pretty dead on. As soon as you sit down at the table, the sight and smell of the food sends a signal to the brain and then down to the stomach to prime your digestive system for the meal, according to Kuemmerle.
That means, at the very first bite, your stomach is primed and ready to go. “When the first bite of food hits the stomach, it’s already revved up: acid and digestive enzymes have been released,” says Kuemmerle. “The stomach starts to expand to accomodate the meal.”
Your mouth plays a role too. “As food is chewed, digestive juice from the salivary glands starts the digestion,” explains Hurley. “The teeth involved in mastication break down the food into protein, carb, fat and then in the stomach, breakdown continues.”
As you eat, your stomach stretches and secretes acid and digestive enzymes to help digest the food. Once you get to a point where your stomach feels full, stretch receptors — a collection of sensory nerves in the stomach — send messages to the brain to tell it that it’s time to stop eating.
Again, this is where your brain can really misguide your body. “When we eat, ghrelin — the hormone that stimulates back to brain to say I’m full or I’m hungry — increases and activates the hunger or satiety centers in the hypothalamus of the brain,” explains Hurley. “But your central nervous system can override the hypothalamus — it’s the same reason we can stay awake, even if our brain is telling us we’re tired.”
Once your body determines fullness, the stomach grinds the food down into two to three millimeter pieces — small enough to fit into the small intestine. As the stomach does this, it begins to contract and reestablish its tone, while pushing the ground up matter and digestive liquid through the pylorus and into the duodenum, which is the upper part of the small intestine.
This process can be slowed, depending on what you ate. “A high fat meal with gravy and butter delays emptying of the stomach because fat is harder to digest,” says Kuemmerle. In other words? Your stomach’s ability to efficiently process its contents may rely on how much butter your Aunt Mable put in those mashed potatoes. This can delay stomach emptying, which is an important step of digestion because the food’s presence in the small intestine signals the release of important enzymes from the pancreas and galbladder. These pancreatic enzymes and bile help to digest carbs and proteins and emulsify fats, breaking the food down into amino acids and simple sugars to be absorbed into the blood stream.
Of note, Hurley explains, our metabolism can actually increase if we eat too much to help with digestion, which requires energy. But don’t get too excited, he says, “it’s not enough to overcome the calories we don’t need — it’s just enough to help us.”
The release of sugar in the blood stream triggers insulin, the hormone responsible for regulating blood sugar. Insulin and another hormone glucagon will store some sugar in the liver as glycogen (some fat is also stored in the liver). Every cell of your body requires glucose and muscles also requires a store of glycogen. What the body doesn’t use for these functions will be sent to fat tissue to be stored as fat — either subcutaneous fat or abdominal visceral fat.
As the digested material hits the end of the small intestine, specific vitamins get absorbed, bile gets reabsorbed and hormonal signals are sent to the brain.
Next, the body performs a really fascinating self-cleaning maneuver: As the matter continues into the colon (where water is reabsorbed and some additional nutrients are absorbed, according to Kuemmerle), the interdigestive period begins. All of the “indigestible material” — the detritus that didn’t make it through the first time — gets pushed through. The pylorus opens widely and the bigger stuff gets swept into the colon. A gallbladder contraction allows the pancreatic duct to get cleaned out. It is, Kuemmerle explains, a form of housekeeping to prep the body for the next meal.
“While the [conscious] brain is involved in chewing and swallowing and ‘starting’ the machinery,” says Kuemmerle. “The vast number of functions occur in the GI tract without us being able to regulate or be aware of it.”
And here you thought you were just sitting on the couch.