Too much turkey and stuffing and pumpkin pie and sweet potatoes and cranberry sauce, and then later, you might spare a thought for the peculiar affliction of the holiday season, the bellyache. And ask yourself: Why this annual curse?
An unfortunate combination of gluttony and anatomy explains the holiday dinner stomachache, with the patchwork of human evolution bearing part of the blame.
“Life is tough all over, and your stomach can only stretch so much,” gastroenterologist Patricia Raymond in Virginia Beach, Virginia, and a fellow of the American College of Gastroenterology, told BuzzFeed News.
The average stomach takes an hour to become half empty, which translates to at least 60 minutes of agony for most people who overdo it at the Thanksgiving table, she said. That’s made worse by stomach fat, which acts like a rubber band constricting your guts as they try to expand. And by fatty food, think pie, which takes longer to digest than lean food.
“You put nice, lean turkey down there and then you follow it up with a lot of gravy and stop things up,” Raymond said.
The basic problem is that evolution hasn’t set us up well to deal with a holiday meal: More than 3 million years ago, stomachs mostly served as an organ for pulping chewy fruit plucked from African jungle trees. Much like many chimps today, our ancestors likely spent hours filling and emptying their stomachs for much of the day, chewing repeatedly on a diet high in fiber from chewy grapes, figs, leaves, and bark. It was the original slow food movement.
Guts don’t fossilize, so paleontologists have had to rely on the evolutionary record of teeth and jaws in human ancestors to figure out this history, along with comparisons to our modern-day cousins, apes. A move toward crunching starchy tubers seems to have started more than 2 million years ago, when a changing climate turned African jungles to savannas and mixed woods. That led to eating more fibrous, tough foods buried underground.
Our ancient ancestors likely had digestive tracts similar to our closest genetic relatives, chimps, which are heavy on the back end to help digest all that food. Chimps have about half of their guts in the very last part of the colon, to handle all the roughage they eat, unlike modern people who have only a fifth.
What happened? The culprit seems to be hunting and gathering, the more recent innovation that really made a difference starting around 2.5 million years ago, when our ancestors started eating more meat. Stone axes that mash up, cut, or kill food started to deliver meat and less fibrous plants to our stomachs, which led to smaller guts that are fed this higher-calorie food.
But not a lot of it. “When a hunter kills something large, like a several-hundred-pound antelope, he distributes meat to everyone in camp,” Harvard evolutionary biologist Daniel Lieberman noted in his 2013 book, The Story of the Human Body: Evolution, Health, and Disease. Sharing was, and is, a “vital strategy” for hunter-gatherers to get fed in return when someone else catches something.
So rather than feasts, this lifestyle featured a lot of vegetarian meals, treated to meat here and again. This high-quality diet and smaller digestive tract is thought to have evolved along with a bigger human brain, the “expensive tissue” hypothesis (brains burn 22 times as many calories as muscle, begging an explanation) that explains why we evolved a mind that has come up with such nutritional marvels as the deep-fried turkey and the cheese dog.
Fire, and cooking, Harvard primatologist Richard Wrangham has argued, delivered softer food more easily handled by the stomach and small intestine. The earliest evidence of controlled fire in early human hearths dates to around 1 million years ago. Cooking food likely spurred more diet-driven evolution that further shortened the human digestive tract, leading to an even bigger brain and, eventually, The Great British Bake Off.
If it is any consolation, our ancient ancestors who stuck us with these less hardy guts and a metabolism unevolved for Thanksgiving likely got theirs too, Harvard’s Lieberman told BuzzFeed News by email.
“I suspect overeaters got stomachaches in the Paleolithic [Stone Age], too, when they gorged on a fortuitous bounty of food — but I have no data to back this up.”
Only in the past 8,000 years does farming come into the picture and the start of the domesticated turkey on the holiday table, which really screwed things up. A 1985 New England Journal of Medicine paper, “Paleolithic Nutrition,” kicked off widespread debate over how much the mismatch between more than a million years of evolution for a hunting and gathering diet, and today’s fast-food diet, was responsible for modern-day ills such as obesity and heart disease. Adult-onset diabetes might be the best example of this. The disease doesn’t develop in people who don’t have enough to eat and who exercise a lot, aka most prehistoric people. Only recently have the genes that make the disease more likely to develop become detrimental to our health. (For diabetes, they also seem to be an import from mating with our Neanderthal cousins in prehistory.)
Archaeological evidence discovered over the last half century (read bones) suggests that eating got even less fun when farming started, with smaller stature and signs of malnourishment becoming more common. Two of the big adaptations that came alongside agriculture, drinking milk and eating grains, are also “mismatches” with the original primate diet, so much so that genetic adaptation to dairy is a relatively recent event, and widespread only in people of European descent.
So here we are, heirs to fruit-munching apes who learned to kill things with rocks and grow plants to stave off famine, blessed with a stomach originally meant to acidify leafy plants and shortened guts meant to carefully process the few morsels of high-calorie food we came across while hunting and gathering. Into that engine we toss the bounty of the holiday meal, filled with food we barely have had time to evolve to eat, or in some cases, chew.
“It's hard to imagine hunter-gatherers feasting on the sheer quantity of food that Americans eat at Thanksgiving,” anthropologist Andrea Wiley of Indiana University told BuzzFeed News.
Despite its relatively small size, the human stomach is a muscular marvel, a grinding and mixing chemistry lab, that can stretch from quart-sized to gallon-sized in the course of a meal. However, all that activity is both a necessity and a “stress” for the body, said Wiley, especially sugars and fats. All of those nutrients are absorbed from our gut by the rest of our organs, which have to work harder to soak them up. “So perhaps the stomachache is a means to limit further consumption until these have been cleared out of the bloodstream,” said Wiley.
It’s a mistake to think the human stomach evolved for either feast or famine, evolutionary biologist Joanna Lambert of the University of Colorado, Boulder, told BuzzFeed News. It’s a working instrument. “Stomachs cannot ‘store’ food — there is no caching for later — so the stomach must operate in a way that maximizes efficiency,” she said by email.
Basically, the ratio of food mass to digestive fluids “gets out of whack when the stomach is stuffed full,” Lambert said. This leads to a roundabout game of telegram that starts with the guts signaling to the back of the brain that things are stopped up, which then gets translated to the front of the brain as the sense that one is “full.”
The last evolutionary catch is that we have evolved to be extremely social creatures, said Lambert, another inheritance from our ape ancestry. At Thanksgiving, or Christmas or the Super Bowl or weddings or whenever, that social context overrides the signals from the gut.
You might be stuffed or feel full, and the brain is saying “stop eating,” she said, but that individual sees their friends and family still chowing away and this sends a different signal: “Keep feeding because everyone else is.” ●