Food supports our brain in many ways, allowing it to perform many important functions, such as memory and concentration. And a balanced diet can also be of great help to our mental health.
But what are the immediate effects on the brain of eating large meals, as we usually do for Christmas dinner?
What happens when we eat too much?
When we eat, several signals throughout our body work together to tell our brain that we are full.
These signals include those sent by hormones released in the gut and metabolites (molecules that break down food to generate energy).
These hormones also signal the release of insulin from the pancreas to control our sugar levels. This whole process is called the “satiety cascade.”
“These signals come from different parts of our gut and act over slightly different periods of time,” explains professor and clinical endocrinologist Tony Goldstone, of Imperial College London.
This cascade of hormones released by the gut and pancreas and sending signals to the brain may also have something to do with the sleepiness we feel after a heavy meal (called “postprandial sleepiness”).
But the exact mechanisms responsible for this process are not yet well understood, according to Professor Aaron Hengist, visiting researcher at the National Institutes of Health in Washington, United States.
This feeling (called a “food coma”) is generally thought to be largely due to the transport of blood from the brain to the stomach. But research indicates that blood flow doesn’t actually decrease after a large meal.
More research is needed to understand sleep after meals, according to Hengist.
“The reaction of intestinal hormones is a cocktail,” according to him. “We don’t know which specific hormones can cause sleep in which brain centers.”
Is overeating dangerous?
Surprisingly, eating too sporadically has little effect on our metabolism, according to Hengist.
In 2020, he published the results of a study that looked at what happens when people eat beyond the amount needed to be comfortably full and when they eat until they’re ready to burst.
To carry out the study, 14 healthy (and very courageous) men volunteered to eat a large quantity of pizza in a single meal.
During one visit, the researchers asked them to eat until they were comfortably full. And, in another, they ate as much as they could.
During this second time, they ate twice the amount of pizza consumed the first time.
The researchers assessed their hormone levels, appetite, mood and metabolic reactions for four hours after the two feasts. They concluded that blood sugar levels were not higher than those observed after a normal meal, nor was the amount of fat in the blood.
“We were surprised to find that, despite doubling energy intake, the body regulated blood sugar levels very well,” says Hengist.
“We found that the body works hard to achieve this, secreting more insulin and several gut hormones, which help release insulin and signal that we are full.”
He explains that this study demonstrates that indulgence alone is not as dangerous as one might think.
But because the study only looked at healthy young men, it’s not possible to extrapolate the research to the general population without studying women and people who are obese or overweight, according to Hengist.
How can we overeat to make a difference?
A pizza binge may not cause immediate harm, but research shows that eating for several hours or a full day of feasting can start to harm your metabolism and put stress on the body, which, in turn, can affect the brain.
In 2021, a study looked at people who ate too much for an extended period of time and found very different results than Hengist’s pizza research.
It draws inspiration from the North American tradition of throwing parties before sports competitions, where people eat a lot and drink alcohol.
Researchers attempted to replicate this tradition by offering alcoholic beverages and foods high in sugar and fat (like burgers, fries, and cakes) to 18 healthy but overweight men to spend an afternoon consuming.
They ate an average of 5,087 calories over five hours. Blood and liver imaging tests revealed that most of them suffered from increased liver fat content after the feast.
Research shows that nonalcoholic fatty liver disease can reduce the amount of oxygen in the brain and cause inflammation of brain tissue, which can increase the risk of brain disease over time.
Foods high in fat and sugar can cause non-alcoholic fatty liver disease in the long term.
“The study shows that men had metabolic dysregulation,” says Hengist. “When you passively consume food and alcohol for several hours, the pressure is too much for the body to handle.”
Why doesn’t a big meal affect us?
Evolution can help us understand why it’s not so dangerous to overeat every once in a while – and how our brains and guts evolved to communicate with each other when we need to eat.
When we’re hungry, many mechanisms kick in and cause us to eat, according to Goldstone.
Our mood, for example, may change and we may feel irritable and hungry. It also increases our propensity to crave high-energy foods.
“We don’t know exactly what drives the irritation of hunger,” says Goldstone. “But current research shows that hunger is a very unpleasant state and that people may be eating to get rid of it.”
Evidence from animal studies demonstrates similar behavior.
Studies highlight how part of the hypothalamus circuit (a part of the brain that controls appetite) is inhibited when rodents see and smell food, even before they start eating.
“Hunger has driven them to look for food, and when they find it, this behavior does not need to continue,” Goldstone says. He points out that much of this process takes place in the subconscious.
Humans have evolved to find ways to combat hunger. After all, without food we die.
But there has been no excess throughout human history. And its effects occur in the long term and are less deadly, at least in the short term, according to Goldstone.
Does it make a difference what we eat too much?
Several studies in rats and mice indicate that long-term high-calorie feeding can affect memory and learning functions.
But in humans, there is less research in this area, according to Stephanie Kullmann, group leader and head of the metabolic neuroimaging division at the University of Tübingen, Germany.
But one study offers some clues about what happens in our brains when we eat foods high in sugar and fat in excess. And their findings could be applied, to some extent, to shorter periods of overeating, according to Kullmann.
In one study, 18 healthy men ate a high-calorie diet for five days, particularly ultra-processed snacks high in sugar and fat, in addition to their normal diet.
On average, they ate 1,200 kcal more per day. 11 other people, belonging to a control group, did not change their diet.
The results demonstrated that the high-calorie diet affected the brain’s response to insulin, in areas that contribute to reducing its response to visual cues about food and memory processes.
The insulin-resistant brain does not sufficiently reduce appetite and food consumption, signals that tell us to stop eating when we are full.
“An important discovery was that the brain changes before the body,” according to Kullmann.
“The participants continued to have the same weight, but when we looked at their brains, we saw that they were much closer to someone who had been obese or overweight for a few years,” she explains.
Research indicates that in obese people, the hypothalamus and the brain’s reward system (which helps regulate our food intake) may be impaired.
This study expands existing research, according to Kullmann. It shows the communication between the gut and the brain and demonstrates how the axis is different in obese people.
Specifically, obese people are more likely to choose larger portions of food when thinking about pleasure.
Participants in Kullmann’s study were asked to return to their normal diet after the five days of research. But a week later, new tests showed that the memory and cognitive parts of the brain were even less responsive than before the higher-calorie diet began.
So can we eat more at Christmas?
It is well known that prolonged periods of eating (particularly high in sugar and saturated fat) are not good for the brain. And while there are fewer studies looking at the impacts of diet alone on our bodies, existing evidence suggests that it is not harmful to the brain.
“Our study shows that indulgence alone is not as harmful as one might expect,” says Aaron Hengist. “So, have a nice Christmas dinner!”
But he points out that going beyond that can start to create strain on the body.
Even five days can be enough to create lasting effects on the brain, according to research by Stephanie Kullmann.
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