What does a huge festive meal do to your brain?
Many of us with throw modesty out the window when it comes to Christmas lunch. But what do these blow-out festive meals do to our body and brain?
There are lots of ways that food supports our brains to do their many important functions, including our memory and concentration. A balanced diet can also be a powerful support to our mental health.
But what immediate effects does eating a huge meal – such as an all-the-works Christmas dinner – have on our brains?
What happens when we overeat?
When we're eating, various signals around the body work together to let our brains know we're full – including from hormones released from the gut, and metabolites (molecules that break down food for energy).
These hormones also signal the release of insulin from the pancreas to control our blood sugar. This whole process is called the "satiety cascade".
"These signals come from different parts of out gut and work over slightly different time frames," says Tony Goldstone, clinical associate professor at Imperial College London and consultant endocrinologist.
This cascade of hormones released from the gut and pancreas and sending signals to the brain may also have something to do with the sleepy feeling we get after a big meal (called "postprandial somnolence"). But the exact mechanisms behind this are not yet well-understood, says Aaron Hengist, a postdoctoral visiting fellow at the National Institutes of Health in Washington, US.
It's been widely believed that this feeling – nicknamed a "food coma" – is largely an effect of blood travelling away from the brain and towards the stomach. But research shows blood flow doesn't actually decrease after a big meal.
But there needs to be more research to understand post-meal sleepiness, Hengist says.
"The gut hormone response is a cocktail; we don't know which specific hormones can cause sleepiness on which centres of the brain," he says.
Is it harmful to overeat?
Eating too much on the odd occasion has surprisingly little effect on our metabolism, says Hengist.
In 2020, he published the results of a study that looked at what happens when people eat beyond comfortably full, and when they eat to about-to-burst full.
Fourteen healthy men (very bravely) volunteered to eat lots of pizza in one sitting. In one study visit they were asked to eat until they were comfortably full, and in the other study visit, as much as they could. They ate double the amount of pizza in the all-you-can-eat experiment.
Their hormones, appetite, mood and metabolic responses were measured for four hours following their feasts. The researchers found that their blood sugar levels were no higher than after a normal meal, and neither was the amount of fat in their blood.
"We were surprised that, despite double the energy intake, the body regulated blood sugar remarkably well," says Hengist. "We found that the body was working hard to do this, by secreting more insulin and various gut hormones that help release insulin and signal that we're full."
This study shows, he says, that a one-off indulgence is not as damaging as you might expect.
However, since the study was only done in young healthy men, the research can't be extrapolated to the general population without first studying women and people who are overweight or obese, says Hengist.
Does it matter how we're overeating?
While one pizza-eating session may not be immediately harmful, there is some research showing that multiple hours or one day of feasting can start to disrupt metabolism and put strain on the body, which could, in turn, affect the brain.
In 2021, a study looking into overeating over a prolonged period found very different results to Hengist's pizza study. The study is titled 'The tailgate study,' named after the American tradition of pre-sports game parties that involve eating lots of food and drinking alcohol.
The researchers tried to replicate this tradition by giving 18 overweight but healthy men alcoholic drinks and high fatty, sugary foods including burgers, chips and cake, to consume over an afternoon. They ate an average of 5,087 calories over the five hours. Blood tests and a liver scan revealed that most of the men had increased fat in the liver following their feasts.
Research shows that non-alcoholic fatty liver disease – which can be caused by a high-fat, high-sugar diet over the longer term – can lead to less oxygen to the brain and inflammation to brain tissues, which can increase the risk of brain disease over time.
"The 'tailgate' study shows the men had metabolic dysregulation. When passively consuming food and alcohol over multiple hours, it's too much strain for their bodies to deal with," Hengist says.
Why does one big meal not affect us?
Evolution may go some way to understanding why it may not be too harmful to overeat once in a while – and how our guts and brains have evolved to communicate with each other when we need to eat.
When we're hungry, lots of things come into play to drive us to eat, Goldstone says – for example, our mood can shift and we can feel 'hangry'. We're also more likely to crave energy-dense food.
"It's unclear exactly what drives 'hanger'," says Goldstone. "But ongoing research shows that hunger is quite an aversive state, and maybe people are eating to get rid of this state."
Evidence in animal studies shows similar behaviour. Studies highlight how some of the appetite circuity in the hypothalamus (a part of the brain that controls appetite) is quietened when rodents see and smell food – even before they've eat it.
"The hunger made them seek food, and once they found it, that behaviour doesn't need to continue," Goldstone says.
Much of this process, Goldstone adds, goes on subconsciously.
Humans have evolved to find ways to cope with starvation, because without food, we'd die. But excess, throughout human history, didn't happen – and its effects are longer term and less lethal, at least in in the short term, says Goldstone.
Does it matter what we're overeating?
Several rat and mouse studies suggest that a long term high-calorie diet can affect memory and learning function. But in humans, there's less research in this area, says Stephanie Kullmann, group lead and head of the metabolic neuroimaging division at University of Tübingen in Germany.
One study, however, gives some insight into what happens in our brains when we overeat food high in sugar and fat. It wasn't quite reduced to one meal, but lasted for five days. However, its findings could potentially be applied to a lesser extent to shorter periods of overeating, according to Kullmann.
Eighteen healthy men ate a high-calorie diet for five days – specifically, ultra-processed snacks high fat and high sugar snacks – on top of their normal diet (on average, they ate 1,200 kcal more per day), while 11 others in a control group didn't change their diets.
The results showed that the high-calorie diet affected how the brain responded to insulin in areas that help to reduce its response to visual food cues and memory processes. A brain that's resistant to insulin doesn't properly reduce appetite and food intake – signals that tell us to stop eating when we're full.
"A key finding was that the brain changes before the body," says Kullman. "The participants still weighed the same, but when we looked at their brains we saw they were much more closely related to someone who had been obese or overweight for a few years," she says.
Research shows that in people who are obese, the hypothalamus and brain's reward systems – which help to regulate our food intake – can be disrupted.
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This study expands on existing research, Kullman says, showing the communication between our guts and brains, and how the axis is different for people who are obese. Specifically, people who are obese are more likely to choose bigger portions of food when thinking about pleasure.
Participants in Kullmann's study were told to return to their normal diet after the five days, but one week later, further tests showed that the memory and cognitive parts of their brains were still less responsive than before beginning their higher calorie diet.
So is it OK to indulge at Christmas?
It is well established that prolonged periods of eating – especially foods high in sugar and saturated fats – aren't good for the brain. While there are fewer studies looking at the impacts of a one-off feast on our bodies, existing evidence suggests it's not detrimental to our brains.
"Our study shows that one-off indulgence is not as damaging as you might expect, so enjoy your Christmas dinner," says Hengist.
However, he adds, any more than this can start to put strain on the body. Even five days might be enough to have longer-lasting effects on the brain, according to Kullman's research.