Is it great to indulge during festive season?

 

What does a huge festive meal do to your brain?

Jessica Bradley

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(Credit: Getty Images)

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.

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Eating a little every now and then will not have a big effect on our metabolism, research suggests (Credit: Getty Images)

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."

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

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.

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.

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.