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A new study reveals the true effect eating late at night can have on the body - but here's what nutritionists really think

Woman&Home logo Woman&Home 12/4/2022 Grace Walsh
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The impact of eating late at night has been under debate for decades now, with many nutritionists, scientists, and dieticians coming to the conclusion that the timing of dinner can have a serious impact on how we gain and lose weight. Others have pointed out that it's all about what you eat, rather than when you eat it.

And while that might be true - after all, it's all about calories in versus calories out for  both weight loss and gain - new evidence suggests it's worth re-evaluating the final conclusion. A new review by Monash University in Australia collated all the recent research on the impact of late-night eating on obesity and weight gain in adults and found that the timing of meals does have a real link to the risk of gaining weight over time. 

While they have noted that further studies are needed to establish exactly why, it's thought to be because three key reasons: our metabolism, appetite hormones, and gut bacteria. But with the idea of a calorie deficit and balance always pushed to the front of our minds if we're dieting but not losing weight, what does this all really mean? Here's what two nutritionists have to say. 

Does eating late at night impact how we gain weight?

The nutritionists agree that the timing of our meals may have an impact on how easily some people gain weight over time but, ultimately, it's important to remember that we gain weight when we eat more energy than we burn for a period of time, says Signe Svanfeldt. "Even when we sleep, we burn energy to support our body’s organs, functions, and tissues. Although we of course burn more when we are active." 

Svanfeldt, who is also the nutritionist for the healthy eating app Lifesum, says, "Simply put, to lose weight we need to burn more energy than we eat. So, it all comes down to how much energy our body burns as well as our daily intake. What we eat and the quantity is the main reason for any weight changes, not so much the time of day it's consumed." 

So if you can only squeeze in your final meal later in the evening, you have no reason to be concerned about the timing of your meal alone for managing any weight concerns you might have. "For some, eating late at night might be the only option to get enough energy in their daily intake," Svanfeldt adds. 

Registered nutritionist Jenna Hope agrees. "Of course, sometimes we eat later in a more social environment or work gets in the way and that’s a normal part of life. It’s consistently eating late which can have negative impacts," she says.

However, that doesn't mean the timing of eating doesn't have any effect at all. Eating late at night isn't the primary reason why we gain weight but when we do eat later into the evening, it can trigger a set of processes that changes the way we want to eat and could lead us to eat more than we need to.

"For example, eating late at night can trigger cravings, and eating more food or sweets and savory snacks than is actually required [may encourage weight gain as you may be eating more calories than you burn]." And this is where the study's findings come through, as Svanfeldt and Hope explain.

Why does eating late at night have an impact at all? 

1. It can disrupt our circadian rhythm

"It’s commonly believed that what you eat is more important than when you eat. However, more recently, evidence has emerged to suggest that eating late at night and outside of regular daytime hours can impact our gut microbiome and glucose metabolism," says nutritionist Hope.

These regular daytime hours correspond to the body's circadian rhythm, which is a 24-hour internal clock that controls everything from our hunger levels and metabolism to how we absorb nutrients and our insulin sensitivity.

Eating outside of these eight to 12 daylight hours, studies by San Diego State University suggest, may mean your body processes calories a little differently and less effectively. This could, in turn, lead to weight gain over time. 

2. Gut bacteria

The change that eating late at night can have on our circadian rhythm goes further, however. As nutritionist Jenna Hope explains, "Evidence has shown that eating outside of regular daytime hours can disturb the relationship between our peripheral circadian clocks and our epithelial cells. Plainly speaking, this can disrupt the link between the circadian rhythm and the gut microbiome." 

As noted, our circadian rhythm is a 24-hour pattern that regulates lots of different biological functions. There's more and more research, including that by the University of Illinois, to suggest that our gut microbiome is also regulated by these internal clocks as well as our eating behaviors. 

We hear about how to improve your gut health so regularly nowadays but many people still underestimate the importance of looking after the gut. "If the health of our gut microbiome is compromised then we may not be able to absorb and utilize nutrients as well. This can increase the amount of energy we store and so could increase the risk of weight gain over a long period of time," she says. 

3. It can change our appetite hormone levels

Eating late may disrupt our appetite hormones, says Hope, pointing to another new study published earlier this year. The research from Brigham Women's Hospital found that overweight participants who ate earlier in the day reportedly had significantly different levels of cravings and feelings of fullness compared to those who ate later. The participants who ate four hours later than their counterparts were found to be twice as likely to be hungry during the day compared to the earlier eaters with more of a desire for starchy and salty foods, rather than dairy or vegetables.

This, the scientists found, was due to the decrease in levels of leptin - the hormone that signals to our brains that we're full - and an increase in ghrelin - the hormone that signals hunger - over the full 24-hour sleep cycle. 

"Additionally, some evidence has shown that consuming higher sugar foods later at night may spike glucose levels more than when the same foods are eaten earlier on in the day," Hope says. When this level eventually dips back to normal, feelings of hunger and the desire to eat - whether or not we've eaten recently - tend to come back pretty quickly. 

How to stop eating late at night

  • Make sure to eat plenty of protein and fiber: As the two most filling nutrients, according to Purdue University, these will be more likely to leave you feeling full later into the evening and so less likely to snack. 
  • Keep snacks out of the house: There's nothing wrong with having a sweet snack later in the evening but if you're looking to stop eating late at night, removing these will probably be a good idea. Out of sight, out of mind, as the saying goes. 
  • Make sure you get enough sleep: Not only will learning how to sleep better mean you feel more refreshed when you wake up, there's plenty of evidence to suggest that sleep deprivation has a huge impact on our cravings.
  • Eat earlier in the evening: If it's late-night cravings we're talking about, choosing to eat earlier on in the evening will be a surefire way to prevent these. "It’s important to note that for some individuals such as shift workers, eating late is unavoidable and therefore it may be better to eat a well-balanced nutritious meal as early as possible and avoid snacking throughout the night," says Hope. 

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