The author of the 5:2 diet tested the popular theory that eating carbs at night is bad for you - and the results suggest we've got it all wrong

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Many people believe that eating carbs in the evening is bad for you, but are they right?

• Most people believe that a carb-heavy diet - particularly in the evening - is bad for you.
• Dr Michael Mosley, author of the popular "5:2 Diet," puts the theory to the test in a new episode of BBC Two program "Trust Me I'm a Doctor."
• A small study done on a group of "healthy volunteers" revealed a clear winner between morning vs. evening carbs - and it proved the theory wrong.

It's a common belief that eating late at night - particularly anything carb-heavy - can ruin an otherwise good diet.

However, when Dr Michael Mosley, author of the popular "5:2 Diet" and "The Blood Sugar Diet," put the theory to the test as part of a new BBC show, the results suggested this might not be true.

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Mosley is famous for the 5:2 diet, which is based on intermittent fasting. It involves eating pretty much whatever you like five days a week, but cutting calories for two non-consecutive days - just 500 calories for women and 600 for men.

However, calorie counting is just one method for weight loss.

Diets high in refined carbs, such as white bread or pasta, have been repeatedly tied to outcomes like weight gain and obesity. This in turn has led to the rise of the low-carb diet.

"[I]f you eat lots of carbohydrates and sugars, particularly the sort without fibre that get quickly absorbed, they will rapidly push up your blood glucose (sugar) levels," Mosley wrote in a BBC article.

If this glucose doesn't get burned off by some form of activity, the pancreas responds by releasing insulin into the bloodstream to bring the levels down again, he explained, and this is done by storing the excess sugar as fat.

"Too much stored fat, particularly visceral fat (inside the abdomen) can lead to serious health problems such as type-2 diabetes," he added.

But Mosley points out that people have also come to believe that when you eat carbs also counts.

"It's widely believed, for example, that eating carbs in the evening is worse for you than having them for breakfast," he wrote.

And the reasoning behind this is pretty self-explanatory: it's thought that if you load up on carbs in the morning, you'll have more opportunity to burn off glucose over the day's activities.

Putting the theory to the test

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Dr Michael Mosley is behind the 5:2 fast diet as well as The Blood Sugar Diet.

In the latest episode of "Trust Me I'm a Doctor," which will air on BBC Two on Wednesday January 17 a 8.30 p.m., Mosley works with Dr Adam Collins from the University of Surrey to put the theory to the test in a "small" study.

They set out to discover how well the bodies of a sample of "healthy volunteers" would cope with eating most of their carb intake in the morning, or in the evening. They also wanted to find out if the body learns to adapt over a period time, he said.

The participants were all given the same daily carb allowance, and in the first phase of the experiment, ate most of this at breakfast time. They then had five days of eating "normally" before swapping to eating high-carb meals in the evenings for another five days.

Their blood sugar levels were monitored throughout.

Ahead of the experiment, Dr Collins said: "It's always made sense to me that we process carbs better if we have a whole day of activity ahead. So, I expect having most of their carbs at breakfast will be easier for their bodies to cope with.

"But we don't really know what happens if you regularly follow an evening-carbs diet," he added. "There's never been a study like this before, and as a scientist I'm excited to see what happens."

The clear winner

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The results surprised Mosley, who said there was a "clear winner," but not the one he was expecting.

The results showed that the average blood glucose response of the participants in the the initial stage of the test was 15.9 units, which was "roughly as predicted."

But during the final five days of the study this went down to 10.4 units, "which was considerably lower than we were expecting," Mosley said.

"[It] could be that what matters is not so much when you eat your carbs but the length of the carbs-free 'fasting' period that precedes your meal," Mosley said.

"If you've had a big gap since your last carb-rich meal, your body will be more ready to deal with it. That happens naturally in the mornings because you've had the whole of the night, when you were asleep, in which to 'fast.'

"But our small study suggests that if you go low-carb for most of the day, that seems to have a similar effect."

He added: "In other words, after a few days of low-carb breakfasts and high-carb dinners your body becomes trained for this - it becomes better at responding to a heavy carb load in the evening."

Collins is now planning a larger study on the topic, according to Mosley, in search of more definitive answers.

For now, he says Collins is advising people not to focus so much on when you eat carbs - provided you are consistent and "don't overload with them at every meal."

"It's more about achieving peaks and troughs," Mosley wrote. "If you've had a lot of carbs in the evening, try to minimise them in the morning.

"On the other hand, if you've had a pile of toast for breakfast, go easy on the pasta that night."