It's a common belief that eating late at night - particularly anything carb-heavy - can ruin an otherwise good diet.
However, Michael Mosley, the doctor behind the so-called 5:2 and blood-sugar diets, put the theory to the test as part of a new BBC show, and the results suggested this may not be true.
The 5:2 diet, based on intermittent fasting, involves eating pretty much whatever you like for five days a week, then for two nonconsecutive days restricting calories to 500 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 tied to outcomes like weight gain and obesity, contributing to the rise of the low-carb diet.
“If 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.
He said that if this glucose isn't burned through some activity, the pancreas responds by releasing insulin into the bloodstream to bring the levels down again, 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 wrote.
But Mosley says many 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.
It's thought that if you load up on carbs in the morning, you can burn the glucose during the day's activities, he said.
Putting the theory to the test
In an episode of Trust Me, I'm a Doctor airing on BBC Two on Wednesday evening, Mosley works with Adam Collins, a doctor at the University of Surrey, to study how a few “healthy volunteers” adapt and cope with eating most of their carbs in the morning or evening.
In the first five days of the experiment, the participants, who were all given the same daily carb allowance, ate most of their carbs at breakfast. They then ate “normally” for five days before swapping to high-carb meals in the evenings for five days.
Their blood sugar levels were monitored throughout.
“It's always made sense to me that we process carbs better if we have a whole day of activity ahead,” Collins said before the experiment. “So I expect having most of their carbs at breakfast will be easier for their bodies to cope with.”
He added: “But we don't really know what happens if you regularly follow an evening-carbs diet. There's never been a study like this before, and as a scientist, I'm excited to see what happens.”
The 'clear winner'
Mosley said there was a surprising but “clear winner” between morning and evening carbs.
Mosley and Collins found that the average blood glucose response of the participants in the initial stage of the test was 15.9 units, which Mosley said was “roughly as predicted.”
But in the final five days of the study, this went down to 10.4 units, which Mosley said “was considerably lower than we were expecting.”
“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.'”
He added: “But our small study suggests that if you go low-carb for most of the day, that seems to have a similar effect. 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.”
Mosley says Collins is planning a larger study on the topic in search of a more definitive answer.
For now, Mosley says, Collins is advising people not to focus so much on when they eat carbs — provided they're 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.”
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