Image copyright Getty Images Image caption Pasta v toast - which can you have?

It's January, which means you may have stood on the scales and decided to go on a diet. But what sort?

For many years, low-carb diets have been in fashion - based on the belief that eating lots of carbohydrates, particularly in the form of sugary treats such as white bread, rice or pasta, is bad for your waist and for your blood-sugar control.

The reasoning is that 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.

Unless you burn this glucose off by doing exercise, your pancreas will pump out lots of the hormone insulin to bring these levels back down to normal.

It does this by storing the excess sugar from the carbs as fat. Too much stored fat, particularly visceral fat (inside the abdomen) can lead to serious health problems such as type-2 diabetes.

As well as concern about the amount of carbs we eat, people also worry about when they get eaten.

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

That's because first thing in the morning your body is raring to go and should soon burn up the glucose released from the carbs. When you eat late at night your body is preparing to sleep, so the body should take longer to clear it.

That's the theory. But is it really true?

Morning v evening carbs

On Trust Me I'm a Doctor, with the help of Dr Adam Collins, from the University of Surrey, we set up a small study.

We recruited healthy volunteers to see how well their bodies coped with eating most of their carbs in the morning, or in the evening.

We also wanted to see if the volunteers' bodies would adapt over time.

All of our volunteers were asked to eat a fixed amount of carbs every day; things such as vegetables, bread and pasta.

For the first five days they were asked to eat most of their carb allowance for breakfast, leaving only a small amount for dinner time.

Then they had five days of normal eating before switching to low-carb breakfasts and high-carb dinners for a final five days.

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Dr Collins's team was monitoring their blood-glucose levels throughout. So what did he think he would find?

"It's always made sense to me that we process carbs better if we have a whole day of activity ahead," he said.

"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.

"There's never been a study like this before, and as a scientist I'm excited to see what happens"

So what did we find?

Well, there was a clear winner. And it wasn't the one I was expecting.

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When the researchers tested the volunteers on the day after a run of high-carb breakfasts and low-carb dinners, they found their average blood glucose response was 15.9 units.

This was roughly as predicted.

But when they did the same tests after five days of low-carb breakfasts and high-carb dinners?

Remarkably, their average glucose response went down to 10.4 units, which was considerably lower than we were expecting.

So what happened? Well, 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.

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.

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.

Dr Collins is now launching a much larger study, which will hopefully provide more definitive answers.

In the meantime, his advice is not to worry too much about what time of day you eat carbs, as long as you're consistent and don't overload with them at every meal.

It's more about achieving peaks and troughs, 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.

The new series of Trust Me I'm a Doctor continues on BBC Two at 20:30 on Wednesday 17 January and will be available on iPlayer afterwards.