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Why exercise might be more beneficial on an empty stomach

Now scientists have said that exercising on an empty stomach may 'promote more favorable' changes in body fat.

Becky Fletcher, Netdoctor, Apr 10, 2017

Before a workout, it can be difficult to know what to eat, or if you should eat anything at all. Now scientists have said that exercising on an empty stomach may "promote more favourable" changes in body fat.

The team from the University of Bath studied 10 overweight male volunteers. The participants were asked to walk for 60-minutes at 60% maximum oxygen consumption first on an empty stomach and, secondly, two hours after consuming a high-calorie carbohydrate-rich breakfast.

The scientists took blood samples after fasting and the high carb breakfast and then after exercise. They also took adipose (fat) tissue samples immediately before and one hour after the walking exercise.

They discovered that the expression of two genes (PDK4 and HSL) significantly increased when the men fasted - but decreased when the men ate a high-calorie breakfast before they exercised. The increase in these genes suggested that fat was used to fuel the exercise, instead of the food consumed at breakfast.

Dylan Thompson, corresponding author of the study explained that this fat tissue often faces competing challenges. After consuming food it is busy responding to the meal and "and a bout of exercise at this time will not stimulate the same [beneficial] changes in adipose tissue. This means that exercise in a fasted state might provoke more favorable changes in adipose tissue, and this could be beneficial for health in the long term."

Although the study supports previous research, it is worthwhile noting the study sample was small and only looked at overweight men.

What do the experts think?

We asked NetDoctor's go-to fitness experts, Laura Williams and James Farmer for their opinions on whether you should eat before exercise:

Laura explains that it depends on the workout you're doing. If you're opting for more intense workouts - or those longer in duration - then eating can fuel your exercise and stop you getting light-headed. She says:

"Exercising on an empty stomach has been shown to enhance or increase fat burning but there's a case for and against. Largely it depends on the type of workout you're doing. If you're participating in a higher intensity session, you'll need your fuel stores for your workout - fat isn't the body's favourite type of fuel - and the chances are you'll burn more calories than when you'rerunning on empty because you'll be able to maintain intensity. But if you're plumping for a low intensity workout, that might even be a brisk walk, then you can try it on empty. Just ensure you stop if you feel lightheaded or weak."

James adds that it's important not to overcomplicate things - and it's important to pay attention to your own needs. "Training fasted or fed is a much talked about subject," he says. "Research has shown that training in a fasted state burns more fat during the workout, but training in a fed state leads to higher fat oxidation over a 24-hour period when compared to a fasted state."

"I think the best 'state' is the one that fits with your lifestyle and the one you can train at your best. If you're training super early before work, eating and letting that meal digest before training is going to be difficult. Chances are, you'll be better training in a fasted state. Later in the day? You'll probably train better having fully digested a meal before hand.

"The simple answer is for the majority of us, it will make little difference when looking at fat loss over a significant period of time. Just pick the state that suits your training time, and the one that makes you feel the best! Don't overcomplicate it."

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