To many it sounds like workout torture, but cardio-fasting — performing a 30 to 45-minute aerobic session in the early morning on an empty stomach — is a growing trend among male gym-goers looking to shed excess pounds quickly. In the online chat rooms of magazines such as Men’s Health and Men’s Fitness, the cardio-fast is a hot topic and US gyms are reporting that more men are hitting the treadmill or cross-trainer before the first calorie of the day passes their lips. But is it really worth the effort?
Enthusiasts claim that with the body in a fasting state during sleep, it is primed for optimal fat-burning when we wake up. Because carbohydrate stores are depleted overnight, so the idea has it, a pre-breakfast cardio workout forces the body to tap into its energy reserves and burn off stored fat. “Exercise at a moderate intensity on an empty tank and the belief is that the body will first burn off protein and fat, sparing any of the limited carbohydrate, stored as glycogen, for later use,” says Dr Neil Walsh, an exercise physiologist at Bangor University. It’s an approach that has been popular among bodybuilders and weightlifters for years and some studies have shown that fat is burnt off more quickly during exercise carried out just after an overnight fast than when doing the same exercise in the afternoon.
“Food releases insulin, a hormone that blunts fat-burning, into the bloodstream,” says Mike Gleeson, Professor of Exercise Biochemistry at Loughborough University. “Insulin levels are at their lowest when you first wake up, which does mean that pre-breakfast is the optimal time for fat to be burnt.”
But Gleeson says that pre-breakfast exercise should not be restricted to cardiovascular activity. “To activate the mechanisms that burn fat and tone muscle, you need to include some resistance or weight training in your fasted workouts. There also needs to be plenty of protein, such as low-fat dairy foods, in your diet and ideally you should consume breakfast when you finish to help your muscles adapt positively to the exercise.”
But other researchers say that any increase in fat burning is offset by the fact that cardio-fasting burns fewer calories because the workouts tend to be shorter. “If you work out in the fasting state, your blood sugar plummets and you are likely to feel tired and ravenously hungry when you finish,” says Louise Sutton, a dietician at the Carnegie Centre for Sports Performance at Leeds Metropolitan University. “That is why most top athletes won’t perform a training session without ingesting some calories, even if it’s just a banana or high-carbohydrate energy drink.”
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A study published in the journal Medicine & Science in Sports & Exercise several years ago appears to prove her point. Researchers asked a group to exercise on indoor bikes one morning after a small breakfast, and another morning after eating nothing at all.
On an empty stomach, subjects got tired more quickly and stopped pedalling 30 minutes earlier than if they had eaten.
But if you can overcome the hunger pangs, cardio-fasting is not necessarily a waste of time. While the extra calories burnt are negligible, it can help to build and firm muscles if you do it two or three times a week. “The exercise you do on an empty stomach can also temporarily curb your appetite so that your overall daily calorie consumption is reduced,” Gleeson says. “But cutting calories too severely is a no-no. Eat too little during the day and you will eventually lose muscle mass, fitness levels will plummet and you will be back to square one.”