Fasted training helps burn fat, study says

Exercising on an empty stomach may amplify the health benefits of exercise, according to a study that looked at the interaction between meal times, metabolic health and movement.

The study, which involved sedentary men and moderate cycling, suggests that whether and when we eat can affect the impact of exercise on the body.

As a general rule, any physical exercise benefits health. But an abundance of recent scientific findings and personal experience indicate that different people react differently to similar exercise routines. Even if everyone completes the same amount of exercise, some people get in better shape than others, either lose more weight or gain better glycemic control.

Most exercise physiologists say that our genetics, diet, physique, temperament and other subtle aspects of life all work together to shape the way the body reacts to exercise.

But some researchers think the timing of meals also plays a role. Working muscles need fuel during exercise, mainly in the form of sugar (glucose) or fat.

That fuel can come from our most recent meal, from when the sugars and fats that made it up into the bloodstream, or from the body’s stored fat and sugar stores. We all have these reserves, especially fat. Some of it resides in the muscles, marbling them like tenderloin.

This muscle fat becomes problematic when we accumulate it in excess. Fatty muscles do not respond well to the hormone insulin, which directs blood sugar to the muscles. Because of this, excess fat can contribute to insulin resistance, high blood glucose levels, and increased risks of type 2 diabetes and other metabolic conditions.

So researchers at the University of Bath in England and other institutions began to wonder whether meal timing might influence how much muscle fat we burn during exercise. This, in turn, would affect the long-term metabolic consequences of exercise and help explain, in part, why some people do better than others when they exercise.

To study these questions they recruited 30 overweight, sedentary men. The researchers tested the men’s physical fitness and their insulin sensitivity and then divided them into three groups.

One of them, the control group, continued to live as usual. The other two groups began doing supervised morning exercise three times a week, riding a stationary bike at moderate speed, using monitors and masks to monitor their heart rate and how much sugar and fat they burned. While they were exercising, the researchers periodically asked them how they were feeling.

Members of one group drank a vanilla-flavored shake two hours before exercising (with nothing else for breakfast), while those in the other group drank a similar-flavored drink, a placebo containing water, flavoring, and no calories. . That is, the men in the placebo group walked on an empty stomach, but did not know it.

After the exercise, each person was given the drink they had not had before. The men who had fasted received the shake, and the other group took the placebo.

This routine continued for six weeks. After that, the scientists studied the numbers and found some telling differences between the groups. The control group’s level of physical fitness and insulin sensitivity remained the same, as predicted. The men in both groups who exercised improved their physical fitness and saw their waistlines shrink, although few had lost weight.

But the men who cycled on an empty stomach had burned about twice as much fat in each cycling session as the men who drank the shake before. They all burned about the same number of calories cycling, but more of those calories came from fat when the men hadn’t eaten before.

By the end of the study, these participants also showed greater improvement in their insulin sensitivity and had developed higher levels of certain proteins in their muscles that influence how muscle cells react to insulin and utilize blood glucose.

Professor of physiology and nutrition at the University of Bath Javier Gonzalez led the study, which was published in the “Journal of Clinical Endocrinology and Metabolism” at the end of 2019. For him, the findings suggest that “if we exercise before having breakfast, morning, we can better enjoy our exercise session without increasing its intensity or duration”.

The reasons for this additional metabolic boost are complex, he said, but likely involve a reduction in muscle fat.

The bodies of the men who were fasting had to draw on internal energy reserves, including muscle fat, for fuel. An interesting fact is that the participants who exercised in a fasted state did not find the exercise session more tiring than those in the other group, according to subjective evaluations of the effort they put in.

But the study looked primarily at insulin sensitivity and not other aspects of exercise and metabolism, including weight loss (other research by some of the same scientists has studied how eating before exercise can affect appetite).

The researchers were also unable to determine whether skipping lunch and exercising in the late afternoon would have the same effects, but that seems likely, according to Gonzalez.

“We believe the key is in the fasting period, not the timing,” he said.

So if you’re thinking about working out to get healthier on vacation, it might be a case of getting up and heading out to exercise earlier in the morning or after hours of fasting. But, said Gonzalez, if your schedule or preferences don’t allow you to exercise in the morning or on an empty stomach, don’t worry too much.

“Any physical activity is better than none,” he explained, whatever time you can fit it in.

Translation by Clara Allain