When you exercise can affect how much fat you burn

With the exception of a few genetic lottery winners, most people have experienced this at some point in their lives: the frustration of stepping on the bathroom scale after a few weeks of diligent exercise, seeing the exact same number as before you even bothered.

It’s enough to make you lose faith in the whole concept of working out – but according to a new study, the key may not be how you work out, but when.

“Our results suggest that late morning exercise may be more effective than evening exercise in terms of increasing metabolism and burning fat,” explained Juleen R. Zierath, professor in the Department of Molecular Medicine and Surgery and the Department of Physiology. and Pharmacology at Sweden’s Karolinska Institutet, in a statement.

“If that’s the case, they could be valuable for overweight people,” she said.

However, there is a big caveat to this conclusion: the effect was only confirmed in mice. Zierath and his team performed RNA analysis of the rodents’ adipose tissue – that is, their body fat – after a bout of high-intensity exercise, looking for specific markers that showed an enhanced metabolic response.

Here’s the problem: The rats were divided into four groups, rationing their time on the small treadmill according to the schedule. This meant that one group of little ones would do the morning shift at the gym – an hour of intense exercise during the “onset of activity” phase of the rodent’s circadian rhythm – while another would complete the same workout later, during the “early rest” phase. ”. . In humans, these times correspond to late morning and late evening, respectively.

The other two groups of mice were much luckier, meanwhile, acting as lazy controls for their sporty brethren.

Although the exercise regimens were the same, the researchers found a big difference between the metabolisms of the former and latter groups of sport mice. Physical activity in the early active phase, the analysis revealed, resulted in increased expression of genes involved in the breakdown of adipose tissue, higher levels of heat production in the body, and more mitochondria in adipose tissue – all indications of a higher metabolism . to assess.

It wasn’t because of that other all-important daily rhythm – the one that lives in your guts – either. The effect held even when the mice were fasted: “Eating status is secondary to time of day as a regulator of exercise-responsive transcriptional activity in adipose tissue,” notes the paper.

So could these findings hold the key to increasing the efficiency of our own workouts? Well, there are a few things to keep in mind before we start drawing those kinds of conclusions. First, while mice and humans share many basic physiological functions – after all, there’s a reason they’re so widespread as lab animals – there’s a big difference when it comes to our circadian rhythms: namely, mice are nocturnal.

However, researchers believe there is enough evidence here to warrant further investigation in humans. Until then – well, if we happen to change our workout to a morning session instead of an evening session, that’s nobody’s business but us.

“Timing appears to be important for the body’s energy balance and for enhancing the health benefits of exercise,” Zierath concluded, “but further studies are needed to draw reliable conclusions about the relevance of our findings to humans. ”.

The results can be found in the journal Proceedings of the National Academy of Sciences.

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