Does exercise fat burning depend on the time of day?

Does exercise fat burning depend on the time of day?

At least in mice, “early” exercise leads to favorable activity in fat cells. © Mary Swift/iStock

When it comes to training, timing may play a role: a study on mice suggests that fat metabolism could be boosted particularly effectively at certain times of the day. If the rodents exercised in their early activity phase - which corresponds to morning exercise in humans - they activated their fat burning more than comparison animals that were put on the exercise bike late. The researchers emphasize that the extent to which the results can be transferred to humans must first be shown by further studies.

Curves with negative effects on health: Many studies show that excess body fat is linked to cardiovascular diseases and metabolic disorders. In addition to reduced food intake, physical activity is known to have a beneficial effect on the metabolism and helps to break down problematic fat deposits. Studies have already shown that training not only leads to increased energy consumption and muscle building: certain programs are also activated in fat cells, which lead to a reduction in their depots. As a result, they become smaller and show improved reactions and functions in the context of metabolism.

A Swedish-Danish team of researchers has now investigated whether the timing of exercise during the day plays a role in these beneficial effects on fat cells. Because it is known from previous studies that the body – and also certain cells – show different reactions to different phases of the daily rhythm. This is because the biological clock system regulates certain genes differently as part of the 24-hour day-night cycle. There are already some indications that disruptions to this system, for example due to shift work, have an unfavorable effect on the human metabolism.

Mice run for science

In order to gain evidence of a connection between the training effect and time, the researchers first turned to the mouse as a model for humans. They had their test animals train intensively on a running bike for one hour during different times of their daily rhythm: some ran in their early active phase and others in the run-up to their typical resting phase. Applied to humans, this would correspond to training in the morning or late in the evening, the scientists explain. After the physical activity, the scientists examined various markers for the metabolism in the test animals and analyzed which genes were active in the adipose tissue.

They report that after the early training in the mice, they found increased levels of certain fatty acids in the blood, which indicate an increased metabolism in the adipose tissue. This was not the case with the “late athletes”. The investigations of gene expression in the fat tissue of the animals underpinned this result: the researchers found a comparatively increased activity of hereditary traits in the animals after the early training, which is associated with increased energy expenditure. The results were not influenced by the diet of the animals, as the researchers were able to show by using groups that were fed differently.

Can training effects be optimized?

Studies of fat cell cultures in the laboratory also provided further indications: they also showed daily activity patterns that could be associated with the breakdown of fat, the team reports. The results thus indicate that, at least in the mouse, the response of adipose tissue to physical activity is time-of-day dependent and may be controlled by the circadian clock, the scientists say. If the results were directly extrapolated to humans, the results would mean: “Late morning exercise may boost metabolism and fat burning more effectively than late evening exercise. So if this is indeed the case, this information could be useful for people who are overweight,” says senior author Juleen Zierath from the Karolinska Institute in Stockholm.

However, the researchers emphasize that further research is now needed to confirm such effects. Because although humans and mice have many physiological functions in common, the results cannot necessarily be transferred. Because, among other things, the rodents have an activity pattern that differs from ours. "Proper timing appears to be important for the body's energy balance and for enhancing the health benefits of exercise, but more studies are needed to draw firm conclusions about the relevance of our findings to humans," concludes Zierath.

Source: Karolinska Institute, professional article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2218510120

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