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Morning exercise burns more body fat, mouse study shows

A man wearing workout clothes goes for a walk or jog in a sunny park.
Mid-morning workouts may just have slightly more benefit than evening ones. BELL KA PANG/ Shutterstock

When you’ve got a busy schedule, you may try to squeeze exercise in whenever you can. But there’s growing evidence that when you workout can make a difference to its effects. Our latest research shows that body fat reacts to exercise differently depending on the time of day – at least in mice.

Our latest study found that when mice did a single exercise session in the late morning (around three hours after waking up), they burned more adipose tissue (body fat) compared to mice who exercised in the late evening. Along with this, we saw changes in the way adipose tissue genes respond following exercise.

Morning exercise is more effective

Using mice to gather evidence about what happens in our own physiology is common because almost every set of genes in humans has a closely related form in mice. This makes it likely that the effects we observe in mice may be similar to what we’d expect to see in humans.

To conduct our study, we had mice engage in an intense exercise session either during their active phase (the equivalent of our late morning) or rest phase (the equivalent of our late evening). We also took samples of adipose tissue every four hours after workouts for a total of 20 hours after exercise to understand the effect of the different timings.

Fatty acids are released into the blood by adipose tissue, which provides the body with energy to use during exercise, while reducing the size of fat cells.

Mice that exercised in the late morning showed increased levels of fatty acids in their blood directly after exercise, and at 12 hours following exercise. But late-evening exercisers showed no such changes. This indicated late-morning exercisers experienced twice as much fat breakdown.

Even more interestingly, the unique timing benefits of exercise were also seen in the way the genes worked. Late-morning exercise increasing genes were related to fat burning (breakdown), heat production (energy use) and blood vessel production. All of these changes are beneficial for fat loss and blood sugar regulation in the body, which may in turn benefit body weight and health.

This research agrees with two previous studies, which have shown that exercise timing matters for many tissues – such as muscle and the liver. Recent work in humans has also suggested that training at different times of the day can affect how the body responds to insulin, a hormone important for regulating blood sugar and therefore body weight.

The body clock

One key reason for the way the body responds to exercise depending on the time of day may be the biological circadian clock.

Every cell in the body has a circadian clock that coordinates metabolism with changes to light, nutrition, and movement throughout the day. Hormones, body temperature and even sensitivity to sounds are all influenced by the body’s clock.

A brain with a clock on it to illustrate our circadian rhythm.
The circadian rhythm regulates our body’s processes. vetre/ Shutterstock

Body fat is no different: it has its own clock, which regulates the expression of many genes. Based on our findings and the findings of other studies, we can speculate that exercise timing may interact with the circadian clock to program fat cells for better fat burning.

The circadian rhythm also controls when food is eaten by making us hungry during daylight hours. This is so that our body has the energy it needs throughout the day to perform all of its functions.

So when we exercise, the body draws upon the foods we’ve recently eaten for energy. However, if there’s no food, it has to mainly use fatty acids released from adipose tissue for energy.

Because of this, we were interested to know if the benefits of late morning exercise were due to exercising with an empty stomach. We had the mice in the late evening group perform exercise in a fasted state, to see if we could make them respond to exercise as well as the morning mice did.

We found that fasted evening exercise did increase signs of body fat breakdown in the blood. However, there were no signs of any gene changes. This surprising finding suggests that the circadian clock may fine-tune how the body reacts to exercise – overriding the effects of meal timing.

When should I exercise?

Our study was only conducted on mice and for only a single exercise session, so it’s difficult to generalise the best time of exercise for people. More research will be needed to know how exercise timing affects fat burning in humans, and if our findings are consistent over many exercise sessions. This is the next step of our research at Karolinska Institute.

In the meantime, the best general advice right now is to be active and engage in exercise – regardless of the time you’re able to workout. Exercise is still a great way of reducing excess body fat, which may help to lower your risk of certain diseases such as heart disease and type 2 diabetes. But if you are able to squeeze your workouts in mid-morning, it may potentially help you to burn more fat.

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