How exercise affects the metabolism of fat tissue can depend on the time of day. A researcher says that over a lifetime this could mean the difference between being slim or not.
People live their lives in a carefully choreographed biological ballet regulated by the circadian rhythm, which fine tunes our biology and regulates our sleep, food intake, body temperature and hormonal fluctuations.
New research now shows that the circadian rhythm also influences how exercise affects the metabolism of fat tissue in mice.
Exercise at certain times of the day thus differentially influences the metabolism of fat tissue. The timing of exercise can therefore be important in regulating weight.
“Our results indicate that the fat cells are more susceptible to the effect of exercise at some times of the day rather than at others. At these times, the fat cells activate the metabolism of fat tissue and, over a lifetime, the timing of exercise may mean the difference between being slim or not,” explains Juleen R. Zierath, Professor, Karolinska Institutet, Stockholm, Sweden and Executive Director, Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen.
The research has been published in the Proceedings of the National Academy of Sciences of the United States of America.
Mice exercised at different times
The researchers wanted to map how exercise affects the metabolic rhythm and therefore induced mice to run on a rodent treadmill at different times of the (24-hour) day.
Some mice ran in the early active phase – 3 hours into their active period (at night), which for a person would correspond to about 10:00 in the morning.
Other mice ran on a treadmill 3 hours into their resting phase, which corresponds to 1:00 in the morning for a person.
The mice had to either run on the treadmill or just rest on the treadmill, and the researchers then investigated how the exercise and the time of day altered the postexercise metabolism of their fat tissue.
“We often think that exercise affects the muscles and lungs, but it also affects the fat tissue. In this study, we examined how exercise affects the gene expression in all fat tissue depots,” says Juleen R. Zierath.
Time of day influences the effect on fat tissue
The researchers found that exercise in the early active phase had by far the greatest effect on gene expression in the fat cells, activating the genes related to mitochondrial function, energy homeostasis and thermogenesis.
According to Juleen R. Zierath, this indicates that the fat cells are more active in exercise in the early active phase than in the early resting phase, which affects how much fat can be burned.
“We were surprised because we did not expect to see a difference. You could argue that the fat cells would not be affected by when you exercise, as long as you exercise, but it does not seem that way. We found that exercise during the active phase led to gene activity that was twice as high in relevant genes in the fat cells,” explains Juleen R. Zierath.
Fasting not the cause
To validate the conclusions, the researchers also investigated whether the result was the effect of the mice having just come out of the resting phase and thus not having eaten that much yet. They hypothesised that fasting could be the cause.
The researchers therefore carried out another exercise-related experiment to examine differences in the expression of genes between mice that were full and mice that were fasting.
The researchers speculated that fasting mice would metabolise more fat in the fat cells if fasting was the trigger but found no difference.
“This leads us to conclude that something about exercise in the early active phase initiates activity in the fat cells. We conclude that the fat cells are more influenced by hormone-signalling processes during this period of the day. The fat cells may be better prepared to be stimulated and increase their potential metabolism during the active phase,” says Juleen R. Zierath.
Also relevant for humans
Juleen R. Zierath emphasises that the experiment was only carried out on mice and that translating the results directly to people is difficult. Nevertheless, she thinks that, even among people, the timing of exercise could be important for how activated the fat cells are and thus the amount of fat burned.
The UK Biobank investigated the effect of exercise at different times of the day. This study showed that exercise between 9:00 and 17:00 is associated with a longer life compared with exercise before 9:00 and after 17:00. The result was most striking among untrained men.
“Overall, the results indicate that exercise at times of the day when the body’s cells and fat tissue are most susceptible to the effect of exercise may provide an additional benefit. Over a lifetime, this can mean that you may have better weight management, which provides various health benefits,” concludes Juleen R. Zierath, emphasising, however, that exercising is healthy regardless of the time of day.
