Time of day may determine the amount of fat burned by exercise

Physical activity at different times of the day can affect the body in different ways since the biological processes depend on the circadian rhythms of the cells. To ascertain how the time of day at which exercise is done affects the burning of fat, researchers at Karolinska Institutet and the University of Copenhagen studied the adipose tissue of mice after a session of high-intensity exercise performed at two points of the daily cycle, an early active phase and early rest phase (corresponding to a late morning and late evening session, respectively, in humans). The researchers studied various markers for fat metabolism and analysed which genes were active in adipose tissue after exercise.

Independent of food intake

The researchers found that physical activity at an early active phase increased the expression of genes involved in the breakdown of adipose tissue, thermogenesis (heat production) and mitochondria in the adipose tissue, indicating a higher metabolic rate. These effects were observed only in mice that exercised in the early active phase and were independent of food intake.

"Our results suggest that late morning exercise could be more effective than late evening exercise in terms of boosting the metabolism and the burning of fat, and if this is the case, they could prove of value to people who are overweight," says Professor Juleen R. Zierath from the Department of Molecular Medicine and Surgery and the Department of Physiology and Pharmacology, Karolinska Institutet.

Improve the health benefits of exercise

Mice and humans share many basic physiological functions, and mice are a well-established model for human physiology and metabolism. However, there are also important differences, such as the fact that mice are nocturnal.

"The right timing seems to be important to the body's energy balance and to improving the health benefits of exercise, but more studies are needed to draw any reliable conclusions about the relevance of our findings to humans," says Professor Zierath.

The study was conducted in close collaboration with the Novo Nordisk Foundation Center for Basic Metabolic Research (CBMR) at the University of Copenhagen and was financed by the Novo Nordisk Foundation, Novo Nordisk, the Swedish Diabetes Foundation, the Swedish Research Council and Karolinska Institutet.