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Late morning exercise may burn more fat

The correct timing of an exercise session may influence the extent of fat metabolism that takes place, as the body strives to keep glucose and energy levels in balance. This is what scientists have discovered in laboratory mice.  A new study from researchers at the Karolinska Institutet in Sweden, and the University of Copenhagen in Denmark, shows that mice that did exercise in an early active phase, which corresponds to morning exercise in humans, increased their metabolism more than mice that did exercise at a time in the day when they usually rest. 

Physiological processes in the bodies of animals are affected by circadian rhythms, the natural 24-hour cycles that respond primarily to input of light and darkness. These changes are mediated by molecular clocks that switch specific genes on or off in cells. A variety of factors including hormones, temperature, food-intake, and exercise can act on tissue-specific molecular clocks to alter the expression of genes that influence metabolism, all in a way that relates to the time of day. This system helps to anticipate and adapt an organism’s physiology and behavior to the activities that take place during different phases of the day. 

In order to find out whether the timing of exercise affects the metabolism of fat, the researchers studied the adipose tissue of mice after a session of high-intensity exercise performed at two different points of the daily cycle – when mice began to get active, and when they were in the early stages of resting. These times correspond to a late morning and a late evening exercise session, respectively, in humans. The researchers studied various markers of fat metabolism and analyzed which genes were active in adipose tissue after exercise.

The results, published in the journal PNAS, showed that certain fatty acids, which are the breakdown products of stored fats, were present at higher concentrations in the mice’s blood after early active phase exercise, but this was not the case after exercise during the early rest phase. This indicates that fat is metabolized as a source of energy more actively when exercise takes place during the early active phase, and that the metabolic response to exercise is dependent on the time of day that exercise takes place. 

The researchers also found that physical activity at an early active phase increased the expression of genes involved in the breakdown of adipose tissue, in thermogenesis (heat production) and in the proliferation of mitochondria (where energy for the exercise is generated) in the adipose tissue, all of which indicate a higher metabolic rate. These effects were observed only in mice that exercised in the early active phase and were independent of their 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,” said Professor Juleen Zierath from the Department of Molecular Medicine and Surgery and the Department of Physiology and Pharmacology at the Karolinska Institutet.

Although experimental results derived from mice may not always be directly applicable to humans, the two species do 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,” said Professor Zierath.

By Alison Bosman, Staff Writer

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