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

Physical activity at the right time of the day seems able to increase fat metabolism, at least in mice. A new study from 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 when they usually rest. The results are published in the journal PNAS.

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.

RNA sequencing results from iWAT collected immediately postexercise or sham intervention during the early rest or early active phase.

(A) Upset plot of altered transcripts at 0 h post exercise at the early rest and early active phases. (B) Differential expression of logFC between early active and early rest phase sedentary (y; green dots) and exercised (x; red dots) groups. Common transcripts between groups are shown as blue dots. (C) Differential expression of logFC between early rest and early active (x; pink dots) phase exercise effects. (D) Gene ontology of transcripts differentially altered within the early active exercise group (versus Sed). (E) Target gene enrichment analysis comparing early rest phase exercise, early active phase exercise, time of day effect exercise, and time of day effect sedentary with public RNAseq data sets from dexamethasone or iso-stimulated 3T3 mouse adipocytes and (CL316) adrenergic-agonist-treated mice. (F) Heatmap of stress-responsive and thermogenesis-associated transcripts from iWAT (FDR < 0.05).

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.

SourceKarolinska Institutet

Pendergrast LA, Lundell LS, Ehrlich AM, Ashcroft SP, Schönke M, Basse AL, Krook A, Treebak JT, Dollet L, Zierath JR. (2023) Time of day determines postexercise metabolism in mouse adipose tissue. PNAS 120(8):e2218510120. [article]

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