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When You Exercise Might Determine the Amount of Fat You Burn

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Researchers from the Karolinska Institute and the University of Copenhagen found that the timing of exercise may fine-tune fat metabolism in mice. The study is published in the Proceedings of the National Academy of Science (PNAS).

Exercise timing affects the body’s response

The time of day in which we exercise can impact the body’s molecular response to physical activity due to the varying circadian rhythm of cells. Scientists led by Professor Juleen Zierath, group leader at the Department of Physiology and Pharmacology at the Karolinska Institute, have explored how exercising at different times of the day can affect the body’s burning of adipose tissue (fat).


The study was conducted in mice, an established laboratory model for studying human physiology and metabolism due to shared characteristics among the two species.

Early active phase exercise increases expression of genes involved in fat breakdown

Zierath and colleagues subjected mice to a session of high-intensity exercise performed at two points in their daily cycle – the early active phase and the early rest phase (corresponding to the late morning and late evening for humans).

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“For the exercise intervention, 10- to 11-wk-old mice were separated into sham- or exercise- treatment at the early rest (ZT3) or early active (ZT15) phase. Exercised mice were exposed to a 1-h exercise bout (ZT3 with lights on; ZT15 in the dark with the use of a red-light lamp) while sham-exercise counterparts were placed on an artificial treadmill for 1 h,” the authors write.


What is a sham treatment?

Sham and placebo treatments are used by scientists to determine the effectiveness of a drug or an intervention. In sham treatments, the subject undergoes the experiment methodology, without performing or receiving the intervention.


To study changes in adipose tissue elicited by exercise, Zierath and team used RNA sequencing – a method that provides insight into a cell’s transcriptome – to explore gene activity and analyzed biomarkers of fat metabolism, including serum non-esterified fatty acids (NEFA), serum catecholamines (adrenaline and noradrenaline), serum corticosterone, serum triglycerides, serum insulin and liver and gastrocnemius glycogen content. 


“Only during the early active phase did exercise elicit an immediate increase in serum NEFA. Furthermore, early active phase exercise increased expression of markers of thermogenesis and mitochondrial proliferation in inguinal adipose tissue,” Zierath and colleagues say. These effects were independent of how much food the mice had consumed.


While mice models are commonly used in research exploring human metabolism, they are nocturnal, which must be considered. “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 Zierath.


Reference: Pendergrast LA, Lundell LS, Ehrlich AM, et al. Time of day determines postexercise metabolism in mouse adipose tissue. PNAS. 2023;120(8):e2218510120. doi: 10.1073/pnas.2218510120.


This article is a rework of a press release issued by the Karolinska Institute. Material has been edited for length and content.

Meet the Author
Molly Campbell
Molly Campbell
Senior Science Writer
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