Source: MedicalNewsToday.com, written by: Corrie Pelc
Exercise plays an important role in overall health.
Not only can it help with weight loss, but previous studies also show physical activity can help lower a person’s risk for diseases like diabetes, cardiovascular disease, stroke, osteoporosis, and high blood pressure.
Although people know they should work out, much debate exists over what is the best time of day to exercise for the most benefits.
For example, one study in October 2021 found exercising in the morning may be more beneficial for people with obesity, while another study in September 2021 found evening workouts more effective for overweight men.
Now, researchers from the Karolinska Institutet in Sweden found that exercising in the morning helps increase fat burn compared to working out in the evening in models of mice.
The study was recently published in the journal PNAS.
Why might the time of day matter?
According to Dr. Juleen R. Zierath, a professor in the Department of Molecular Medicine and Surgery and the Department of Physiology and Pharmacology at the Karolinska Institutet in Sweden, and the lead author of this study, many of our internal rhythms are governed by an intrinsic molecular clocke.
“This clock coordinates a great deal of our physiology, including body temperature, blood pressure response, hormone release, and even strength and alertness,” she explained to Medical News Today.
“Therefore, if we can align what we do — for example, our daily exercise routine — with this clock, we may be able to fine-tune the health benefits,” she added.
Measuring adipose (fat) tissue
For this study, Dr. Zierath and her team examined the adipose tissue of mice after they had a session of high intensity exercise at one of two times during the day — a late morning session or a late evening session.
Upon analysis, researchers found that early exercise increased the expression of genes involved in the breakdown of adipose tissue and some other functions associated with a higher metabolic rate.
“The early active phase exercise increased the expression of genes related to fat burning, heat production, and blood vessel production,” Dr. Zierath said.
“These changes occurred even when levels of exercise-induced hormones were similar between phases, meaning that the sensitivity of body fat to exercise-induced hormones may differ depending on the time of day when the physical activity is performed,” she explained.
Scientists only found these signs of increased metabolism in the mice that exercised in the morning.
“We were surprised that early active phase exercise — corresponding to late morning exercise in humans — had such a profound impact on boosting metabolism in fat cells. Initially, we thought this time-of-day-dependent effect was because of nutritional status.”
— Dr. Juleen R. Zierath
“However, we later confirmed that the gene signature of a higher metabolic rate was not affected by food intake,” she said.
Next steps: Trials in humans
When asked what the next steps for this research would be, Dr. Zierath said the limitation of the current study was that the work was only performed in healthy mice.
“Our next step is to perform a similar study in men and women with [healthy] weight or obesity. It is important to understand if our findings can be applied to fine-tune the well-known benefits of exercise on metabolism,” she continued.
“The right time seems to be important to the body’s energy balance and to improving the health benefits to exercise, but more studies are needed to draw any reliable conclusions about the relevance of our findings to humans.”
— Dr. Juleen R. Zierath
No scientific agreement on ‘best time’
Medical News Today also spoke with Ryan Glatt, a senior brain health coach and director of the FitBrain Program at Pacific Neuroscience Institute in Santa Monica, California, about this study.
He agreed that the time of day might affect how physical activity impacts the body based on individual circadian rhythms — or biological clocks based upon time of day — and the various changes in hormones that may normally fluctuate throughout the day in relation to the endocrine effects of exercise.
“There is no current scientific agreement on the ‘best time of day’ for exercise, as what is likely more important is what is most behaviorally sound for that particular individual. It is also unlikely that (the) time of day to perform exercise outperforms the concept of ‘calories in versus calories out’ for overall fat loss.”
— Ryan Glatt, senior brain health coach
For the next steps in this research, Glatt said he would like to see this type of study conducted in humans, as there is a difference in mouse vs. human physiology.
“Prior studies have conducted such studies in humans for outcomes such as gaining muscle, but the differences among the times of day and the outcomes may not be as significant as some might expect. Unique variables, such as hormonal profile, circadian rhythm, and behavioral preferences, may mediate the interactions of exercise and time of day,” he explained.
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