Depending on the time we exercise, different hormones, neurotransmitters, local transmitters and pheromones are produced, explains Juleen Zierath, a physiologist at the Karolinska Institute in Sweden. “[These] have a broad impact on health, influencing sleep, memory, exercise performance and metabolic homeostasis.”
This has led Zierath, and other researchers, to wonder whether they can harness this knowledge to optimise the benefits of exercise. And because the muscle clock is also involved in the activation of genes that regulate glucose and fat, several new studies have honed in on exercise timing as a therapeutic strategy in metabolic diseases, like type 2 diabetes.
“The best time of the day to exercise is when you can! It is important to establish a habit of daily exercise for your overall health and wellbeing,” stresses Zierath. “However, evidence… indicates that there are certain times of the day when one can ‘fine-tune’ the response to exercise.”
Earlier this year Zierath’s research found that, in mice, exercise in the morning led to “a more robust metabolic impact” than at night: The mice were better able to break down fats and use carbohydrates.
“This study suggests that the timing of exercise during the day may prove to be a valuable therapy for patients with metabolic disorders,” she says.
In a follow-up study, she looked at how exercising in the morning or afternoon affected men with type 2 diabetes.
While changes in body composition were similar, afternoon exercise was better at improving blood glucose levels while morning exercise seemed to be better for burning fat.
There could be many reasons for the difference, she says, including the intrinsic daily rhythm of our circadian clock, hormonal differences at different times of the day or whether someone has eaten before exercise, which is more likely in the afternoon than the morning.
While her lab attempts to tease out these effects, much remains unknown, says Karyn Esser, a professor in Physiology and Aging at the University of Florida, whose new research also explores the impact of exercise on circadian rhythm and whose review paper looks at the evidence around timing exercise and metabolic disorders.
“There is going to be enough variation among humans and possibly the types of exercise that making blanket statements that one time or other is best for everyone will not be correct,” Esser says. “What we do know is that exercise at [night] would have different metabolomic outcomes than if performed in the middle of the day/active period. Is this a risk? Unlikely, is my best guess for the healthy subjects … but could it increase risk for people with metabolic disease? We don’t know.”
Dr David Mizrahi, a research fellow at The Daffodil Centre, The University of Sydney, is circumspect.
“Although the findings showing ‘precision’ or personalised exercise programs based on metabolic health may optimise the benefits, I am cautioning putting all the focus on this message and instead promoting that any exercise, and at any time, is better than none for the general population.”
Regular exercise at any time of the day makes tissues more insulin sensitive, adds Zierath, which benefits everyone including those with metabolic disorders. As a prescription, however, there may be time-of-day benefits.
“In response to afternoon exercise, people with type 2 diabetes might achieve better nocturnal glucose control,” she says. “There is also some evidence that in the middle-aged men and women, there may be greater gains in strength and aerobic fitness in response to afternoon exercise.”
With more people living with metabolic disorders – the number of Australians living with diabetes almost tripled in the last 20 years and about 90 per cent of those cases are type 2 diabetes – understanding how tweaks to diet and exercise can help is an increasing focus.
“We are still at the early stages of learning the potential benefits (and maybe risks) with exercising at different times of the day,” Esser says. “In general, exercise when you can as we know it is good for us… But timing can matter in terms of health and performance.”
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