The data come from 775 participants with a mean body mass index (BMI) of 26.2 kg/m2 in the observational Netherlands Epidemiology of Obesity (NEO) study. Use of activity monitors for four consecutive days showed that performance of MVPA (defined as activity with intensity of > 3 metabolic equivalents of task) in the afternoon or evening was associated with up to 25% reduced insulin resistance compared with an even distribution of activity during the day.
“This is one of the first studies where in humans the relation between timing of physical activity and insulin resistance was examined,” lead author Jeroen van der Velde of the department of clinical epidemiology, Leiden (the Netherlands) University Medical Center, said in an interview.
Moreover, he noted that, while previous intervention studies have shown greater blood glucose reduction with high-intensity exercise performed in the afternoon, compared with the morning, in people with impaired glucose metabolism or type 2 diabetes, “as far as I am aware, we were the first to use a population-based study in a general population to study this.”
Katarina Kos, MD, PhD, senior lecturer in diabetes and obesity, University of Exeter (England), said: “This study is novel in that it relates the timing of physical activity if performed in the morning, afternoon, or evening to insulin resistance and fat content. This is from a cohort of middle-aged Dutch people between ages 45-65 studied 10 years ago and based on self-reports of weight and eating behavior and who were found to be generally overweight.”
Is it down to circadian rhythm?
“The results are of interest in that if the chosen timing was in the afternoon [63% of studied population] or evening (8% of the studied population), it seemed to relate with improved metabolism when compared to the morning exercising [16% of population]. ... Whether this was due to the (timing) of activity is yet to be shown,” Dr. Kos told the UK Science Media Centre.
Mr. van der Velde agrees that the effect may be explained at least in part by the circadian rhythm of the body. “Physical activity may act as ... a cue for the activation of clock genes. Previous research has suggested that our body’s muscular system and oxidative system are also affected by our circadian rhythm and their peak activity seems to be in the late afternoon. So, being mostly active in this time period ... may elicit greater metabolic responses compared to being active in the morning.”
But, he cautioned, “I think it is important to realize that we are just beginning to understand the potential impact of physical activity timing. At this stage, I believe it is most important to be physically active in general. So ... if the morning is the only time of the day to go for a walk or a run, certainly do this.”
Dr. Kos concurred: “As this is not an intervention study, further research is needed to explain the cause of the observed association.”
Mr. van der Velde also added that it’s not yet clear which individuals or subgroups might experience additional benefits from timed activities. That’s the current research focus of a large consortium of several research institutes in the Netherlands and Canada.