Diabetes Mellitus is a growing health concern that is expected to affect 366 million people globally by 2030. Multiple approaches to addressing this health concern have been documented, with increased habitual physical activity being one of them. Several mechanisms have been demonstrated to improve cellular insulin sensitivity. The interaction between insulin sensitivity and insulin resistance is important in the development and maintenance of the diabetic state, which is directly related to levels of physical activity.
When compared to an even distribution of physical activity throughout the day, a new study published in Diabetologia (the journal of the European Association for the Study of Diabetes [EASD]) finds that afternoon or evening physical activity is associated with reduced insulin resistance (and thus better blood sugar control). Dr. Jeroen van der Velde and colleagues from the Department of Clinical Epidemiology at Leiden University Medical Center in Leiden, the Netherlands, concluded that morning physical activity provided no benefits.
The current global obesity pandemic is caused in part by a lack of physical activity combined with sedentary behavior (long periods of sitting) during the day. This type of behavior has been linked to an increased risk of developing metabolic diseases such as type 2 diabetes (T2D), whereas previous research has found that short breaks in sedentary behavior are associated with an improved cardiometabolic profile. Experiment studies show that frequent interruptions of prolonged sitting with standing or light physical activity result in lower triacylglycerol levels and lower blood glucose, indicating an improved blood sugar profile.
High fasting serum triacylglycerol levels may be linked to higher concentrations of fat in the liver, which in turn is strongly associated with insulin resistance. Previous studies have demonstrated that exercise is linked to reduced liver fat and improved insulin sensitivity.The authors hypothesised that taking breaks from sedentary behaviour may reduce liver fat, resulting in decreased insulin resistance and ultimately preventing T2D.
Physical activity timing is a relatively unexplored field in human biology, and the mechanisms underlying the potential benefits of physical activity timing remain unknown. Earlier research found that metabolic responses to high-intensity exercise differed depending on the time of day.
In addition to the duration of sedentary periods, it has been suggested that the timing of physical activity throughout the day may play a role in metabolic health. In-vitro studies and animal research have revealed daytime-dependent changes in exercise capacity as well as associated metabolic risk markers, but few such studies have been conducted in humans, and the results have been inconsistent. The researchers wanted to look into the relationships between the timing of physical activity and breaks in sedentary time and liver fat content and insulin resistance in a middle-aged population.
The data came from the Netherlands Epidemiology of Obesity (NEO) study, a population-based prospective cohort study designed to look into the processes that lead to the development of obesity-related diseases. Between 2008 and 2012, men and women living in the greater Leiden area were invited to participate if they were 45 to 65 years old and had a self-reported body mass index (BMI) of 27kg/m2 or higher. Invitations were also sent to all residents aged 45 to 65 from one municipality in the region to serve as a reference population with a BMI representative of the general Dutch population, resulting in a study population of 6,671 people.
Participants underwent a physical examination during which blood samples were taken to measure fasting and postprandial (after meal) blood glucose and insulin levels, while demographic, lifestyle and clinical information were obtained via questionnaire. They were also screened for suitability for an MRI scan, and roughly 35% of those able to undergo the procedure were randomly selected to have their liver fat content measured using this technique.
A further random subsample of 955 participants were given a combined accelerometer and heart rate monitor to wear for four consecutive days and nights to monitor movement and activity. Acceleration and heart rate measurements were used to estimate physical activity energy expenditure (PAEE, measured in kJ/kg/day), allowing the team to determine time spent at various activity intensities. These were expressed as metabolic equivalents of task (MET), which is a ratio of PAEE during an activity to that while resting (and a standard method for measuring physical activity).
Sedentary periods (excluding sleep) were defined as being ≤1.5 MET, while a break in sedentary time was indicated by a spell of activity with accelerations >0.75 m/s2 (as such accelerations have been established by previous research as an accurate indicator of breaking up sedentary time). An intensity of more than 1.5 MET up to 3 MET was defined as light physical activity (LPA), with still higher intensities classed as MVPA.
The day was divided into three blocks: morning (06:00-12:00); afternoon (12:00-18:00); and evening (18:00-24:00), with the proportion of total daily MVPA occurring in each revealing the most active period. If the share of MVPA in each block differed from the others by less than 5%, then it was classified as being an even distribution of activity throughout the day.
This study is based on analysis of results obtained from those 775 participants for whom complete data sets were available. The group were 42% male and 58% female, had an average age of 56 years and average BMI of 26.2 kg/m2. After adjusting for variables such as age, sex, ethnicity and total body fat, the researchers observed that higher total PAEE and particularly MVPA were associated with both reduced liver fat content and reduced insulin resistance. An association was also found between insulin resistance and the timing of MVPA during the day: performing MVPA in the afternoon or evening was linked to reduced insulin resistance, by 18% and 25% respectively, compared to an even distribution of activity throughout the day, even after adjustment for total amount of MVPA. There was no significant difference in insulin resistance between morning activity and activity spread evenly over the day.
There was no link found between the amount of sedentary time or the number of breaks in sedentary behavior and liver fat content or insulin resistance. The authors propose: “It’s possible that the intensity of the activity during the breaks in our study was too low to elicit metabolic responses. Most daily activities are of low intensity, and because we found no link between LPA and insulin resistance, this could explain the lack of a link between breaks and insulin resistance.”
Physical activity timing is a relatively unexplored field in human biology, and the mechanisms underlying the potential benefits of physical activity timing remain unknown. Earlier research found that metabolic responses to high-intensity exercise differed depending on the time of day. Furthermore, both muscular strength and metabolic function of skeletal muscle cells peak in the late afternoon, implying that being most active during this time period may result in a more pronounced metabolic response than activity earlier in the day.
The researchers reach a conclusion “In addition to total daily MVPA, timing of MVPA during the day was associated with reduced insulin resistance: performing the majority of MVPA in the afternoon or evening was associated with up to 25% lower insulin resistance compared to an even distribution of MVPA during the day. These findings imply that the timing of physical activity during the day is important for the beneficial effects of physical activity on inulin sensitivity. Further research should be conducted to determine whether the timing of physical activity is indeed important in the development of type 2 diabetes.”
