The current global obesity pandemic is partially the result of a lack of physical activity combined with sedentary behavior (prolonged sitting) during the day. Such behavior is linked to an increased risk of developing metabolic diseases including type 2 diabetes (T2D), while earlier research has found that short breaks in sedentary behavior are associated with an improved cardiometabolic profile. This evidence is further supported by experimental studies showing that frequent interruptions of extended sitting with standing or light physical activity resulted in lower triacylglycerol levels and reduced 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.
As well as the importance of the duration of sedentary periods, it has been argued that the timing of physical activity throughout the day may be a factor in metabolic health. In-vitro studies and research in animals have revealed daytime-dependent changes in exercise capacity as well as associated metabolic risk markers, however few such investigations have been performed in humans and their results are inconsistent.
A new study finds that afternoon or evening physical activity is associated with reduced insulin resistance
The researchers used data from the Netherlands Epidemiology of Obesity (NEO) study, a population-based prospective cohort study designed to investigate processes involved in the development of obesity-related diseases. Study participants were recruited between 2008 and 2012 with men and women living in the greater Leiden area being invited to participate if they were aged between 45 and 65 years and had a self-reported body mass index (BMI) of 27kg/m2 or higher.
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. Measurements of acceleration and heart rate were used to estimate physical activity energy expenditure (PAEE, measured in kJ/kg/day), which in turn allowed the team to determine time spent at different activity intensities. These were expressed as metabolic equivalents of task (MET).
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.
The researchers observed that higher total PAEE and particularly MVPA were associated with both reduced liver fat content and reduced insulin resistance.
There was no significant difference in insulin resistance between morning activity and activity spread evenly over the day.
In addition, muscular strength as well as the metabolic function of skeletal muscle cells show a peak in the late afternoon, suggesting that being most active during this period may result in a more pronounced metabolic response than activity earlier in the day.
These results suggest that timing of physical activity throughout the day is relevant for the beneficial effects of physical activity on inulin sensitivity. Further studies should assess whether timing of physical activity is indeed important for the occurrence of type 2 diabetes.
Jeroen H. P. M. van der Velde, Sebastiaan C. Boone, Esther Winters-van Eekelen, Matthijs K. C. Hesselink, Vera B. Schrauwen-Hinderling, Patrick Schrauwen, Hildo J. Lamb, Frits R. Rosendaal, Renée de Mutsert (November 1, 2022). Timing of physical activity in relation to liver fat content and insulin resistance. Diabetologia. Retrieved from: https://link.springer.com/article/10.1007/s00125-022-05813-3
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