Physical activity later in the day may reduce type 2 diabetes risk
A new study reported in Diabetologia looks at how breaks in sedentary behaviour and timing of physical activity affect liver fat and insulin resistance. The findings could inform lifestyle interventions to prevent type 2 diabetes. Dr Susan Aldridge reports.
Sedentary behaviour has been linked with an increased risk of cardiometabolic diseases, including type 2 diabetes. Several studies have shown that interrupting prolonged sitting with standing or light activity improves glycaemic responses and lipid levels. Meanwhile, moderate-to-vigorous-physical activity (MVPA) is associated with reduced liver fat, improved insulin sensitivity and reduced type 2 diabetes risk.
The timing of physical activity may also have an impact on metabolic health.
People at risk of type 2 diabetes are often told to move more to help prevent or delay the onset of the condition. If breaking up sedentary time and planning the timing of physical activity do have an influence, incorporating this information into lifestyle intervention could help clarify goals, increase motivation and hopefully increase the success of type 2 diabetes prevention efforts.
Accordingly, Jeroen van der Velde of Leiden University Medical Center in The Netherlands, and colleagues have looked at whether breaks in sedentary time and the timing of physical activity influence liver fat content and insulin resistance in a group of people taking part in an obesity study.
Measuring sedentary time and physical activity
The researchers studied 775 participants drawn from the Netherlands Epidemiology of Obesity study, involving men and women aged 45 to 65 with a BMI of 27kg/m2 or more. This is a population-based prospective study looking at the pathways leading to obesity-related diseases.
Daily levels of activity, including sedentary time, were measured with a combined acceleration and heart-rate monitor. The participants wore these for four consecutive days and nights and carried on with their usual activities, so this was very much a study of everyday life without any particular lifestyle intervention. Levels of activity were defined as sedentary time, light physical activity (LPA) and MVPA. For timing of physical activity, morning was 06.00 to 12.00 hours, afternoon 12.00 to 18.00 hours and evening 18.00 to 00.00 hours. Blood samples were used to measure insulin resistance, while a subgroup of participants had their liver fat measured by MRI.
Sedentary time and breaks
This study found that neither the length of sedentary time nor the number of breaks were associated with a reduction in liver fat or insulin resistance, contrary to what might have been expected because of previous research. So why were no benefits observed here? The researchers suggest that experimental studies are usually set up to compare regular breaks in sedentary time with continuous sitting. This situation is not usual in the free-living situation that was being monitored in the present study. Another explanation could be that in this free-living situation, the intensity of the activity carried out in the breaks was too light to elicit any improvements in liver fat or insulin resistance.
Surprisingly, the number of activity breaks in sedentary time was even associated with an increase in liver fat – the opposite of what might have been expected. The researchers suggest that, since there wasn’t a corresponding increase in insulin resistance, the increase in liver fat might have occurred by chance, given that the sample size was relatively small, at 206 participants.
These findings add to the existing debate over the value of taking breaks during sedentary time. It may be that a longer trial – this study only lasted for four days – would reveal some benefit. And maybe boosting the intensity of the break activity would help too.
Timing of physical activity
As you might expect, higher overall physical energy expenditure and MVPA were both associated with reduced liver fat and insulin resistance, whereas no effect was seen for LPA. Timing of exercise is a relatively unexplored area in obesity and diabetes. In this study, MVPA in the afternoon and evening was associated with up to 25% reduced insulin resistance compared with MVPA in the morning or distributed evenly throughout the day. However, there was no impact of MVPA timing on liver fat.
These findings are supported by earlier research showing that, in men with type 2 diabetes, high-intensity exercise in the afternoon improved blood glucose more than high-intensity exercise in the morning. Another study showed that aerobic exercise in the evening, but not in the morning, decreased blood pressure in hypertensive men. And an analysis of data from the Look AHEAD trial of lifestyle intervention in people with type 2 diabetes showed that those doing MVPA in the morning had an increased risk of cardiovascular disease compared with those doing MVPA later in the day.
Taken together, all of these findings suggest benefit on metabolic health to physical activity later, rather than earlier, in the day. The underlying mechanisms remain unclear, however. It is possible that physical activity could activate body clock genes, enhancing our circadian rhythms, and this, in turn, improves metabolic factors like insulin resistance. This is supported by two earlier studies showing that metabolic responses vary with time of day and are regulated by clock genes. Furthermore, muscular strength and the function of mitochondria in skeletal muscle peak in early afternoon, which would also help maximise the impact of exercise.
What does this mean for type 2 diabetes prevention?
This study did not show any metabolic benefit in taking breaks from sedentary behaviour. That was not surprising, given that it was short in duration and involved participants doing their usual activities. It would be interesting to see the impact of taking more structured and intense exercise breaks, such as running on the spot or doing squats and press-ups during the non-sedentary time. And a longer trial of weeks or months monitoring sedentary time and breaks may provide new insights.
The impact of timing of physical activity on insulin resistance is particularly interesting. Further studies are needed but it may be that advising people to be more active in the afternoon and evening, rather than in the morning, would be more effective in terms of type 2 diabetes prevention.
To read this paper, go to: van der Velde J, Boone S, Winters-van Eekelen E, Hesselink M, Schrauwen-Hinderling V, Schrauwen P, Lamb H, Rosendaal F, de Mutsert R. Timing of physical activity in relation to liver fat content and insulin resistance. Diabetologia online 1 November 2022.
Any opinions expressed in this article are the responsibility of the EASD e-Learning Programme Director, Dr Eleanor D Kennedy.