The impact of postprandial exercise on blood glucose in type 1 diabetes
A new review, reported in Diabetologia, looks at the glycaemic impact of different forms of exercise performed after a meal in people with type 1 diabetes. The findings should help inform more detailed guidelines as well as identifying some crucial research gaps, so that those living with type 1 diabetes can enjoy the benefits of physical activity while minimising the risks. Dr Susan Aldridge reports.
Physical activity can be challenging for people with type 1 diabetes because of concerns around managing blood glucose levels, with exercise-induced hypoglycaemia being a particular concern. Glycaemic responses to exercise depend upon whether it is performed in a fasted or fed state, how much insulin is on board and the timing, type and intensity of the physical activity being undertaken. This level of detail does not yet appear in current guidelines. If it did, people with type 1 diabetes could enjoy more flexibility in scheduling their exercise sessions and the type of activity undertaken, which would make them more likely to enjoy its benefits, rather than avoiding it for fear of adverse consequences on blood glucose.
Accordingly, Simon Helleputte, University of Ghent, Belgium, Jane Yardley, University of Alberta, Edmonton, Canada, and colleagues elsewhere, have summarised the current state of research into the glycaemic impact of different types of postprandial exercise (taken within two hours of a meal, with insulin administration) in people with type 1 diabetes. They reviewed 20 clinical trials on the acute (during exercise), sub-acute (within two hours of exercise) and late (more than two hours and up to 24 hours after exercise) glycaemic effects of postprandial exercise.
Findings were organised according to four types of exercise – walking, continuous moderate intensity, continuous high intensity and interval exercise. The researchers also noted whether guidelines on reducing insulin before exercise to avoid hypoglycaemia were followed in the reviewed studies and the impact it had. They did not find any studies on resistance exercise, which is an obvious gap in the research as this is often recommended for overall fitness.
Reductions in blood glucose across the board
The review found that blood glucose decreased during all modalities of postprandial exercise, seemingly regardless of pre-exercise bolus insulin reduction, but depending on exercise duration and intensity. Walking, outside or on a treadmill, was defined as low-intensity aerobic exercise. There were only two studies in this category and both showed that a short walk – between 15 and 30 minutes – shortly after a meal lowers blood glucose during and for two hours after exercise, thus avoiding meal-related hyperglycaemia.
Continuous moderate-intensity exercise was defined as exercise at less than 70% of maximal aerobic capacity. The eight studies in this category showed decreases in blood glucose ranging from 2.3 mmol/l to 5 mmol/l. The larger decreases were found with longer and more intense bouts of activity. There were also many reports of premature termination of exercise because of low blood glucose, particularly when no insulin-reduction strategy was used.
Seven studies looked at the impact of high-intensity exercise, defined as exercise between 70% and 80% of maximal aerobic capacity. When carried out for between 45 and 60 minutes, large decreases in blood glucose, ranging from 3 mmol/l to 8 mmol/l were seen. Hypoglycaemia was infrequent, presumably because insulin-reduction strategies were used in most of these studies. The risk of late nocturnal hypoglycaemia persisted, sometimes even when participants were protected by a post-exercise meal.
Interval exercise consisted of either intermittent high-intensity exercise (IHE) or high-intensity interval training (HIIT). IHE is defined as a combination of continuous, moderate-intensity exercise interspersed with very short high-intensity sprints at regular time intervals. HIIT is brief, intermittent periods of vigorous, near-maximal capacity exercise, interspersed with low-intensity recovery periods of similar duration. There were five studies – three on IHE and two on HIIT. All led to declines in blood glucose during exercise between 1.9 mmol/l and 3.9 mmol/l and, notably, none used an insulin-reduction strategy.
So exercise is a useful blood-glucose management tool in type 1 diabetes, the authors say, with the largest effects seen in high-intensity exercise and the lowest with HIIT, but the exact decline depending on intensity and duration of exercise. However, physical exercise does carry a risk of hypoglycaemia, which can be mitigated by reductions in the insulin taken to cover the pre-exercise meal. This creates higher blood-glucose levels at the start of exercise, protecting against hypoglycaemia, but similar declines in blood glucose are seen, regardless of the insulin-reduction strategy used.
These findings suggest that non-insulin mediated uptake of glucose by muscle is an important mechanism for driving blood-glucose decreases during exercise and it is perhaps even more important than the amount of circulating insulin. Another important point is that pre-exercise insulin reductions should not come at the expense of significant hyperglycaemia before, during or after exercise because this, over time, could increase the risk of diabetes complications. Blood glucose before exercise and the precise timing of postprandial exercise are crucial in maintaining the balance between hypoglycaemia and hyperglycaemia, and further research is now needed to look at this aspect of physical activity in type 1 diabetes.
Future directions
This review revealed some gaps in the research on postprandial exercise in type 1 diabetes, besides getting the balance between hypoglycaemia and hyperglycaemia right, as mentioned above. Individual characteristics should be taken into account in future research, as there was considerable heterogeneity among the participants in the review studies. For instance, blood-glucose declines and risks of hypoglycaemia might vary according to an individual’s physical fitness or gender.
Pre-exercise meal composition is another important factor. It may be that higher protein or fibre content could be protective against hypoglycaemia. In the review studies, there was great variation in the meals consumed – some being high carbohydrate and others being a more standard composition. Some did not even report the detailed composition of the meal beyond its carbohydrate content, so it is difficult to draw firm conclusions on the impact of the pre-exercise meal composition on glucose excursions thereafter. If we knew the optimal pre-exercise meal composition, it would be easy for a person with type 1 diabetes to incorporate into their exercise plan.
The studies also had an overall lack of detailed information on the late effects of postprandial exercise, although there was evidence of nocturnal hypoglycaemia and the authors point to the value of a post-exercise snack or meal. However, continuous glucose monitoring (CGM) was not widely available when most of the review studies were carried out. With the improved current availability, future research should be able to describe the risks of post-exercise hypoglycaemia in more detail and advise people with type 1 diabetes accordingly. There will also hopefully be opportunities for research on postprandial exercise in hybrid closed-loop users, who have options to decrease their circulating insulin before, during and after exercise, simplifying glucose management around physical activity.
So, in conclusion, this review provides new insights into the glycaemic effects of postprandial exercise to improve glucose management. Postprandial exercise results in a consistent decline in blood glucose, whatever the exercise and regardless of insulin-reduction strategy. The authors recommend substantially reducing the prandial insulin dose at the pre-exercise meal to achieve higher blood glucose before, during and after exercise, thereby preventing exercise-induced hypoglycaemia. The magnitude of this insulin reduction should be proportional to exercise duration and intensity, but pre-meal blood-glucose levels and timing of exercise should also be considered to avoid pronounced hyperglycaemia. With these strategies, people with type 1 diabetes should be able to exercise safely and enjoy its benefits, without compromising their glycaemic management.
To read this paper, go to: Helleputte S, Yardley JE, Scott SN, Stautmans J, Jansseune L, Marlier J, De Backer T, Lapauw B, Calders P. Effect of postprandial exercise on blood glucose levels in adults with type 1 diabetes: a review. Diabetologia 4 April 2023. https://doi.org/10.1007/s00125-023-05910-x
To learn more, listen to Professors Miles Fisher and Mike Riddell discuss how technology is making a difference for athletes with diabetes in ‘The long and the short of it’.
To learn more, enrol on the EASD e-Learning course ‘Lifestyle intervention’, with ‘Module 2: Promoting physical activity for people with diabetes’ launching soon.
Any opinions expressed in this article are the responsibility of the EASD e-Learning Programme Director, Dr Eleanor D Kennedy.
