Adjusting insulin and carbohydrates for physical exercise in type 1 diabetes

Research studies have explored adjustments to diet and medication that improve the acute glycaemic responses to physical activity in people with type 1 diabetes. These changes may form the basis of an individualised “acute exercise management strategy” that instils more confidence for people with type 1 diabetes and their healthcare professionals to engage in physical activity and obtain the many metabolic and psychological health benefits of being regularly active.

On the assumption that individuals have been assessed by their GP or hospital specialist and present minimal complications that preclude them from starting a physical activity programme in line with Department of Health (2011) guidelines (see Box 1), a person with type 1 diabetes may need to make individualised adjustments to insulin and carbohydrate intake before, during and after a bout of exercise (Figure 1 provides an example).

Insulin and carbohydrate adjustments for aerobic exercise
Safe blood glucose ranges for exercise
According to the International Diabetes Federation (2014), pre-exercise blood glucose should ideally be >6 mmol/L. If the level is >14 mmol/L, ketones should be tested and exercise delayed until the values decline. A note of caution here is that, since account is not made of the direction of change in blood glucose in the hours prior to measurement, a 60-minute pre-exercise blood glucose sample may provide additional confidence.

Basal–bolus insulin reduction
In people with relatively well-controlled type 1 diabetes, insulin detemir is associated with less hypoglycaemia during and after exercise than insulin glargine (but not NPH insulin; Arutchelvam et al, 2009). However, in the majority of cases, basal insulin corrections are not normally made.

Research examining adjustments to pre-exercise rapid-acting (bolus) insulin has explored reductions ranging from 10% to 90% (Rabasa-Lhoret et al, 2001; Mauvais-Jarvis et al, 2003; Grimm, 2005; De Feo et al, 2006; West et al, 2010; 2011b). Based on these experiments, a prudent start point appears to be an approximate 50% reduction in rapid-acting insulin made 30–60 minutes before activity (West et al, 2011b).

Carbohydrate ingestion
For aerobic activity, bolus insulin reductions are usually made alongside consumption of carbohydrate. Carbohydrates come in different forms. Some that enter the blood stream rapidly (i.e. high-glycaemic-index [high-GI] carbohydrates such as glucose and maltodextrin) are important in alleviating hypoglycaemia, but slower alternatives (i.e. low-GI carbohydrates such as fructose and isomaltulose) have been shown to provide equal energy, reduce glycaemic fluctuations and produce equivalent exercise performance (West et al, 2011a; Bracken et al, 2012). Current recommendations suggest ingesting an upper limit of 1 g carbohydrate per kg of body mass per planned hour of exercise, in a 6–10% solution. As an example, an 80 kg male aiming to perform 30 minutes of aerobic cycling might consume a 10% solution of a low-GI carbohydrate (0.40 L) that delivers 40 g of available carbohydrate. After initial blood glucose monitoring, further refinement on amounts may be applied.

The pre- and post-exercise periods
A combined insulin reduction and carbohydrate feeding strategy 30–60 minutes before running can preserve blood glucose concentration after exercise in people with type 1 diabetes who are undertaking physical activity (West et al, 2011b).

Importantly, there is a need to adjust insulin and carbohydrate following physical activity because performing exercise sensitises tissues to insulin and independently increases muscle and liver glucose uptake. Accordingly, in addition to pre-exercise adjustments, consumption of a low-GI meal and a 50% post-exercise rapid-acting insulin dose (1 hour after exercise) reduces glycaemic fluctuations and protects against hypoglycaemia for up to 8 hours (Campbell et al, 2013; 2014).

Insulin and carbohydrate adjustments for strength exercise
There is much less information available on the adjustments necessary for people with type 1 diabetes who are performing strength exercise. In contrast to aerobic exercise, strength exercises can cause large acid–base shifts and counter-regulatory responses that increase blood glucose concentrations following the period of physical activity. The amount of weight-lifting in a session can determine the degree of hyperglycaemia (Turner et al, 2014a). Thus, it seems (for morning exercise at least) that there is a minimal need to consume carbohydrates for strength exercise lasting approximately 30 minutes. However, if exercise-induced hyperglycaemia regularly occurs it may be useful to deliver a small rapid-acting insulin dose (Turner et al, 2014b).

Conclusion
Regular exercise is promoted as a cornerstone of good glycaemic management in type 1 diabetes, yet much work remains to be done to promote physical activity so that it is on an equal footing with diet and medication. We need to instil more confidence in people with the condition and primary care staff to manage blood glucose before, during and after exercise, in order to allow the many health benefits of being active to be achieved. Everyone’s response to exercise will be different, but it is hoped that knowledge of the typical glycaemic response to aerobic or anaerobic exercises may help in the development of an individualised “acute exercise management strategy”.

Questions to test your knowledge
The answers are not always to be found in this article.

1. Fructose is a high-glycaemic-index carbohydrate.
   Is this true or false?
2. It is usual to adjust rapid-acting insulin before exercise.
   Is this true or false?
3. Insulin requirements remain unaltered following aerobic activity.
   Is this true or false?
4. Thirty minutes of strength exercise is likely to cause an increase in blood glucose concentrations.
   Is this true or false? 

Answers: 1 – false; 2– true; 3 – false; 4 – true.