Impact of acute hyperglycaemia on exercise performance

Guidelines recommend people with type 1 diabetes to undertake regular physical activity, and to aim for blood glucose levels around 7–10 mmol/L during exercise. However, many people exercise at higher glucose levels, for reasons including stress, unplanned exercise, poor background glycaemic control and deliberate attempts to reduce the risk of hypoglycaemia. Previous research has suggested that aerobic fitness, as measured by peak VO₂, is reduced in people with chronic hyperglycaemia. The present authors sought to evaluate the effects of acute hyperglycaemia, in the presence of either high or low plasma insulin levels, on exercise performance.

In this randomised crossover study, 12 people with type 1 diabetes (mean age 18 years, diabetes duration 7 years, BMI 24 kg/m² and HbA_1c 61 mmol/mol [7.7%]) undertook a series of exercise and reaction time tests whilst under a glucose/insulin clamp set at the following levels:

• Blood glucose at 5 mmol/L and insulin at 20 mU/min per m² of body surface area (equivalent to levels at 2 hours post-bolus).
• Blood glucose at 17 mmol/L and insulin at 20 mU/min per m² of body surface area.
• Blood glucose at 17 mmol/L and insulin at 5 mU/min per m² of body surface area (equivalent to levels at the basal state).

Compared with the euglycaemia arm, the primary outcome, peak VO₂, was significantly reduced in the hyperglycaemia/high insulin arm, but not in the hyperglycaemia/low insulin arm. However, the difference was small, with only a 6.6% reduction in VO₂, and was unlikely to be clinically relevant. Small differences in sprint cycling power, total work and reaction time were also observed between the different arms, but again these were unlikely to dramatically affect performance unless at the elite athlete level. No effects on the other measured indicators of exercise performance were observed.

While the study was not designed to evaluate safety, there were no safety concerns in either of the hyperglycaemic conditions. Thus, the authors conclude that acute hyperglycaemia does not have a consistent adverse effect on exercise performance and, in the absence of ketosis, should not discourage individuals with type 1 diabetes from continuing to exercise.

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