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Endurance cycling, glycaemia and reduced heart rate variability

Type 1 diabetes – September 2020 digest

Over multi-day endurance exercise, time spent in hyperglycaemia increases and is associated with reduced parasympathetic cardiac tone, a risk factor for cardiovascular morbidity and mortality.

Cardiac autonomic neuropathy (CAN) can occur in people with diabetes and is associated with increased risk of arrhythmia and cardiovascular morbidity and mortality. A preclinical sign of CAN is reduced heart rate variability (HRV). In people without diabetes, exercise is known to improve HRV; however, in those with type 1 diabetes, exercise can lead to sharp glycaemic variability, which in turn reduces HRV. Thus, the overall effects of exercise on HRV in people with type 1 diabetes are uncertain.

The aim of this observational study was to examine the effects of exercise, specifically long-distance endurance cycling, on HRV. Twenty adults with uncomplicated type 1 diabetes who took part in the 2015 mHealth Grand Tour, comprising a 9-day cycle ride of nearly 1500 km, used heart rate and continuous glucose monitoring to assess their HRV and blood glucose levels.

As the 9-day tour progressed, the time spent in hypoglycaemia was found to decrease; however, dramatic increases in the time spent in hyperglycaemia were observed, both in the day while cycling and at night while resting. This was despite no changes in insulin administration and a reduction in daily carbohydrate intake.

This hyperglycaemia was associated with a reduction in parasympathetic cardiac tone, with the more time spent in hyperglycaemia the lower the tone. Overall HRV tended to decrease with the number of kilometres traveled on the previous day, although this was not influenced by time in hyperglycaemia or by exercise intensity or duration.

Given the potential future risk of cardiac mortality associated with these findings, the authors conclude that more research to understand and manage exercise-induced hyperglycaemia would be beneficial, particularly as endurance sport events become more popular.

Lespagnol E, Bocock O, Heyman J et al (2020) In amateur athletes with type 1 diabetes, a 9-day period of cycling at moderate-to-vigorous intensity unexpectedly increased the time spent in a state of hyperglycemia, which was associated with impairment in heart rate variability. Diabetes Care 30 Jul [epub ahead of print]. https://doi.org/10.2337/dc19-1928

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