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Carbs before sprinting exercises do not disrupt glycemic management

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February 23, 2019

Glycemic management for young people with type 1 diabetes may not be negatively affected by carbohydrate intake before exercise that includes intermittent sprints, according to findings published in Diabetic Medicine.

“Carbohydrate ingestion before, during and after exercise can reduce the hypoglycemia risk associated with both planned and unplanned physical activities. Another effective means to reduce hypoglycemia risk during moderate exercise is to incorporate repeated short sprints in a pattern of activity referred to as intermittent high-intensity exercise,” W.H.K. Soon, PhD, a research assistant at the Telethon Kids Institute in West Perth, Australia, and colleagues wrote. “These findings raise the issue of whether the pre-exercise ingestion of carbohydrates at levels recommended to maintain stable glycemia during moderate-intensity exercise might result in an excessive rise in blood glucose level if combined with repeated sprints.”

Soon and colleagues recruited eight participants aged 14 to 35 years (mean age, 18.8 years; five females; mean HbA1c, 61 mmol/L; mean BMI, 25.9 kg/m2) from the Princess Margaret Hospital diabetes clinic in Perth, Australia, to measure blood glucose response during and after four separate exercise routines. These routines were completed in a random order on 4 separate days. Participants fasted overnight and took morning insulin before exercise.

The first routine was moderate-intensity exercise of 40 minutes of cycling at 50% peak oxygen consumption. The second was intermittent high-intensity exercise of 40 minutes of cycling at 50% peak oxygen consumption with interspersed sprints every 2 minutes and a 10-second sprint at the conclusion. The third was the moderate-intensity exercise after the ingestion of 3.5 mg carbohydrates per hour of exercise. The final routine was the high-intensity exercise after the same carbohydrate intake.

During each exercise, blood glucose response was measured by time-averaged area under the blood glucose curve and by peak blood glucose level during each exercise. Participants were measured for blood glucose, lactate and heart rate at zero, 10, 20, 30 and 40 minutes after exercising.

The researchers found significant differences in time-averaged blood glucose AUC when comparing moderate exercise vs. moderate exercise with carbohydrate intake (–1.7 mmol/L; 95% CI, –2.6 to –0.8), moderate exercise vs. intense exercise with carbohydrate intake (–1.5 mmol/L; 95% CI, –2.4 to –0.5), moderate exercise with carbohydrate intake vs. intense exercise (1.5 mmol/L; 95% CI, 0.5-2.4) and intense exercise vs. intense exercise with carbohydrate intake (–1.3 mmol/L; 95% CI, –2.2 to –0.3).

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However, no significant differences in mean time-averaged blood glucose AUC and peak blood glucose levels were found when comparing moderate exercise with carbohydrate intake and exercise that included both intermittent sprints and pre-routine carbohydrate consumption. The same was true when comparing the time-averaged blood glucose AUC and peak blood glucose level between the non-carbohydrate moderate-exercise and intense-exercise routines. In addition, blood glucose levels were elevated for at least 30 minutes after both pre-carbohydrate exercises.

“Against expectations, the present study showed that when carbohydrates were ingested prior to moderate-intensity exercise in the fasted state under basal insulin conditions, there was no evidence to suggest that adding repeated sprints caused any further significant increase in blood glucose levels during and after exercise,” the researchers wrote. – by Phil Neuffer

Disclosures: The authors report no relevant financial disclosures.

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Some prior studies in type 1 diabetes have shown that either short (4 sec) sprints interspersed into moderate aerobic exercise or carbohydrate ingestion before it can largely prevent a decrease in blood glucose, or limit how far it falls, during the activity and possibly for a couple hours afterward. In this study, Soon and colleagues hypothesized that pre-exercise ingestion of carbohydrates would result in excessive hyperglycemia if combined with repeated sprints done during moderate aerobic exercise.

However, neither the combination of pre-exercise carbohydrate and intermittent sprinting during moderate activity nor sprinting without prior carbohydrates resulted in any significant elevation in blood glucose compared with continuous exercise under similar carbohydrate conditions. The authors seemed surprised by these findings (calling them “against expectations” and “another unexpected finding”), but I found them completely unsurprising. The most likely reason for this outcome is the relatively small volume of intense sprinting done over the 40 minutes of exercise: Intermittent sprints were short in duration (4 sec) and undertaken every 2 minutes over 40 minutes (4 x 20, or 80 sec total), far less than done in many other high-intensity interval training studies involving 1-minute sprint intervals (60 sec x 10, or 600 sec total).

What’s more, the authors specifically chose to study moderate exercise during basal (fasting) insulin conditions with the expectation that both sprints and carbohydrate intake would limit any decrease in blood glucose levels arising from 40 minutes of moderate activity not surprisingly, this was confirmed. In addition, prior carbohydrate ingestion (albeit only about 10 g) led to elevated blood glucose above baseline from the start of exercise until 45 min into recovery during both the moderate exercise-only trial and from the start of exercise until 30 min into recovery for the trial with sprinting interspersed. Again, this was expected due to the timing of the activity. Most exercisers with type 1 diabetes have reported a need for additional insulin, not carbohydrate, when undertaking any intense activities in a fasted state.

Pre-breakfast physical activity limits fluctuations in blood glucose during the activity due to higher levels of cortisol and other glucoregulatory hormones and lower circulating insulin at that time of day. In fact, many with type 1 diabetes preferentially exercise at times when their “insulin on board” is lower, either when fasting or 2 to 3 hours after the last bolus dose. The wisdom of this practice has again been confirmed by this study.

Sheri R. Colberg, PhD, FACSM

Professor Emerita of Exercise Science,
Old Dominion University

Disclosure: Colberg reports no relevant financial disclosures.