Only High-Intensity Exercise Improves Glucose Tolerance

Jenni Laidman

March 03, 2015

High- and low-intensity workouts were associated with similar weight loss and reductions in waist circumference, but only high-intensity workouts improved 2-hour glucose tolerance, according to a study published in the March 3 issue of the Annals of Internal Medicine. High-intensity workouts were also associated with the greatest reduction in risk for cardiovascular mortality.

The results of the single-center randomized controlled trial suggest that American Diabetes Association consensus recommendations for exercise may not be sufficient to influence 2-hour glucose tolerance.

Robert Ross, PhD, professor, School of Kinesiology and Health Studies, Queen's University, Kingston, Ontario, Canada, and colleagues randomly assigned 300 abdominally obese men and women into four groups: 75 participants were assigned to a no-exercise group; 73 to a low-amount, low-intensity (LALI) exercise group; 76 to a high-amount, low-intensity (HALI) exercise group; and 76 to a high-amount, high-intensity group (HAHI). All groups participated in five weekly exercise sessions.

Individuals in the LALI group had exercise sessions that expended 180 kcal for women and 300 kcal for men at 50% of maximum oxygen consumption (VO2peak), for a mean exercise time of 31 minutes per session. Those in the HALI group had sessions expending 360 kcal for women and 600 kcal for men at 50% VO2peak, for a mean exercise time of 58 minutes per session. The HAHI group sessions expended 360 kcal for women and 600 kcal for men at 75% VO2peak, for a mean exercise time of 40 minutes per session.

Among the 217 participants who completed the trial, waist circumference reduction did not differ significantly among the three exercise groups (P > .43), although all exceeded the no-exercise group (P < .001 for each). After adjusting for age and sex, participants in the LALI group dropped an average of 3.9 cm in waist circumference(95% confidence interval [CI], −5.6 to −2.3 cm; P < .001) at 24 weeks. In the HALI group, participants lost an average of 4.6 cm in waist circumference (95% CI, −6.2 to −3.0 cm; P < .001). Similarly, participants in the HAHI group lost an average of 4.6 cm in waist circumference (95% CI, −6.3 to −2.9 cm; P < .001).

Weight loss was also greater in the exercise groups than the control group (P < .001) but did not differ among the exercise groups (P > .182).

Where exercise intensity appeared to leave a mark was in measures of 2-hour glucose tolerance. The reduction was greatest in the HAHI group, with a between-group difference from baseline to 24 weeks of −0.7 mmol/L (95% CI, −1.3 to −0.1 mmol/L; P = .027) compared with the control group. However, there was no difference between the control group and the LALI or HALI groups (P = .94 and .159, respectively).

"Perhaps the most important finding was that benefit with respect to reduction of 2-hour glucose level was restricted to the higher-intensity exercise group," the authors write. Vigorous exercise was associated with a 9% improvement in glucose tolerance. However, they note that the clinical importance of reducing 2-hour glucose scores is uncertain.

The fact that lower-intensity exercise failed to influence glucose tolerance or other measures of insulin action contradicts the American Diabetes Association consensus statement that suggests 150 minutes per week of low-intensity exercise combined with modest weight loss of 5% to 10% would reduce 2-hour glucose levels. In this study, weight loss fell within the recommended range (with 150 minutes per week of low-intensity exercise), at 5% to 6%, but failed to alter glucose tolerance.

The increase in cardiorespiratory fitness (CRF) was greatest for the HAHI group at 24 weeks (19.6%; 2.2 metabolic equivalents) compared with both low-intensity groups. However, the results showed that the amount of also exercise mattered, with 300 minutes per week having a larger effect on CRF than 150 minutes per week in the two low-intensity groups. Previous studies show that an improvement in CRF is clinically meaningful, such that an improvement of 1 metabolic equivalent in CRF reduces the risk for death from cardiovascular disease 15% to 20%. The risk reduction was greatest in the high-intensity group (30% - 40%).

Support for this research provided by the Canadian Institutes of Health Research. The authors have disclosed no relevant financial relationships.

Ann Intern Med. 2015;162:325-334. Abstract

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