Summary written by Whitney Neal

Lifestyle interventions, including those focused on aerobic exercise training and diet, are essential for the prevention of type 2 diabetes. Current American Diabetes Association (ADA) guidelines recommend that adults with prediabetes engage in moderate-intensity exercise for at least 150 minutes per week or vigorous-intensity exercise for at least 90 minutes per week. However, results may differ depending on whether one follows the moderate-intensity versus vigorous-intensity exercise recommendations. Several studies have shown greater improvements in reducing type 2 diabetes risk through high-intensity compared with moderate-intensity exercise training. Additionally, high-intensity exercise training has emerged as a primary alternative to traditional exercise programs because less time commitment is required.

This study examined the effects of an isocaloric diet (moderate carbohydrate, moderate fat diet) combined with moderate-intensity exercise, high-intensity exercise, or no exercise at all. It was hypothesized that short-lived exercise would improve the metabolic response to an oral glucose tolerance test after eating a meal, and that high-intensity exercise would be more effective than moderate-intensity exercise due to improved insulin sensitivity (how sensitive the body is to insulin). People with low insulin sensitivity require larger amounts of insulin, whereas those with high insulin sensitivity require less.

Eighteen middle-aged men and women who met the ADA criteria for prediabetes and were sedentary (exercised less than 30 minutes per day, less than three days per week) volunteered for the study. All participants completed a peak O2 consumption (VO2 max)/lactate threshold protocol prior to beginning the exercise sessions. A VO2 max/lactate threshold test analyzes a person’s cardiorespiratory function by measuring his or her heart rate, exhaled gases and amount of lactate in his or her blood while performing an exercise. Each participant’s VO2 max was chosen as the highest consumption attained during the exercise protocol. The lactate threshold was determined by looking at the power output at which lactate started to accumulate in the blood stream. Participants consumed an isocaloric diet for 72 hours prior to the study.

The control group session included no exercise; participants remained in the seated position for one hour. In the moderate-intensity exercise session, subjects performed 200 kcal of bicycle ergometer exercise at the power output and VO2 associated with the individual’s lactate threshold. Participants in the high-intensity exercise session performed the same bicycle ergometer exercise at the power output and VO2 associated with 75 percent of the difference between the lactate threshold and the peak power output. Metabolic rate, heart rate, and data on the ratings of perceived exertion were monitored during each exercise session. After the exercise period, subjects received an oral glucose tolerance test. Blood samples were also taken in 10-minute intervals during exercise and during the one-hour recovery period to measure glucose, insulin, and C-peptide levels.

In support of the original hypothesis, when compared with the control group, insulin sensitivity improved by 51 percent in the moderate-intensity exercise group and by 85 percent in the high-intensity exercise group. Improvements in insulin sensitivity corresponded to improvements in glucose and insulin responses to an oral glucose tolerance test. The data also suggested that high-intensity exercise improves insulin sensitivity after a meal better than moderate-intensity exercise. Another important finding was the indication that glucose-lowering benefits of exercise may occur in as little as one hour after exercise. Overall, the study showed that long-term training interventions similar to this study should be conducted to investigate whether or not high-intensity exercise training can reduce the conversion of pre-diabetes to type 2 diabetes.