Obesity is a growing global health risk for cardiovascular disease, type 2 diabetes, metabolic syndrome and all-cause mortality. Indeed, central adiposity (visceral fat), the fat tissue around the major organs in the torso, generally elevates the risk of chronic diseases. Ample research shows that high-volume, moderate-intensity exercise is an effective way to reduce central obesity (Zhang et al. 2017). However, until recently, little has been known about the influence of high-intensity interval training (HIIT) on weight and fat loss in young adults with obesity.
This month, we explore how HIIT affects weight and fat loss in this population. Two recent studies reveal HIIT’s ability to produce high-impact fitness gains in a short time frame, while two older studies show how HIIT improves calorie-burning capacity in as little as a half-dozen sessions over 2 weeks.
Zhang, H., et al. 2017. Comparable effects of high-intensity interval training and prolonged continuous exercise training on abdominal visceral fat reduction in obese young women. Journal of Diabetes Research, 5071740.
Purpose. The researchers compared the effects of HIIT vs. moderate-intensity continuous exercise (MICT) on three classifications of fat—whole-body, abdominal visceral and abdominal subcutaneous—in a 12-week training study with young women who were obese.
Participants. Forty-three healthy Chinese women with obesity (aged 18–22; BMI > 25; body fat percentage > 30) were randomly divided among a HIIT group (n = 15), an MICT group (n = 15) and a no-training control group (n = 13). HIIT and MICT participants did no other exercise during the study.
Training protocol. For the first 4 weeks, the two exercise groups trained 3 days a week. For weeks 5–12, they trained 4 days a week. Researchers used continuous gas analysis collection during each exercise trial, ensuring that the women in both exercise groups expended identical amounts of energy. All exercisers burned 200 kilocalories per workout in the first 4 weeks and 300 kilocalories per session in weeks 5–12.
The MICT group performed continuous, steady-state exercise on a cycle ergometer at a comfortably hard intensity (60% of VO2max). Also using a cycle ergometer, the HIIT group performed 4-minute high-intensity intervals (90% of VO2max) alternating with 3-minute passive-recovery intervals until each volunteer reached her targeted kilocalorie expenditure.
At the end of weeks 4 and 8, the researchers reassessed participants’ VO2max to make sure training intensities reflected their rising fitness levels. All workouts began with a warmup and finished with a cooldown.
During the study, each woman recorded her daily dietary intake, and a dietitian evaluated it. This process ensured that none of the participants went on a diet, which would have skewed the outcome.
Results and Discussion
Body mass, percent body fat, abdominal visceral fat (AVFA) and abdominal subcutaneous fat (ASFA) were measured using dual-energy X-ray absorptiometry and computed tomography. Table 1 shows the results: The HIIT and MICT groups saw identical—and significant—changes in all measured body fat parameters. The control group showed no changes.
Interestingly, since the researchers matched energy expenditure in the active groups’ workouts, the most significant difference between HIIT and MICT was workout duration. Average time to complete the workout sessions during weeks 5–12 was 36 minutes for the HIIT group vs. 68 minutes for the MICT group. And the HIIT group had the same improvement in VO2max as the MICT group in about 50% less exercise time.
Although it’s unclear why the HIIT group attained similar results in a much shorter time, the authors propose that HIIT workouts elevate the postworkout metabolic rate (often called excess postexercise oxygen consumption or exercise “afterburn” ). The researchers also say high-intensity exercise activates major hormones (epinephrine, norepinephrine, growth hormone) involved in fat metabolism.
Major take-away: In sedentary women with obesity, a 12-week HIIT program (4-minute work intervals alternating with 3-minute passive-recovery intervals) produced improvements in body composition and aerobic capacity in about half the time it took a continuous-exercise program to yield the same benefits.
Heydari, M., Freund, J., & Boutcher, S.H. 2012. The effect of high-intensity intermittent exercise on body composition of overweight young males. Journal of Obesity, 480467.
Purpose. The scientists tested how HIIT affected the body composition of men with excess weight who exercised for 20 minutes three times a week over the 12-week study.
Participants. Thirty-eight inactive men with excess weight (aged 25±5; BMI > 28; percent body fat ∼35) completed the study (from the original 46 recruits). They came from a university population and were randomly split into an exercise group (n = 20) and a nontraining control group (n = 18).
Training protocol. The men in the exercise group completed supervised intervals of 8-second sprints (at 80%–90% of peak heart rate) alternating with 12 seconds of recovery throughout a 20-minute session on a cycle ergometer. During the 8-second sprint, they pedaled at 120–130 revolutions per minute with a load on the cycle. Then they pedaled at 40 rpm for the 12-second recovery. The load stayed the same for the entire session. All workouts began with a 5-minute warmup and finished with a 5-minute cooldown.
Results and Discussion
Body mass, percent body fat, abdominal fat and abdominal trunk fat were measured with dual-energy X-ray absorptiometry and computed tomography. The exercising HIIT group saw significant reductions in all body fat measures. The authors did a before-and-after dietary analysis of the subjects to verify that they all ate similar diets during the study. Aerobic capacity, as measured by maximal oxygen consumption tests, improved 15% with the HIIT intervention. See Table 2 for details.
Major take-away: In sedentary men who are overweight, a 12-week HIIT program (20 minutes of 8-second work intervals alternating with 12-second recovery intervals) produced impressive improvements in body composition and aerobic capacity.
How Long Does It Take for the Body to Start Improving Fat Metabolism With HIIT?
Fitness enthusiasts and professionals often wonder how many HIIT workouts people must do before they start burning fat. Two investigations have taken up this question. Talanian et al. (2007) examined the effect of seven HIIT workouts in 2 weeks on eight healthy women (aged 22). The HIIT protocol consisted of 4-minute work bouts at approximately 90% of VO2max alternating with 2 minutes of active rest on a cycle ergometer, for a total of 10 bouts. The 2-week regimen produced marked increases in the fat-burning capacity of these women, who were moderately active.
In the second study, Jacobs et al. (2013) examined the effect of six HIIT sessions in 2 weeks on 16 untrained men (aged 27±3). The HIIT protocol initially consisted of 60-second work bouts at 100% of peak power alternating with 75 seconds of recovery at a low intensity (30 watts) on a cycle ergometer. The men completed eight work/recovery intervals in the first two training sessions, 10 during the third and fourth sessions, and 12 in the final two sessions. The six HIIT sessions spread over 2 weeks yielded significant increases in skeletal muscle mitochondria (the energy powerhouse of cells where fat burning occurs), content, function and oxidative capacity.
For an overview of the HIIT protocols reviewed in this column, see Table 3.
A HIIT for the Time-Crunched
Studies suggest HIIT produces meaningful benefits in reducing abdominal and total-body fat in half the time of steady-state training programs. Also, in as little as six or seven workouts over a couple of weeks, HIIT has produced marked increases in fat-burning capacity. For time efficiency, HIIT is a big hit.