Exercise Intensity and Fat-Loss: Nothing in Moderation

by Sam Knox

It’s hard to understate the benefit of moderate-intensity exercise for weight-loss. Aerobic exercise alone can produce a modest decrease in weight, but rarely achieves clinical significance. The benefit is especially modest considering the amount of time required to achieve it. When aerobic exercise is combined with a calorie-restricted diet, the benefit is even less, and becomes smaller still as the restriction increases. Resistance exercise alone is no more likely to produce fat-loss than not.

All is not lost.

Low-volume, high-intensity interval training (HIIT) combines brief periods of all-out effort with periods of reduced effort or complete rest. It has been shown to provide the same or similar benefits as continuous aerobic exercise, plus an improvement in anaerobic fitness, with considerably less time and effort.

Because a lack of time is the reason most often given for failing to adhere to an exercise program, low-volume HIIT has generated some interest in research and the popular press as an alternative to continuous aerobic exercise. Perhaps because it has long been assumed that the effect of exercise on weight-loss is strictly a function of caloric expenditure, low-volume HIIT has received little attention as an alternative for weight-loss. The attention it has received, however, has produced some interesting results.

In one study, twenty-seven men and women aged 18-32 were divided into two groups. One group completed a 20-week program of moderate to vigorous aerobic exercise. The other group completed a 15-week program combining moderate aerobic exercise and low-volume HIIT. Subcutaneous fat was assessed by skinfold measurements. The loss of subcutaneous fat was greater in the HIIT group even though the aerobic exercise group expended roughly twice as many calories. Adjusted for energy expenditure, the loss of subcutaneous fat in the HIIT group was nine times greater than the aerobic exercise group. (Neither group lost weight, suggesting that lean mass was increased, especially in the HIIT group.)

In another study, thirty-four women aged 18-30 were divided into two exercise groups and one control group. One exercise group performed low-volume HIIT, the other moderate aerobic exercise, both for a period of 15-weeks. There was no loss of weight or fat in either the aerobic exercise group or the control group, while the HIIT group lost an average of ~6 pounds, all from fat. Also, improvements in measures of insulin sensitivity (fasting insulin, HOMA-IR) were roughly three times greater in the HIIT group compared to the aerobic group, although only fasting insulin was significantly different from the control. The energy expenditure of both exercise groups were similar, but the HIIT program required only half the time as the aerobic exercise program. Interestingly, members of the HIIT group lost fat and gained lean mass primarily in the abdomen and thighs, although total lean mass was unchanged.

Given the differences in the way we respond to moderate vs. high-intensity exercise, both in magnitude and in kind, it isn’t clear why low-volume HIIT has this effect, but it does not appear to be the number of calories expended during exercise. The dramatic reduction in measures of insulin resistance, however, raises the possibility that whatever effect exercise has on fat-loss may be the result of lower average levels of circulating insulin.

For dieters and non-dieters alike, aerobic exercise and resistance training have well-established benefits for the prevention or treatment of chronic disease and quality of life. Resistance training in particular can help preserve muscle mass and bone density. Both should be recommended for those reasons alone.

While there are additional risks associated with high-intensity exercise, especially for some groups with chronic disease, we should at least consider the possibility that low-volume HIIT might be superior to high-volume, moderate-intensity aerobic exercise for those of us engaged in an exercise program primarily for weight-loss.


“… a moderate decrease …”: Wing: “In 6 of the 10 studies reviewed here, the differences were significant. However, in all cases [except one] the effect of exercise was very modest.” Garrow, et al: “Aerobic exercise without dietary restriction among men caused a weight loss of 3 kg in 30 weeks compared with sedentary controls, and 1.4 kg in 12 weeks among women …”. (Both cited in the ACSM Position Stand.)

“… clinical significance …”: ACSM Position Stand: “Few studies with sedentary overweight or obese individuals using PA [Physical Activity] as the only intervention result in ≥3% decreases of baseline weight.”

(Clinical significance in this case means “beneficial improvements in chronic disease risk factors”.)

“The benefit is especially modest …”: ACSM Position Stand: “… it seems that PA [Physical Activity] <150 min·wk-1 results in minimal weight loss compared to controls, PA > 150 min·wk-1 results in modest weight loss of ∼2-3 kg, and PA between 225 and 420 min·wk-1 results in 5- to 7.5-kg weight loss.

In one study cited by the ACSM Position Stand as an example of “laboratory studies generally [finding] greater levels of weight loss in response to PA” 225 minutes/week of aerobic exercise (~80% MHR) over 16 months produced an average weight loss of 11 pounds in men and no weight loss in women. (Donnelly, et al) For the men, this was a monthly return of less than one pound of weight-loss in exchange for 15 hours of exercise.

“… is combined with …”: Wing: “Only two of the 13 studies showed a statistically significant difference in the weight loss obtained in the diet plus exercise condition compared with the diet condition alone…with [one study] finding the significant difference in men only.” (Cited in the ACSM Position Stand.)

“… becomes smaller still …”: ACSM Position Stand: “When energy intake is reduced severely, diet and diet and PA [Physical Activity] groups tend to have similar results.”

“Resistance training alone …”: ACSM Position Stand: “Resistance training will not promote clinically significant weight loss.” “Neither randomized controlled … nor intervention studies without a sedentary comparison group … provide evidence for a reduction in body weight when resistance training is performed without any modification of diet. However, the studies since 1999 are equally split concerning whether resistance training will cause a loss of body fat.”

“Low-volume, high-intensity …”: In research, most often thirty-second sprints between four minute rest periods, repeated four to six times (“Wingate Protocol”).

The length and intensity of both the sprint and rest periods are infinitely variable. This allows for modifications to suit individual needs, and almost unlimited opportunities for market differentiation among trainers and gym owners. (“Thirty seconds is useless! Thirty-five seconds will make you a god!”)

“… the same or similar benefits …”: Martin J. Gibala, et al.

“… anaerobic fitness …”: Tabata I, et al.

“… considerable savings …”: Martin J. Gibala, et al: “Training time commitment over 2 weeks was ∼2.5 h for SIT [sprint interval training] and ∼10.5 h for ET [endurance training] and total training volume was ∼90% lower for SIT versus ET (∼630 versus ∼6500 kJ).”

“… the popular press …”: How 1-Minute Intervals Can Improve Your Health

“… it has long been assumed …”: Given the number of chronic physiological responses to regular exercise, not the least of which is improved insulin sensitivity, there is at least some reason to believe that this is incorrect.

“… produced some interesting results.”: Stephan H. Boutcher

“One study …”: Tremblay, et al

“In another study …”: Trapp, et al

“Given the differences …”: Marliss, Vranic; Boutcher

“… groups with chronic disease …”: Exercise-Intensity and Diabetes: HIIT Me?


ACSM Position Stand: Appropriate Physical Activity Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults, Medicine & Science in Sports & Exercise: February 2009 – Volume 41 – Issue 2 – pp 459-471

Wing, Rena R.: Physical Activity in the treatment of the adulthood overweight and obesity: evidence and research issues, Medicine & Science in Sports & Exercise: November 1999 – Volume 31 – Issue 11 – p S547. (Cited in the ACSM Position Stand.)

Garrow JS, Summerbell CD.: Meta-analysis: effect of exercise, with or without dieting, on the body composition of overweight subjects, Eur J Clin Nutr. 1995 Jan;49(1):1-10. (Cited in the ACSM Position Stand)

Joseph E. Donnelly, et al: Effects of a 16-Month Randomized Controlled Exercise Trial on Body Weight and Composition in Young, Overweight Men and Women, Arch Intern Med. 2003;163:1343-1350. (Cited in the ACSM Position Stand)

Martin J. Gibala, et al: Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance, September 15, 2006 The Journal of Physiology, 575, 901-911.

Tabata I, et al: Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max, Med Sci Sports Exerc. 1996 Oct;28(10):1327-30.

Errol B. Marliss and Mladen Vranic: Intense Exercise Has Unique Effects on Both Insulin Release and Its Roles in Glucoregulation Implications for Diabetes, Diabetes February 2002 vol. 51 no. suppl 1 S271-S283

Stephen H. Boutcher: High-Intensity Intermittent Exercise and Fat Loss, J Obes. 2011; 2011: 868305. Published online 2010 November 24.

Tremblay A, Simoneau JA, Bouchard C.: Impact of exercise intensity on body fatness and skeletal muscle metabolism, Metabolism. 1994 Jul;43(7):814-8.

Trapp EG, Chisholm DJ, Freund J, Boutcher SH.: The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women. Int J Obes (Lond). 2008 Apr;32(4):684-91. Epub 2008 Jan 15.