There is a militant camp that believes high-intensity interval training (HIIT) is the absolute best means of achieving fat loss, accounting for total energy expenditure over a 24-hour period. It certainly works well for some people. But recent findings suggest that the extra calories burned during HIIT may not be coming from stored fat, and that HIIT training doesn’t make the body a 24-hour fat-burning machine compared to steady-state cardio (SSC)— meaning the intensity is moderate and doesn’t vary during the exercise session.
Both HIIT and SSC improve fitness as measured by exercise capacity and VO2 max.1,2 The effect is very similar, nearly equal in fact. If one were to compare sprint performance, vertical leap or some other power-based measure, HIIT would likely be better due to the training style. HIIT also has an appetite-suppressing effect, partially due to lowering circulating concentration of the hormone ghrelin.3 HIIT may be more effective at improving glucose disposal and insulin sensitivity.4 Lastly, the challenge of HIIT and variety likely make it attractive to those with a higher fitness level, accustomed to vigorous training.
Steady-State Training a Better Fat Buster
The vast majority does “cardio” with the hopes of dropping an inch or two from the waist and burning more fat. The Journal of Obesity published a study looking at inactive, overweight adults, comparing SSC to HIIT in regard to changes in fitness (work capacity) and body composition (three sessions per week for 12 weeks).1 Whereas both SSC and HIIT training styles improved fitness equally, only SSC resulted in a reduction of abdominal fat and total body fat (2.7% reduction).
A second study from the Scandinavian Journal of Medicine & Science in Sports provided some insight to this difference.2 Fat is stored in muscle and fat cells for long-duration energy needs. The rate of fat burning is dependent upon enzymes that break down stored fat, transporting fatty acids to the mitochondria, and then the fat-burning rate of the mitochondria— small compartments in cells that generate energy from food calories. It has been noted that HIIT results in a greater number of mitochondria in muscle— so the question is, does this result in greater capacity to burn fat? In fact, it does not. Sugars are also burned in the mitochondria, as well as breakdown products of protein during catabolic conditions. The mitochondria from healthy, overweight HIIT-trained subjects (three sessions weekly for six weeks) do not adapt to greater fat burning at rest. It appears from this study that the mitochondria undergo a training-specific adaptation, accommodating the influx of sugar, fat and amino acid-sourced calories in the skeletal muscle (e.g., increased mitochondrial content). However, once the stress of exercise is gone, no change occurs in the “normal” conditions and functions of the muscle. The acute (meaning the intensity is not maintained for a long period) nature of the energy demands of HIIT appear to favor the acquisition and use of “immediate” energy from sugars, as opposed to the slower and more complex energy produced by the mitochondria.
HIIT Does Not Provide the Same Training Response
Basically, fat burning increases when the body realizes it does not have much “sugar” energy left, and the process is slow. HIIT forces the body to “redline,” resulting in a state where the “sugar” calories cannot get bottlenecked at the mitochondria for a few minutes, but once the pace slows down there are still enough “sugar” calories available. Thus, the need to tap into stored fat does not exist because the fatty acids could not process through the mitochondria any faster than the “sugars,” so there is no need to increase fatty acid availability or transport. That is an oversimplification, but it rationalizes why HIIT doesn’t provide the same training response.
The authors of that study did note that while other HIIT studies agreed with their findings, others disagreed with reports of greater fat burning with HIIT training. This issue remains to be resolved, and sides will be taken in the verbal battles between SSC and HIIT advocates. However, it is clear that SSC is equally effective in meeting targets relevant to fat loss and fitness. So, do what you enjoy and will stick with; try the other pattern once in a while for variety, and realize that what works best for others may not be what works best for you.
Fat-Burning Workout Plan
A great training plan for fat loss combines steady-state cardio and high-intensity interval training. Trainer Nick Tumminello, owner of Performance University in Florida and author of Strength Training for Fat Loss, shared a basic weekly workout plan you can follow if fat loss is your goal:
HIIT: 8-10 minutes total of sprints, on rower or upright bike. 2-minute warm-up and 3- to 5-minute cool down of easy work
Steady-state cardio: 30 to 40 minutes at approximately 70 percent of MHR on treadmill or elliptical.
For more information on Nick, visit www.NickTumminello.com.
1. Keating SE, Machan EA, et al. Continuous exercise but not high-intensity interval training improves fat distribution in overweight adults. J Obes 2014:834865(12 pp).
2. Larsen S, Danielsen JH, et al. The effect of high-intensity training on mitochondrial fat oxidation in skeletal muscle and subcutaneous adipose tissue. Scand J Med Sci Sports 2014 May 21. [E-pub, ahead of print]
3. Sim AY, Wallman KE, et al. High-intensity intermittent exercise attenuates ad-libitum energy intake. Int J Obes 2014;38:417-22
4. Babraj JA, Vollaard NB, et al. Extremely short duration high-intensity interval training substantially improves insulin action in young healthy males. BMC Endocr Disord 2009;9:3(8 pp).
5. Zinner C, Wahl P, et al. Acute hormonal responses before and after 2 weeks of HIT in well-trained junior triathletes. Int J Sports Med 2014;35:316-22.
6. Mitchell CJ, Churchward-Venne TA, et al. Muscular and systemic correlates of resistance training-induced muscle hypertrophy. PLoS One 2013;8(10):e78636.