HIIT for fat burning

High-intensity interval training (HIIT) has long become the «star» of the fitness industry: short, intense and supposedly far more effective for fat burning than ordinary running. The reality, as always, is more interesting than the marketing. Our editorial team examined what meta-analyses show, how the «afterburn effect» works and who HIIT really suits.

What HIIT is and how it differs from cardio

HIIT is the alternation of short segments of very intense work with periods of rest or light activity. During the work interval the pulse approaches maximal values and talking is practically impossible. Rest allows partial recovery so that the next segment can again be performed at high intensity.

The opposite of HIIT is moderate-intensity training at a steady pace (in the literature MICT — moderate-intensity continuous training): steady running, cycling or uphill walking for 30–60 minutes. Here the intensity is lower but the duration is much greater.

There is also a separate variety — sprint interval training (SIT), when the work segments are performed «all out», for example 30-second sprints on a cycle ergometer. It was precisely such protocols that Martin Gibala’s group studied. Their review (2012) showed that even a small volume of intense intervals improves aerobic conditioning and metabolic indicators comparably to much longer moderate-intensity training.

The well-known «Tabata protocol» comes from a study by Tabata and colleagues (1996): 8 intervals of 20 seconds of work on a cycle ergometer at an intensity above the level of maximal oxygen consumption and 10 seconds of rest. It is important to understand that a true Tabata is extremely hard work, and many «Tabata workouts» in fitness clubs have little in common with it.

Fat and HIIT: what meta-analyses say

A meta-analysis by Wewege and colleagues (2017), which included studies of people with overweight and obesity, compared HIIT and steady-state cardio. Both approaches reduced fat mass and waist circumference, and no significant difference was found between them. The main advantage of HIIT is less time spent for a similar result.

A later meta-analysis by Viana and colleagues (2019) found that interval training produced a slightly greater reduction in absolute fat mass than steady-state cardio. At the same time, in kilograms this difference is small, and the studies were short and heterogeneous. So intervals may have a small advantage, but it is not the «magic pill» it is sometimes advertised to be.

The key conclusion of both works: overall energy balance is decisive. No workout compensates for a steady calorie surplus. HIIT can be a useful tool in a weight-loss program, but without dietary control the effect will be modest regardless of the type of cardio chosen.

A systematic review by Weston, Wisløff and Coombes (2014) showed that in patients with cardiometabolic diseases HIIT improves maximal oxygen consumption more than moderate continuous training. This is important, since aerobic fitness is one of the strongest predictors of overall health. However, in such patients the intervals were carried out under medical supervision.

The afterburn effect (EPOC): myth or reality

One of the main marketing promises of HIIT is «fat burning for a whole day after the workout». This refers to EPOC — excess post-exercise oxygen consumption. After intense work, the body spends some time using more energy to restore creatine phosphate stores, clear lactate, and normalize body temperature and hormonal levels.

A review by LaForgia, Withers and Gore (2006) analyzed how the intensity and duration of a load affect EPOC. The authors confirmed that the effect exists and grows with intensity. However, in absolute numbers the additional energy expenditure after a workout is usually only a small fraction of the expenditure during the session itself.

In other words, EPOC is real, but its contribution to weight loss is modest. It does not turn a 15-minute workout into the equivalent of an hour of running. If the total amount of energy burned during HIIT is small, the «afterburn» does not compensate for it.

Interestingly, intense work can temporarily reduce appetite in some people and, on the contrary, increase it in others. The reaction is individual, so the only reliable way to assess a program’s effectiveness is to monitor weight, girths and well-being over weeks.

Protocols and how to apply them

Studies have used very different protocols: from 4–6 sprints of 30 seconds to 4 intervals of 4 minutes at an intensity of 85–95% of maximal heart rate. What they have in common is one thing — the work segments are much more intense than ordinary cardio, and the total duration of the session is short.

• «4×4»— 4 intervals of 4 minutes at high intensity with 3 minutes of active rest; often used in clinical studies.
• «10×1»— 10 intervals of 1 minute intensely with 1 minute of light work; convenient for HIIT beginners.
• Sprint intervals— 4–6 segments of 20–30 seconds «all out» with long rest; for trained people.

For a person without serious limitations it is sensible to start with 1–2 sessions a week. Most studies that showed an effect used 3 sessions a week, but on top of strength training three HIIT sessions can overload recovery. Frequency matters more than the «heroism» of a single session: two regular workouts are better than one exhausting session every two weeks.

The choice of machine affects the injury risk. A stationary bike, a rowing machine and an elliptical are the safest options for HIIT: they have no impact load and allow precise dosing of effort. Sprints on a treadmill or outdoors are effective but carry a greater risk of injury to the hamstrings and the Achilles tendon.

Every HIIT session should begin with a full warm-up of 5–10 minutes with a gradual increase in intensity and end with a cool-down. After intense intervals a sharp stop can cause dizziness, so a few minutes of light movement are mandatory.

Limitations, risks and who HIIT does not suit

HIIT is a stressful load for the cardiovascular system. People with known heart disease, uncontrolled hypertension, arrhythmias or a family history of sudden cardiac death may do intense intervals only after a check-up and with a cardiologist’s permission. In clinical studies such patients trained under medical supervision.

People with pronounced obesity and joint problems should better start with steady-state cardio and strength training and introduce intervals gradually on non-impact machines. Beginners should also first build an endurance base over several weeks.

An excessive number of HIIT sessions is a common mistake. Intense intervals every day lead to accumulating fatigue, worsening sleep, reduced motivation and worse results in the gym. Signs of overload are an elevated resting pulse in the morning, constant fatigue and frequent colds.

Finally, HIIT is psychologically hard. For many people regular walking or cycling that brings enjoyment will produce a better long-term result than intervals a person will give up after a month. The best fat-burning program is the one you will stick to.

Editorial conclusions

HIIT effectively improves aerobic fitness and promotes a reduction in fat mass, but its advantage over steady-state cardio in fat burning is small. The main plus is time saving.

The «afterburn effect» exists, but its contribution to total energy expenditure is modest. The decisive factor in weight loss remains energy balance.

For most people it is optimal to combine 1–2 HIIT sessions a week with strength training and moderate daily activity, choosing non-impact machines and not neglecting the warm-up.

We also recommend reading our materials on cardio during a bulk, on training for definition and on cooling down and stretching after a workout.

References

1. Wewege M, van den Berg R, Ward RE, Keech A. The effects of high-intensity interval training vs. moderate-intensity continuous training on body composition in overweight and obese adults: a systematic review and meta-analysis. Obes Rev. 2017;18(6):635–646.
2. Viana RB, Naves JPA, Coswig VS, et al. Is interval training the magic bullet for fat loss? A systematic review and meta-analysis comparing moderate-intensity continuous training with high-intensity interval training (HIIT). Br J Sports Med. 2019;53(10):655–664.
3. Gibala MJ, Little JP, Macdonald MJ, Hawley JA. Physiological adaptations to low-volume, high-intensity interval training in health and disease. J Physiol. 2012;590(5):1077–1084.
4. Tabata I, Nishimura K, Kouzaki M, et al. Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max. Med Sci Sports Exerc. 1996;28(10):1327–1330.
5. LaForgia J, Withers RT, Gore CJ. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. J Sports Sci. 2006;24(12):1247–1264.
6. Weston KS, Wisløff U, Coombes JS. High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis. Br J Sports Med. 2014;48(16):1227–1234.