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Interval training vs. classic "cardio": difference in fat oxidation

To improve their fitness and health, many people engage in cardiovascular activities. The WHO recommends at least 150 minutes of moderate intensity physical activity or at least 75 minutes of vigorous activity per week. Unfortunately, it is estimated that more than 1.4 billion people do not meet these thresholds. Since lack of time is the main barrier mentioned by these people, for more than a decade, many studies have been devoted to exploring the health potential of short duration efforts.

HIIT (High-Intensity Interval Training) has thus become very popular with the general public and has appeared as a miracle solution to improve one's VO_2MAX, metabolic health, and body composition, its main advantage being the duration of a session compared to Moderate-Intensity Continuous Training (MICT).

Interval training (IT) involves intense or maximal efforts of short duration, repeated several times in a session. This type of training has been shown to increase fatty acid oxidation. And better fat oxidation is generally associated with better metabolic health, especially in people with obesity. But how big is this effect on body fat? Is it greater than that achieved with Moderate-Intensity Continuous Training?

The Study

To answer these questions, an international team conducted a meta-analysis to compare the effects of IT and MICT on fat oxidation. To do this, they selected 18 studies involving 511 participants (287 women and 224 men) aged from 20 to 63 years old. The studies did not include trained athlete, the interventions had to last at least 2 weeks or 6 sessions, the intensities of the sessions were greater than or equal to 75% of VO_2MAX/VO_2PEAK/FC_PEAK/FC_PEAK, total or maximal fatty acid oxidation (FatOx) was assessed via indirect calorimetry measurement during continuous or incremental submaximal exercise, and finally, the effects of HIIT/SIT on FatOx were compared to those of MICT or a non-exercising control group.

Of these studies, 9 compared the effects of HIIT vs. MICT, 7 evaluated the effects of HIIT vs. MICT, and 2 included a control group and a MICT group.

Results & Analyzes

The main results of this study show that IT training increases fatty acid oxidation with an average increase of 0.05 to 0.13 g/min. These effects were significantly greater in overweight and obese individuals. Furthermore, a significant effect on FatOx can be expected after at least 4 weeks of training. The magnitude of the effect increases with the duration of the training protocol, with each additional week increasing FatOx by 0.004-0.017 g/min.

The effects on FatOx would appear to be very slightly greater for IT than for MICT (0.03-0.12 g/min), however this difference only becomes significant (+0.03 g/min) for individuals with body mass above the norm (>25 kg/m²). This could be explained by a higher level of intramuscular triglycerides and a higher plasma concentration of fatty acids.

Practical Applications

This meta-analysis shows once again that there is almost no difference between interval training and classic cardio in terms of the fat oxidation process, and ultimately fat loss. With an average FatOx of 0.10 g/min, at a rate of 150 min of effort per week, the loss of fat mass will be 780g in one year... Not really an exceptional result considering the investment in terms of effort.

The results reflect those of various meta-analyses on the subject, which lead to the same conclusion: exercise alone, i.e. without an associated and calibrated diet, which does not compensate for the caloric deficit created by the exercise, is not sufficient for effective fat loss. On average, the loss is 0.2 kg whether with an interval program or continuous effort [Steele et al., 2021]. This will have very little clinical or aesthetic effect. Both activities help to lose body fat through different physiological processes that take place in different ways, but the result is the same. And on the condition that the diet is controlled and that the energy intake is always the same.

The fact remains that cardiovascular exercise will improve cardiac, metabolic and cognitive functions, it can help preserve muscle mass during periods of caloric restriction, and it will facilitate fat loss if (and only if) a diet adapted in quality and quantity is respected. The choice of interval or continuous effort should therefore be based on personal preference: easy but long or short but hard. HIIT is unfortunately not the magic pill that we were all waiting for and that some people sell without scruples (or knowledge).

References

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