In 2016, overweight and obesity affected 1.9 billion adults worldwide. Of these, 650 million suffered from obesity, a figure that has almost tripled in 40 years. These conditions are often the result of a sedentary lifestyle, a poor diet (unbalanced, of poor quality and in excess of calories) and other socio-economic factors. Overweight and obesity have profound repercussions on health. Indeed, beyond their aesthetic appearance, overweight and obesity are closely associated with an abnormal ectopic fat deposit, visceral fat mass, which itself is a major risk factor for a multitude of chronic diseases, including metabolic syndrome, cardiovascular disease, type 2 diabetes and certain types of cancer.
Obesity is a complex condition resulting primarily from a lasting imbalance between energy consumed and energy expended. Physical exercise is one of the cornerstones of obesity prevention and management. The intensity of exercise is particularly crucial, as it influences changes in fat oxidation during activity. Over the past twenty years, maximal fat oxidation and the exercise intensity at which this maximal oxidation occurs (FatMAX, as a percentage of VO2MAX) have become hotly debated topics in the scientific literature for use as tools in weight management, metabolic health and choice of training type. Some authors have determined that FatMAX corresponds to 43 ± 18% of VO2PEAK in men and 61 ± 10% of VO2PEAK in women, in overweight and obese populations (VO2PEAK is a similar variable but lower than VO2MAX, more suitable for evaluation in populations with health problems). Observations show that high maximal fat oxidation is positively correlated with weight loss maintenance. Moreover, increased fat oxidation is beneficial for improving insulin sensitivity and reducing metabolic risk factors. In addition, the rate of fat oxidation during exercise provides an indication of metabolic flexibility, i.e. the body's ability to adapt to changes in metabolic or energy demand as a function of the physical activity performed. Overweight and obese people seem to have less metabolic flexibility than people of normal weight.
Maximum fat oxidation and FatMAX are also highly dependent on a number of factors, such as age, gender, body fat percentage, cardiorespiratory fitness level and type of exercise performed. The scientific literature suggests that maximal fat oxidation can be enhanced by both moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT). The increase in fat oxidation induced by MICT results from adaptations in adipose tissue lipolysis, muscle triglyceride lipolysis and/or fatty acid uptake by mitochondria. HIIT, on the other hand, improved fat oxidation by, among other things, increasing enzymatic activity and the number of mitochondria. However, the results of studies concerning the effectiveness of HIIT and MICT in improving maximal fat oxidation are sometimes incomplete and offer only a partial answer. So, can these interventions be adapted to overweight or obese adults? Which is more beneficial, MICT or HIIT?
To answer these questions, an international team of researchers carried out a meta-analysis to compare the effects of MICT and HIIT on maximum fat oxidation in overweight and obese people. To do this, the researchers analyzed the results of 13 randomized controlled trials involving 519 overweight or obese people (274 women and 245 men) aged between 19 and 57. Participants were divided into 3 groups: HIIT (136), MICT (235) and Control (148). Three articles evaluated the effects of HIIT compared with a control group. Six articles evaluated the effects of MICT compared with a control group. And 4 articles compared the effects of HIIT and MICT on maximal fat oxidation.
Training protocols lasted at least 2 weeks. HIIT sessions were performed at an intensity of at least 75% of VO2PEAK. MICT sessions were performed at a moderate intensity equivalent to 45-75% VO2PEAK. Maximum fat oxidation was assessed during submaximal or progressive continuous exercise tests using indirect calorimetry before and after the interventions.
The main results of this study show that both HIIT and MICT are effective in improving maximal fat oxidation in overweight and obese individuals. HIIT delivered an average fat oxidation rate of 0.07 g/min, while MICT delivered an average fat oxidation rate of 0.1 g/min. In obese people, regardless of the training method used, the results were statistically similar.
In sub-analyses, the researchers found that MICT, when not performed at FatMAX, was more effective in improving maximal fat oxidation (0.13 g/min) in overweight and obese people than when performed at FatMAX (0.04 g/min). As indicated at the beginning of this article, for overweight and obese populations, FatMAX intensity would be assumed to be between 43% and 61% of VO2PEAK. The data from this meta-analysis show that the optimum intensity for inducing an increase in maximal fat oxidation during exercise is 65-70% of VO2PEAK.
The results also show that MICT improves maximal fat oxidation more in overweight people (0.13 ± 0.03 g/min) than in obese people (0.04 ± 0.01 g/min). Other studies have also observed a positive correlation between maximum fat oxidation and BMI, with a higher rate of oxidation observed in obese adults compared to normal-weight and overweight adults, suggesting that the overweight population may have greater potential to improve fat oxidation rates compared to obese adults following training.
Finally, the results showed that there was no significant difference between the MICT for running (0.09 ± 0.02 g/min) and the MICT for cycling (0.10 ± 0.03 g/min). However, some studies have shown that running can induce greater maximal fat oxidation than cycling. This may be linked to the amount and type of muscle primarily recruited by the different modes of exercise. However, for overweight or obese people who do not engage in regular physical activity, it may be preferable to start with a less traumatic activity such as cycling.
This study highlights the effectiveness of HIIT and MICT in improving maximal fat oxidation during exercise in overweight or obese people. The choice between HIIT and MICT should not be based solely on physiological effects but should take into account individual preferences, advantages such as time efficiency, and potential disadvantages such as long-term adherence challenges with HIIT. For obese adults, HIIT is more demanding in terms of perception but also physiologically. Some studies have observed a lack of enjoyment compared to HIIT, a higher risk of lower-limb muscle and joint damage, and for some very intense HIIT protocols dizziness and nausea. Regarding long-term adherence, a recent study compared HIIT and MICT, over a 12-month follow-up period. Those initially assigned to HIIT tended to exercise at lower intensities than prescribed, and HIIT showed no advantage in terms of long-term adherence.
Analyzing the results of MICT-related studies, the researchers found that an optimal increase in the maximal rate of fat oxidation could be achieved with an intensity of 65-70% VO2PEAK, at three 60-minute sessions per week. For HIIT, the ideal protocol for overweight or obese people is less clear because of the many HIIT formats, but in general, 3 weekly sessions of 15 to 45 min each, featuring 3 to 5 repetitions of 20s to 4 min at an intensity ranging from 85% of maximal heart rate to all-out sprints, with inter-set rests of 60 to 180s.
And don't forget, as we reminded you in another article, exercise alone, i.e. without an associated, calibrated diet, which does not compensate for the caloric deficit created by exercise, is not enough for effective fat loss. Nevertheless, cardiovascular exercise can improve cardiac, metabolic and cognitive functions, help preserve muscle mass during periods of caloric restriction, and facilitate fat loss if (and only if) a diet adapted in quality and quantity is respected.
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