Metabolic and Nutritional Responses to Acute High-intensity Interval Exercise Performed Under Hypoxic vs. Normoxic Conditions at Matched External Workload in Adults With Overweight and Obesity. (HYPOXHIIT)

Obesity management involves lifestyle and dietary measures that include appropriate nutrition and increased physical activity (PA). Among the various PA options available, High-intensity interval training (HIIT) is now recognized as a well-tolerated, safe, and effective exercise modality for improving body composition. In 2016, our pioneering study demonstrated that HIIT is more effective than moderate-intensity continuous training (MICT) in inducing abdominal fat loss in postmenopausal women. A subsequent meta-analysis, conducted by our team in 2018 and encompassing 39 articles involving 617 participants, confirmed the significant effects of HIIT on reducing total and abdominal fat mass in overweight or obese individuals More recently, our work has shown significant results from HIIT + resistance training or HIIT cycling vs. running on improving body composition, and more specifically on reducing abdominal and visceral fat mass, in overweight or obese subjects.

With a focus on performance, HIIT has been combined with simulated hypoxia to maximize training adaptations, particularly to improve cardiovascular, respiratory, circulatory, hematological, and metabolic capacities at the muscular level. Live Low Train High (LLTH) allows athletes to maintain normoxia while being exposed to acute periods of hypoxia during training via hypoxic chambers (simulated altitude). More recently, research has explored the effects of HIIT under hypoxic conditions in patients with metabolic disorders and/or physical deconditioning, focusing primarily on cardiovascular and ventilatory adaptations. To date, in overweight or obese individuals, only three studies investigated the benefit of HIIT under simulated hypoxia for modulating body composition, and more specifically for reducing total or abdominal fat. These studies generally hypothesize a higher energy expenditure during hypoxia, associated with a transient loss of appetite that may be linked to hormonal disturbances, central nervous system adaptability, or metabolic flexibility altering hunger and satiety signals.

In parallel with these hypotheses, hypoxia stimulates HIF-1 production, thus promoting a shift from oxidative metabolism to glycolysis. At the same external mechanical load, the increased use of carbohydrates via glycolysis, combined with increased cardiovascular stress, should promote, during the recovery phase, greater oxygen consumption and lipid oxidation following a hypoxic session. To date, no study has yet evaluated these hypotheses by comparing these parameters following two acute HIIT sessions performed under normoxia or hypoxia.

The aim is to compare the influence of these two sessions on oxygen consumption, the corresponding energy expenditure, and carbohydrate and lipid utilization during recovery (+2h). Given the potential influence of hypoxia on appetite, food intake and spontaneous energy expenditure post-24h will also be measured. Finally, since enjoyment and perceived exertion are crucial parameters for adherence to an adapted physical activity (APA) program, these two concepts will also be assessed through the participants' experiences.

The authors propose the following hypotheses:

Main hypothesis:

1. Lipid oxidation (g/min) during recovery (+2h) will be higher post-hypoxic session.

   Secondary hypotheses:

2. Oxygen consumption and energy expenditure will be increased during post-hypoxic session recovery.

4/ Energy intake from the post-exercise meal may be lower post-hypoxic condition.

5/ The effort will be perceived as slightly more difficult in hypoxia due to increased cardiovascular demand.