High-Intensity Interval Training in Low Versus Normal Oxygen: Effects on Body Fat and Gut Health in Adults Aged 50-75 With Overweight or Obesity (HYPOXFAT) (HYPOXFAT)

High-intensity Interval Training Under Hypoxic Versus Normoxic Conditions: Effects on Fat Mass Loss and Gut Microbiota in Adults Aged 50-75 With Overweight or Obesity

Overweight and obesity are major public health challenges worldwide, and their prevalence increases with age, partly because of age-related metabolic and hormonal changes. Beyond total fat mass, abdominal and particularly visceral fat accumulation is strongly associated with chronic low-grade inflammation, insulin resistance, hypertension and cardiovascular disease. Men are more likely to display an android fat distribution, while premenopausal women generally show a more protective gynoid distribution. However, declining sex hormone concentrations after menopause promote abdominal and visceral fat accumulation, increasing cardiometabolic risk.

Physical activity is a cornerstone of overweight and obesity management. High-intensity interval training (HIIT) is an effective strategy for reducing total, abdominal and visceral fat and may produce greater benefits than moderate-intensity continuous training. Since 2016, our team has conducted several studies and reviews showing that HIIT performed on a cycle ergometer or treadmill, alone or combined with resistance training, reduces total and abdominal fat mass in people with overweight or obesity, regardless of age, sex or hormonal status.

HIIT can also be performed under real or simulated hypoxic conditions to enhance cardiovascular, respiratory, circulatory, haematological and skeletal muscle adaptations. The Live Low-Train High model allows participants to live under normoxic conditions while being exposed to hypoxia only during exercise. Hypoxic HIIT has proved feasible and safe in people with metabolic disorders or physical deconditioning, including at simulated altitudes of 4,000-5,000 m. However, only two studies have suggested that hypoxic HIIT may enhance fat mass loss in people with overweight or obesity. Neither specifically assessed abdominal or visceral fat, and neither focused on postmenopausal women.

Several mechanisms may contribute to HIIT-induced fat loss, including increased catecholaminergic stimulation and lipid oxidation during recovery, excess post-exercise oxygen consumption and changes in appetite regulation. The gut microbiota may also contribute to the regulation of adipose tissue and cardiometabolic health. Physical activity can increase bacterial diversity and promote beneficial taxa, particularly short-chain fatty acid-producing bacteria. Our previous HIIT studies identified associations between changes in selected bacteria and reductions in total or abdominal fat mass. Hypoxic exposure may also alter gut microbiota composition and abundance, with effects depending on the severity and type of hypoxia.

This study will assess the effects of a 3-month cycle-ergometer HIIT programme, performed three times per week under hypoxic versus normoxic conditions, in adults aged 50-75 years with overweight or obesity. The primary objective is to determine whether hypoxic HIIT produces a greater reduction in abdominal and visceral fat mass than the same training performed in normoxia.

Secondary outcomes will include total and regional fat and lean mass; fasting glucose, insulin, HbA1c and HOMA-IR; total cholesterol, triglycerides, HDL-C, LDL-C, ApoB, ApoA1 and the ApoB/ApoA1 ratio; high-sensitivity C-reactive protein and fibrinogen; systolic and diastolic blood pressure; blood biophysical properties; and faecal microbiota richness, diversity and taxonomic composition. Lipid oxidation during prolonged moderate-intensity exercise, maximal oxygen uptake and maximal aerobic power will also be assessed before and after the intervention.

We hypothesise that hypoxic training will enhance improvements in body composition, metabolic and cardiovascular risk factors, aerobic fitness and lipid oxidation compared with normoxic training. We further hypothesise that hypoxia will modify faecal microbiota composition and that changes in selected bacterial taxa will be associated with changes in body composition and cardiovascular risk markers.

The ultimate aim is to improve the management of adults aged 50-75 years with overweight or obesity by developing an effective 3-month HIIT programme performed under controlled hypoxic conditions.

Study Overview

• Status: Recruiting
• Conditions: Obesity & Overweight; Fat Mass
• Intervention / Treatment: Behavioral: Hypoxic High-Intensity Interval Training; Behavioral: Normoxic High-Intensity Interval Training

• Study Type: Interventional
• Enrollment (Estimated): 32
• Phase: Not Applicable

Contacts and Locations

• Study Contact: Name: Nathalie BOISSEAU, Professor; Phone Number: +33684331693; Email: nathalie.boisseau@uca.fr

Study Locations

• France
  • Bellerive-sur-Allier, France, 03321
    • Recruiting
    • CREPS Auvergne Rhône-Alpes / Vichy
    • Contact: Nathalie BOISSEAU, Pr; Phone Number: +33 0473405519; Email: nathalie.boisseau@uca.fr

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

Aged 50-75 years, inclusive Male or postmenopausal female, defined as no menstruation during the previous 12 months Body mass index between 25 and 35 kg/m², inclusive Non-smoker or smoking no more than two cigarettes per day Stable body weight for at least three months before study initiation, defined as a change of less than 4 kg Stable dietary habits and physical activity level for at least three months before study initiation Willing to maintain usual dietary habits and physical activity level throughout the study Able and willing to comply with the study protocol Able and willing to provide written informed consent Affiliated with a social security scheme Willing to be registered in the French National Register of Volunteers Participating in Biomedical Research

Exclusion Criteria:

Medical or surgical history considered by the investigators to be incompatible with participation in the study Any medical condition considered by the investigators to be incompatible with participation in the study Medical contraindication to vigorous physical activity Current use of hormone replacement therapy Current use of beta-blocker therapy Diagnosed moderate-to-severe sleep apnoea syndrome that is untreated or inadequately controlled High-risk cardiovascular history, including symptomatic coronary artery disease, recent myocardial infarction or stroke, unstable heart failure, clinically significant arrhythmia, or uncontrolled hypertension Severe respiratory disease, including advanced chronic obstructive pulmonary disease classified as group B or E according to the 2023 GOLD classification, chronic respiratory failure, or uncontrolled asthma Diabetes of any type Chronic kidney disease stage 3B, 4 or 5 according to the KDIGO classification Severe liver disease, defined by an international normalised ratio greater than 1.2-1.3 Antibiotic treatment during the three months preceding the first stool collection More than eight hours of moderate-to-vigorous physical activity per week Any treatment that, in the investigators' opinion, could interfere with assessment of the study outcomes Low-energy diet during the three months preceding the study or intention to begin an energy-restricted diet during the study Extreme dietary habits Stay at an altitude above 2,500 m for more than eight days during the three months preceding the study Participation in another clinical study or currently within an exclusion period following participation in a previous clinical study Total compensation received for participation in research since the beginning of the calendar year exceeding €4,500, or the applicable regulatory limit Linguistic or psychological inability to provide informed consent Refusal to provide written informed consent Legally protected adult Deprived of liberty by an administrative or judicial decision Under guardianship or curatorship

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Hypoxic HIIT (HIIT-H); Participants will complete a 12-week cycle-ergometer HIIT programme, with three sessions per week, under normobaric hypoxia (FiO₂: 15%; simulated altitude: 3,000 m).	Behavioral: Hypoxic High-Intensity Interval Training; Participants will complete 36 cycle-ergometer HIIT sessions over 12 weeks (three sessions per week) under normobaric hypoxia (FiO₂: 15%; simulated altitude: 3,000 m). Each session will include a 5-minute warm-up followed by 60 repetitions of 8 seconds of acceleration at approximately 80-85 rpm and 12 seconds of deceleration at approximately 60 rpm. The 8-second efforts will be performed at 75-80% of maximal aerobic power.; Other Names: Hypoxic HIIT; HIIT-H
Active Comparator: Normoxic HIIT (HIIT-N); Participants will complete the same 12-week cycle-ergometer HIIT programme, with three sessions per week, under normoxic conditions (FiO₂: 21%).	Behavioral: Normoxic High-Intensity Interval Training; Participants will complete 36 cycle-ergometer HIIT sessions over 12 weeks (three sessions per week) under normoxic conditions (FiO₂: 21%). Each session will include a 5-minute warm-up followed by 60 repetitions of 8 seconds of acceleration at approximately 80-85 rpm and 12 seconds of deceleration at approximately 60 rpm. The 8-second efforts will be performed at 75-80% of maximal aerobic power.; Other Names: Normoxic HIIT; HIIT-N

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Relative Change in Total Abdominal Fat Mass	Total abdominal fat mass will be measured by dual-energy X-ray absorptiometry (DXA) at baseline and after the 12-week intervention. The relative change will be calculated as: (abdominal fat mass at 12 weeks - abdominal fat mass at baseline) / abdominal fat mass at baseline. More negative values indicate a greater reduction in abdominal fat mass. The change will be compared between the hypoxic and normoxic HIIT groups.	Baseline and immediately after the 12-week intervention

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Relative Change in Body Mass	Description: Body mass will be measured in kilograms at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between the hypoxic and normoxic HIIT groups.	Baseline and after 12 weeks of intervention
Relative Change in Visceral Abdominal Fat Mass	Description: Visceral abdominal fat mass will be measured by DXA at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in Total Lean Mass	Description: Total lean mass will be measured in kilograms by DXA at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in Waist-to-Hip Ratio	Description: The waist-to-hip ratio will be calculated by dividing waist circumference by hip circumference at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in metabolic Plasma parameters (TG, HBA1C, glucose, insulin, HDL, LDL)	Fasting concentration will be measured at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in Whole Blood Viscosity	Whole blood viscosity will be assessed using standardised haemorheological procedures at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in Red Blood Cell Deformability	Description: Red blood cell deformability will be assessed using standardised haemorheological procedures at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in Red Blood Cell Aggregation	Description: Red blood cell aggregation will be assessed using standardised haemorheological procedures at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Change in the Relative Abundance of Prespecified Gut Bacterial Taxa	Description: The relative abundance of prespecified bacterial taxa will be determined from faecal microbiota samples collected at baseline and after the 12-week intervention. Changes in relative abundance will be compared between the hypoxic and normoxic HIIT groups.	Baseline and after 12 weeks of intervention
Change in Faecal Gut Microbiota Alpha Diversity	Description: Alpha-diversity indices reflecting within-sample bacterial richness and diversity will be calculated from faecal microbiota samples collected at baseline and after the 12-week intervention. Changes will be compared between the hypoxic and normoxic HIIT groups.	Baseline and after 12 weeks of intervention
Change in Faecal Gut Microbiota Beta Diversity	Description: Beta diversity, reflecting differences in bacterial community composition, will be assessed using faecal samples collected at baseline and after the 12-week intervention. Changes in community composition will be compared between the hypoxic and normoxic HIIT groups.	Baseline and after 12 weeks of intervention
Relative Change in Maximal Aerobic Power	Description: Maximal aerobic power, expressed in watts, will be determined during a maximal exercise test at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in Lipid Oxidation Rate During Submaximal Exercise	Description: Lipid oxidation rate, expressed in grams per minute, will be calculated by indirect calorimetry during 45 minutes of fasted exercise performed at 50% of V̇O₂max, at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention
Relative Change in Carbohydrate Oxidation Rate During Submaximal Exercise	Description: Carbohydrate oxidation rate, expressed in grams per minute, will be calculated by indirect calorimetry during 45 minutes of fasted exercise performed at 50% of V̇O₂max, at baseline and after the 12-week intervention. Relative change will be calculated as: (week 12 value - baseline value) / baseline value and compared between groups.	Baseline and after 12 weeks of intervention

Collaborators and Investigators

• Sponsor: Laboratoire des Adaptations Métaboliques à l'Exercice en conditions Physiologiques et Pathologiques
• Collaborators: CREPS Auvergne Rhône-Alpes (AURA)

Study record dates

Study Major Dates

• Study Start (Actual): August 26, 2026
• Primary Completion (Estimated): July 5, 2028
• Study Completion (Estimated): July 5, 2028

Study Registration Dates

• First Submitted: August 27, 2026
• First Submitted That Met QC Criteria: August 27, 2026
• First Posted (Actual): September 1, 2026

Study Record Updates

• Last Update Posted (Actual): September 1, 2026
• Last Update Submitted That Met QC Criteria: August 27, 2026
• Last Verified: August 1, 2026

More Information

Terms related to this study

• Keywords: health; obesity; overweight; HIIT; fat mass; exercise prescription; normobaric hypoxia
• Additional Relevant MeSH Terms: Nutrition Disorders; Overnutrition; Body Weight; Pathological Conditions, Signs and Symptoms; Nutritional and Metabolic Diseases; Signs and Symptoms; Overweight; Obesity
• Other Study ID Numbers: ID-RCB : 2025-A01744-45

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No