The obesity epidemic calls for new therapeutic opportunities to prevent and treat obesity and its comorbidities amongst which are insulin resistance and cardiovascular diseases. Recent evidence suggests that tissue oxygenation plays an important role in cardiometabolic health. Remarkably, individuals residing at high altitude (hypobaric hypoxia) are less prone to develop type 2 diabetes mellitus as compared to individuals living at sea-level (normobaric normoxia). Furthermore, there is evidence to suggest that normobaric hypoxia exposure may improve glucose homeostasis and insulin sensitivity in both rodents and humans.

The level of physical activity is an important determinant of insulin sensitivity and glucose homeostasis. It is well established that performing physical activity improves glucose uptake in the short term, and glycemic control in the long term. Interestingly, recent studies have demonstrated that an acute bout of exercise under hypoxic conditions (inhalation of air containing less oxygen) may lead to a more pronounced improvement in plasma glucose concentrations and/or insulin sensitivity as compared to normoxic exercise. However, the effects of repeated hypoxic exercise bouts on glucose profile throughout the day (i.e. 24h continuous glucose monitoring) remain elusive. In the present randomized, placebo-controlled, single-blind, cross-over study study, the investigators will investigate the effects of exercise under mild normobaric hypoxic conditions (FiO2, 15%) for 4 consecutive days (2 x 30-min cycling session at 50% WMAX) on postprandial substrate metabolism and 24h-glucose level in overweight/obese subjects with impaired glucose tolerance. The investigators hypothesize that 4 consecutive days of exposure to mild hypoxia while performing moderate intensity exercise improves glucose homeostasis in overweight and obese individuals with impaired glucose homeostasis.

In the present randomized, single-blind, placebo-controlled cross-over study, subjects will be exposed to normobaric 1) mild hypoxia (oxygen level: 15%) and 2) normoxia (oxygen level: 21%) during exercise (2 x 30min/day on a cycle ergometer) of the same relative exercise intensity (equal to 50%WMAX under normoxic conditions) for 4 consecutive days. Subjects will be randomly assigned to each condition (computer-generated randomization plan; block size, n=4), separated by a washout period (3-6 weeks). To accomplish this, subjects will exercise in an oxygen chamber in which oxygen concentration of the ambient air and, as such, oxygen levels can be tightly controlled and monitored. Subjects will cycle two times a day for 30 minutes at 50% WMAX, determined by an incremental workload test. Since we will allow 5-10 min for subjects to get ready to start the 30-min exercise session, and take into account a 5-min cooling down period before leaving the hypoxic room again, subjects will be in the room for 45 min for each session.

After initial screening, subjects are asked to visit the university for two periods of 5 consecutive days each with a washout period of 3-6 weeks. During the first 4 days (time investment: 4.5 hours/day), subjects will be undergoing the exercise regimen, as described above.

• At day 1, on the first morning of each regimen, a glucose sensor (Enlite Glucose Sensor MiniMed; Medtronic). The sensor will be inserted subcutaneously, will be inserted subcutaneously, at 5 cm from the umbilicus, on the right side of the abdomen, and will be connected to a continuous glucose monitor (iPro2 Professional CGM MiniMed; Medtronic, Northridge, CA, USA). The sensor will remain inserted throughout the study (days 1-5). Furthermore, a physical activity monitor (ActivPAL3 micro monitor) will be applied at the same moment, to monitor physical activity of participants. At the end of day 5, the glucose sensor, and the physical activity monitor will be removed.
• At days 1-5 (time investment: 4.5 hours), fasting blood samples will be collected to determine plasma metabolites and inflammatory markers, and blood pressure and body weight will be monitored.
• At day 5 (time investment: 8 hours), a mixed liquid meal challenge will be performed to determine fasting and postprandial metabolite concentrations, and substrate oxidation (using indirect calorimetry). A skeletal muscle biopsy (m. vastus lateralis) will be collected under fasting conditions. Moreover, HOMA-IR will be used to estimate insulin resistance, using fasting plasma glucose and insulin values measured on the day after completion of the 4 day regimen.

After initial screening, the assessment of basal metabolic rate (BMR) and the incremental workload test (to determine the maximal workload, WMAX), subjects will have to invest approximately 52 hours.

Inclusion Criteria:

• overweight or obese (BMI >28 kg/m2)
• impaired glucose tolerance (2h glucose: >7.8 - 11.1 mmol/L)
• subjects have to be weight-stable for at least 3 months prior to participation (no change in bodyweight: <3kg change)

Exclusion Criteria:

• cardiovascular disease (determined by questionnaire, blood pressure (Subjects with moderate to severe hypertension (grade 2 or 3 based on WHO criteria)
• type 2 diabetes mellitus
• cancer
• asthma
• bronchitis
• chronic obstructive pulmonary disease
• lung fibrosis
• obstructive sleep apnea
• use of oxygen at home situation
• resting SpO2 ≤93%
• abnormal pre-bronchodilator forced expiratory volume (FEV1) and forced vital capacity (FVC)
• liver or kidney malfunction (determined based on ALAT and creatinine levels, respectively)
• disease with a life expectancy shorter then 5 years
• lactose intolerance
• abuse of products (alcohol consumption > 15 units/week)
• smoking
• plans to lose weight (subjects will be asked if they have weight loss plans (e.g. to increase their physical activity level or change diet)
• use of high doses of anti-oxidant vitamins
• use of any medication that influences glucose metabolism and inflammation
• shift working