A ketogenic diet (KD) reduces daily carbohydrates (CHOs) ingestion by replacing most calories with fat. KD is of increasing interest among athletes because it may increase their maximal oxygen uptake (VO2max), the principal performance limitation at high altitudes. The investigators examined the tolerance of a 4-week isocaloric KD (ICKD) under simulated hypoxia and the possibility of evaluating ICKD performance benefits with a maximal graded exercise bike test under hypoxia and collected data on the effect of the diet on performance markers and arterial blood gases.

In a randomised single-blind cross-over model, 6 recreational mountaineers (age 24 to 44 years) will complete a 4-week ICKD followed or preceded by a 4-week habitual mixed western diet (HD). Performance parameters (VO2max, lactate threshold [LT], peak power [Ppeak]) and arterial blood gases (PaO2, PaCO2, pH, HCO3-) will be measured at baseline under two conditions (normoxia and hypoxia) as well as after a 4-week HD and 4-week ICKD under the hypoxic condition.

Inclusion Criteria:

• Recreational mountaineers
• Familiarity with altitude (>2500 m above sea level) and males/females 20 to 45 years old.

Exclusion Criteria:

• high training load (such as professional athletes) or new planned training
• dietary restrictions.