Cycling for Weight Loss May Clear Carbohydrates Rather Than Fat, Irrespective of Normal or Mildly Reduced Normobaric Oxygen

A single-center randomized, controlled cross-over exercise intervention in 20 women willing to reduce weight (20–40 y, BMI: 27.4 ± 2.1), with the aim to examine potential benefits for weight loss under normal (N-Ox: 20.9%) and mildly reduced (R-Ox: 17.0%) normobaric oxygen in an “Altitude Simulation Chamber”. O2 consumption (VO2), CO2 production (VCO2), blood oxygen saturation (SaO2), blood glucose and lactate (mM) were studied before, during and after cycling for 22 min at a mean personalized workload of 54.2 ± 11.7 watts, about 40% of VO2max. Despite lower absolute SaO2 values and a greater decline from rest to exercise under R-Ox (time x treatment interaction p < 0.01), VO2 did not differ from N-Ox (time x treatment interaction p = 0.178). Average net VO2, 13.8 mL O2 per watt, reflected fairly normal aerobic cycling, irrespective of O2 regime. The Respiratory Exchange Ratio (RER; VO2/VCO2), 0.83 at rest, increased for both treatments to a ratio close to or beyond unity during and directly after exercise (treatment effect p = 0.407). The tendency of cycling for weight loss to clear carbohydrates rather than fat, irrespective of normal or mildly reduced normobaric oxygen, is discussed as a lactate-mediated and phenotype-specific consequence of apparent anaerobic glycolysis with adverse perspectives for weight loss and metabolic health.