Fitness and therapeutic potential of intermittent hypoxia training: a matter of dose

The introduction of different methods of intermittent hypoxic training (IHT) into fitness, sports, military and medical practice has raised a lot of questions about the most beneficial regimens of such treatment and their optimal instrumental implementation. Low doses of hypoxia might not be sufficient stimuli to mobilize adaptive mechanisms, while severe or prolonged hypoxia may provoke dangerous pathological processes. In this review, we pay attention to narrow practical question of the most effective and convenient technology of IHT implementation, notably the inhalation of hypoxic gas mixtures. Data strongly suggest that in humans the training with 15-13% inhaled oxygen (FiO2) at various time characteristics does not provide marked positive changes. Short-term daily sessions consisting 3-4 bouts of 5-7 min exposures to 12-10% FiO2 alternating with equal durations of normoxia for 2-3 weeks have been shown as a most beneficial without maladaptive consequences for fitness and treatment of some diseases. More severe or longer intermittent hypoxia protocols must be accompanied by strict monitoring of blood oxygen saturation (SpO2), electrocardiogram, breathing pattern and arterial blood pressure in order to avoid unexpected undesirable individual reactions. For sports purposes, the reduction of oxygen content to individually tolerable level for some minutes is justified as far as it maximizes benefits. However, such regimen requires preliminary diagnostics of individual hypoxic tolerance and cardio-respiratory reactivity as well as rigorous monitoring of vital functions during IHT and good feedback device. The use of oxygen concentrations below 12% for treatment of diseases, especially in children and the elderly, are required substantial additional research. Recently, a new mode of adaptive training was explored, which combines periods of hypoxia (12-10% FiO2) and hyperoxia (30-35% FiO2). Limited evidences suggest that such regime can reduce the time of recovery periods, that is shorten the duration of sessions. However, there is still no strong comparative evidence for humans that this method is much more efficient than hypoxic-normoxic mode. We appeal to all scientists working in the field of IHT not to hide their negative results but publish all observations in the open press. It will make a significant contribution in developing of common guidelines for IHT implementation to improve public health of our Planet.