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Mirjam Jeanette Limmer, Markus de Marées, Petra Platen

• \(\bf Background\) The present study investigated the effects of chronic sodium bicarbonate (NaHCO\(_3\)) ingestion on a single bout of high-intensity exercise and on acid-base balance during 7-day high-altitude exposure. \(\bf Methods\) Ten recreationally active subjects participated in a pre-test at sea level and a 7-day hiking tour in the Swiss Alps up to 4554 m above sea level. Subjects received either a daily dose of 0.3 g/kg NaHCO\(_3\) solution (\(\it n\) = 5) or water as a placebo (\(\it n\) = 5) for 7 days. Anaerobic high-intensity exercise performance was assessed using the portable tethered sprint running (PTSR) test under normoxic and hypoxic conditions (3585 m). PTSR tests assessed overall peak force, mean force, and fatigue index. Blood lactate levels and blood gas parameters were assessed pre- and post-PTSR. Urinary pH and blood gas parameters were further analyzed daily at rest in early morning samples under normoxic and hypoxic conditions. \(\bf Results\) There were no significant differences between the bicarbonate and control group in any of the PTSR-related parameters. However, urinary pH (\(\it p\) = 0.003, \(\eta\)p\(^{2}\) = 0.458), early morning blood bicarbonate concentration (\(\it p\) < 0.001, v = 0.457) and base excess (\(\it p\) = 0.002, \(\eta\)p\(^{2}\) = 0.436) were significantly higher in the bicarbonate group compared with the control group under hypoxic conditions. \(\bf Conclusions\) These results indicate that oral NaHCO\(_3\) ingestion does not ameliorate the hypoxia-induced impairment in anaerobic, high-intensity exercise performance, represented by PTSR-related test parameters, under hypobaric, hypoxic conditions, but the maximal performance measurements may have been negatively affected by other factors, such as poor implementation of PTSR test instructions, pre-acclimatization, the time course of hypoxia-induced renal [HCO\(_3\)\(^{−}\)] compensation, changes in the concentrations of intra- and extracellular ions others than [H\(^{+}\)] and [HCO\(_3\)\(^{−}\)], or gastrointestinal disturbances caused by NaHCO\(_3\) ingestion. However, chronic NaHCO\(_3\) ingestion improves blood bicarbonate concentration and base excess at altitude, which partially represent the blood buffering capacity.