Sodium bicarbonate reduces ventilation without altering core temperature threshold or sensitivity of hyperthermia-induced hyperventilation in exercising humans

Hyperthermia stimulates ventilation (hyperthermia-induced hyperventilation). In exercising humans, once the core temperature reaches ~37°C, minute ventilation (V E ) increases linearly with rising core temperature, and the slope of the relation between V E and core temperature reflects the sensitivity of the response. We previously reported that sodium bicarbonate ingestion reduces V E during prolonged exercise in the heat without affecting the sensitivity of hyperthermia-induced hyperventilation. Here, we hypothesized that reductions in V E associated with sodium bicarbonate ingestion reflect elevation of the core temperature threshold for hyperthermia-induced hyperventilation. Thirteen healthy young males ingested sodium bicarbonate (0.3 g/kg body weight) (NaHCO 3 trial) or sodium chloride (0.208 g/kg body weight) (NaCl trial), after which they performed a cycle exercise at 50% of peak oxygen uptake in the heat (35°C and 50% relative humidity) following a pre-cooling. The pre-cooling enabled detection of an esophageal temperature ( T es : an index of core temperature) threshold for hyperthermia-induced hyperventilation. The T es thresholds for increases in V E were similar between the two trials (P = 0.514). The slopes relating V E to T es also did not differ between trials (P = 0.131). However, V E was lower in the NaHCO 3 than the NaCl trial in the range of T es = 36.8-38.4°C (P < 0.007, main effect of trial). These results suggest that sodium bicarbonate ingestion does not alter the core temperature threshold or sensitivity of hyperthermia-induced hyperventilation during prolonged exercise in the heat; instead, it downshifts the exercise hyperpnea.