The acute reversal of a diet-induced metabolic acidosis does not restore endurance capacity during high-intensity exercise in man

The present experiment was designed to investigate whether a diet-induced metabolic acidosis was a major factor in the earlier onset of fatigue during high-intensity exercise. Six healthy males cycled to exhaustion at a workload equivalent to 95% of maximum oxygen uptake on four separate occasions. Exercise tests were performed after an overnight fast and each test was preceded by one of four experimental conditions. Two experimental diets were designed, either to replicate each subject's own normal diet [N diet, mean (SD) daily energy intake (E) = 13 (0.7) MJ, 14.5 (0.8)% protein (Pro), 37.5 (2.2)% fat (Fat) and 47.5 (2.1)% carbohydrate (CHO)], or a low-carbohydrate diet [E = 12.6 (0.8) MJ, 33.6 (1.3)% Pro, 64.4 (1.5)% Fat and 2.2 (0.4)% CHO]. These diets were prepared and consumed within the department over a 3-day period. Over a 3-period prior to the exercise trial subjects ingested either NaHCO₃ or CaCO₃ (3.6 and 3.0 mmol · kg body mass), thus giving four experimental conditions: N diet and treatment, N diet and placebo, low-CHO diet and treatment and low-CHO diet and placebo. Treatments were assigned using a randomised protocol. Arterialised venous blood samples were taken for the determination of acid-base status and metabolite concentrations at rest prior to exercise and at intervals for 30 min following exhaustion. Consumption of the low-CHO diet induced a mild metabolic acidosis which was reversed by the ingestion of NaHCO₃. Blood pH, bicarbonate (HCO₃) and base excess (BE) were higher following NaHCO₃ ingestion after the normal diet than all of the other experimental conditions (P < 0.01). Exercise time following the low-CHO diet was less than on the normal diet conditions (P < 0.05): bicarbonate ingestion had no effect on exercise time on either of the diet conditions. Post-exercise blood pH, HCO₃ ^- and BE were higher following the ingestion of NaHCO₃ irrespective of the pre-exercise diet (P < 0.05). Blood lactate concentration was higher 2 min after exercise following the N diet with NaHCO₃ when compared to the low-CHO diets with either NaHCO₃ or placebo (P < 0.05). Plasma ammonia accumulation was not significantly different between experimental conditions. These data confirm previous data showing that the ingestion of a low-CHO diet reduces the capacity to perform high-intensity exercise, but it appears that the metabolic acidosis induced by the low-CHO diet is not the cause of the reduced exercise capacity observed during high-intensity exercise under these conditions.