The influence of dietary manipulation on plasma ammonia accumulation during incremental exercise in man

Summary

The influence of a pattern of exercise and dietary manipulation, intended to alter carbohydrate (CHO) availability, on pre-exercise acid-base status and plasma ammonia and blood lactate accumulation during incremental exercise was investigated. On three separate occasions, five healthy male subjects underwent a pre-determined incremental exercise test (IET) on an electrically braked cycle ergometer. Each IET involved subjects exercising for 5 min at 30%, 50%, 70% and 95% of their maximal oxygen uptake ($\dot VO_{2max}$) and workloads were separated by 5 min rest. The first IET took place after 3 days of normal dietary CHO intake. The second and third tests followed 3 days of low or high CHO intake, which was preceded by prolonged exercise to exhaustion in an attempt to deplete muscle and liver glycogen stores. Acid-base status and plasma ammonia and blood lactate levels were measured on arterialised venous blood samples immediately prior to and during the final 15 s of exercise at each workload and for 40 min following the completion of each IET. Three days of low CHO intake resulted in the development of a mild metabolic acidosis in all subjects. Plasma ammonia (NH₃) accumulation on the low-CHO diet tended to be greater than normal at each exercise workload. Values returned towards resting levels during each recovery period. After the normal and high-CHO diets plasma NH₃ levels did not markedly increase above resting values until after exercise at 95% $\dot VO_{2max}$. Plasma NH₃ levels after the high-CHO diet were similar to those after the normal CHO diet. Blood lactate levels did not markedly increase with each treatment until after exercise at 70% $\dot VO_{2max}$. There was a trend for blood lactate accumulation after the low-CHO diet to be lower than that after the normal and high-CHO diets during the IET and for the majority of the recovery period. Levels after the high-CHO diet tended to be similar to those after the normal diet. The present experiment demonstrates that an alteration in pre-exercise substrate availability will influence the accumulation of plasma NH₃ during exercise. In this situation, the use of plasma NH₃ as an index of exercise intensity and the onset of fatigue should be viewed with some caution.