THE INFLUENCE OF DIETARY MANIPULATION ON PLASMA AMMONIA ACCUMULATION DURING INCREMENTAL EXERCISE IN MAN

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 (VO2max) 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% VO2max. 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% VO2max. 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.