Ammonia accumulation during highly intensive long-lasting cycling: individual observations.

In a number of individual cycling tests lasting 2.5-5 h with alternating exercise intensities of 50%-85% of maximal working capacity, it was observed that plasma ammonia levels may rise above 250 mumol/l when reaching exhaustion, while lactate levels remain relatively low. Acute quantitative ammonia production during intensive endurance exercise may be enhanced by a reduced glycogen availability in muscle. However, adequate amounts of glycogen itself do not prevent ammonia production when exercise is at high intensity and long-lasting. The continuous ammonia accumulation in blood during endurance exercise in trained individuals may be the result of a relatively low blood flow to the liver and thereby low clearance in contrast to lactate which may not accumulate due to a high clearance rate in both active and nonactive oxidative muscle fibers. In a number of subjects it was observed that exhaustion, when performing endurance exercise at high exercise intensities, occurred when plasma ammonia levels were high. Muscle cramps occurred in subjects who reached their highest individual ammonia values and seemed not to be related to serum potassium, plasma lactate, or muscle glycogen. These individual observations give rise to the hypothesis that high intramuscular ammonia levels may be related to the etiology of muscle exhaustion and muscle cramping during highly intensive endurance exercise.