The effects of diet on muscle pH and metabolism during high intensity exercise

Summary

Five healthy male subjects exercised for 3 min at a workload equivalent to 100% $\dot V_{{\text{O}}_{{\text{2 max}}} }$ on two separate occasions. Each exercise test was performed on an electrically braked cycle ergometer after a four-day period of dietary manipulation. During each of these periods subjects consumed either a low carbohydrate (3±0%, mean ±SD), high fat (73±2%), high protein (24±3%) diet (FP) or a high carbohydrate (82±1%), low fat (8±1%), low protein (10±1%) diet (CHO). The diets were isoenergetic and were assigned in a randomised manner. Muscle biopsy samples (Vastus lateralis) were taken at rest prior to dietary manipulation, immediately prior to exercise and immediately post-exercise for measurement of pH, glycogen, glucose 6-phosphate, fructose 1,6-diphosphate, triose phosphates, lactate and glutamine content. Blood acid-base status and selected metabolites were measured in arterialised venous samples at rest prior to dietary manipulation, immediately prior to exercise and at pre-determined intervals during the post-exercise period. There was no differences between the two treatments in blood acid-base status at rest prior to dietary manipulation; immediately prior to exercise plasma pH (p<0.01), blood $P_{{\text{CO}}_{\text{2}} }$ (p<0.01), plasma bicarbonate (p<0.001) and blood base-excess (p<0.001) values were all lower on the FP treatment. There were no major differences in blood acid-base variables between the two diets during the post-exercise period. Compared with the CHO diet, the FP diet resulted in plasma alanine (p<0.05), blood lactate (p<0.05), and plasma glutamine (p<0.01) levels being lower immediately prior to exercise; plasma free fatty acids (FFA; p<0.05), glycerol (p<0.01), urea (p<0.001) and blood 3-hydroxybutyrate (3-OHB; p<0.01) levels were all higher. After the FP diet blood alanine, lactate and plasma glutamine levels were lower for the whole or the majority of the post-exercise period, while the concentrations of plasma FFA, glycerol, urea and blood 3-OHB and glucose were higher. There was no difference between the diets in pre-exercise glucose and insulin levels and post-exercise insulin levels. There was no difference in muscle pH between the two diets immediately prior to exercise; the decline in muscle pH was 104% greater during exercise on the FP diet resulting in a significant difference in post-exercise pH (p=0.05). The FP diet resulted in 23% decline in muscle glutamine levels, resulting in lower levels (p<0.05) immediately prior to exercise. Exercise had no influence on muscle glutamine levels after the FP diet but produced a 17% decline on the CHO diet. Muscle glycogen content increased by 23% on the CHO diet, but was unchanged after the FP diet. This resulted in levels being significantly different prior to exercise (p<0.05). The decline in muscle glycogen content during exercise was 50% greater on the CHO diet. There were no differences when comparing the two dietary treatments in any of the pre-exercise glycolytic intermediates measured. Immediately post-exercise glucose 6-phosphate levels were 22% higher and fructose 1,6-diphosphate levels were 130% lower on the FP diet. There were no differences between the two diets in muscle triose phosphate or lactate levels at any point of the study. The present study demonstrates that a FP diet can induce metabolic acidosis and may reduce pre-exercise muscle buffering capacity, which may then influence subsequent exercise performance. However, this appears not to influence the efflux of H⁺ from muscle during and after high intensity exercise.