The peroxisome proliferator-activated receptor α (PPARα) is a nuclear receptor playing a pivotal role in the expression of proteins involved in transport and metabolism of fatty acids. This study aimed to investigate the effect of training and fat or carbohydrate diet on the expression of PPARα in human muscle.

Thirteen sedentary males trained the knee extensors of one leg for 4 weeks while the other leg served as control. Training was performed on 18 days and the total amount of training was 31.5 h. During the 4 weeks six subjects consumed a fat diet (58 % fat, 24 % carbohydrate and 18 % protein) and seven a carbohydrate diet (32 % fat, 55 % carbohydrate and 12 % protein). Dietary intake was controlled by dietary diaries. Before and after 4 weeks, a biopsy from both legs was obtained from m. vastus lateralis approximately 40 h after the last exercise bout. PPARα protein expression was analysed by Western blotting and PPARα mRNA levels by Northern blotting. Two-way ANOVA was applied and data are expressed as means ± S.E.M. Ethical approval with informed consent was obtained.

The PPARα protein expression was maintained after fat diet whereas a significant decrease was observed after carbohydrate diet (P < 0.05, Fig. 1). In contrast, training had no effect on PPARα protein expression. The PPARα mRNA levels were not different from baseline after 4 weeks diet and training. Across the 4 weeks citrate synthase activity was significantly increased in the trained legs after fat and carbohydrate diet (15 %, P < 0.05), whereas it remained unchanged in the non-trained legs. The β-hydroxy-acyl-CoA-dehydrogenase (HAD) activity was not affected by either diet or training across the 4 weeks. In contrast the HAD mRNA was significantly increased in both the untrained (2.95-fold) and the trained leg (1.95-fold) after fat diet (P < 0.05), whereas after carbohydrate diet no change was evident. Whole body maximal oxygen uptake remained unchanged across the training period, whereas the one leg maximal workload (W_max) was increased by 6 % in the trained leg after 14 days (P < 0.05). Energy intake was significantly different between groups and averaged 17 ± 1 and 12 ± 1 MJ after consumption of the fat and carbohydrate diet, respectively.

In conclusion, across a 4 week period PPARα protein expression was influenced by carbohydrate diet and not training. Although this diet induced difference in PPARα, protein expression did not manifest into an increased HAD activity after fat diet; the significant increase in HAD mRNA indicates that prolonged fat diet exposure may lead to increased enzyme activity, consistent with our previous findings (Helge & Kiens, 1997).