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1 Pediatrics and Medicine, McMaster University, Hamilton, Canada
* To whom correspondence should be addressed. E-mail: tarnopol{at}mcmaster.ca.
We examined whether endurance exercise training and sex influenced IMCL and mitochondrial morphology using electron microscopy, whole-body substrate use, and mitochondrial enzyme activity. Untrained men (N=5) and women (N=7) were tested before and after seven weeks of endurance exercise training. Testing included 90 min of cycle ergometry at 60% VO2peak with pre-exercise muscle biopsies analyzed for IMCL and mitochondrial size/area using electron microscopy and short-chain 
-hydroxyacyl-CoA dehydrogenase (SCHAD) and citrate synthase (CS) enzyme activity. Training increased the mean lipid area density (P = 0.090), the number of IMCL droplets (P = 0.055), the number of IMCL droplets in contact with mitochondria (P = 0.010), the total mitochondrial area (P < 0.001), and the size of individual mitochondrial fragments (P = 0.006). Women had higher mean lipid area density (P = 0.030) and number of IMCL droplets (P = 0.002), as compared with men. Women oxidized more fat (P = 0.027) and less carbohydrate (P=0.032) throughout the study. Training increased VO2peak (P < 0.001), % fat oxidation (P = 0.018) and SCHAD (P = 0.003) and CS activity (P = 0.042). In summary, endurance exercise training increased IMCL area density due to an increase in the number of lipid droplets, while the increase in total mitochondrial area was due to an increase in the size of individual mitochondrial fragments. Women have higher IMCL content as compared with men due mainly to a greater number of individual droplets. Finally, endurance exercise training increased the proportion of IMCL in physical contact with mitochondria.
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