Both higher fitness level and higher current physical activity level may be required for intramyocellular lipid accumulation in non-athlete men.

Accumulation of intramyocellular lipid (IMCL) is observed in individuals with insulin resistance as well as insulin-sensitive endurance athletes with high peak oxygen consumption (VOpeak), which is called the athlete's paradox. It remains unclear whether non-athletes with higher fitness levels have IMCL accumulation and higher insulin sensitivity in general. In this study, we investigated the association between IMCL accumulation and muscle insulin sensitivity (M-IS) in subjects with high or low VOpeak. We studied 61 nonobese (BMI, 23 to 25 kg/m), non-athlete Japanese men. We divided the subjects into four groups based on the median value of VOpeak and IMCL in the soleus muscle. We evaluated M-IS using a two-step hyperinsulinemic-euglycemic clamp. Among subjects with higher VOpeak (n = 32), half of those (n = 16) had lower IMCL levels. Both High-VOpeak groups had higher M-IS than the Low-VOpeak groups. On the other hand, M-IS was comparable between the High-VOpeak/High-IMCL and High-VOpeak/Low-IMCL groups, whereas the High-VOpeak/High-IMCL group had IMCL levels that were twice as high as those in the High-VOpeak/Low-IMCL group. On the other hand, the High-VOpeak/High-IMCL group had significantly higher physical activity levels (approximately 1.8-fold) than the other three groups. In conclusion, in nonobese, non-athlete Japanese men, subjects with higher VOpeak and higher IMCL had higher physical activity levels. IMCL accumulation is not associated with insulin resistance in individuals with higher or lower fitness levels.