Role of Exercise Intensity on Intramyocelluar Lipid Level After Exercise in Subjects with Moderate Insulin Resistance

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It is known that the increased intramyocellular lipid (IMCL) levels observed in obese subjects are closely associated with insulin resistance (IR) in muscle ¹⁾. Interestingly, some reports suggested low intensity exercise (LIE) decreased IMCL and improved IR ²⁾. On the other hand, a few reports showed vigorous intensity exercise (VIE) improved IR, but increased IMCL level ^{3) 4)}. This phenomenon is a reminiscent of endurance-trained athletes, who possess a high oxidative capacity and enhanced insulin sensitivity, also have higher IMCL content known as athlete’s paradox (AP) ^{5) 6)}. From these findings, we hypothesized exercise intensity is one of the determinants of AP.

To test this hypothesis, we recruited 20 men with moderate insulin resistance (HOMA-R >1.6) and randomly assigned to LIE (40% VO₂ peak) or VIE (70% VO₂ peak) group. Each group performed with ergometer for 5 consecutive days. Before and 3-day after completion of protocol, IR was evaluated by glucose clamp. IMCL was measured by ¹H-MRS. The IMCL was also evaluated immediately after the exercise at day 5. Our preliminary data showed that in VIE group IMCL level was not significantly changed after exercise at day 5. Although IMCL level was decreased at 3-day after last bout of exercise in LIE group, that in VIE group increased about 50% from baseline. Interestingly, insulin resistance was similarly improved in both groups. These data suggested exercise intensity is a determinant of change of IMCL.

Although, changes in IMCL level after exercise were opposite between LIE and VIE, the improvement of insulin resistance was similar. IMCL exists mostly as triacylglycerol (TAG), which may not impair insulin sensitivity in muscle. On the other hand, intramyocellular diacylglycerol (DAG) concentration is considered to induce insulin resistance, which is generally increased in parallel with the amount of intramyocellular TAG. It has been shown that one bout of aerobic exercise increased the expression level of diacylglycerol acyltransferase (DGAT)-1 in muscle and to prevent FFA-induced muscle DAG accumulation and insulin resistance in healthy humans. Thus, we speculated that DAGT1 expression and decreased DAG levels play roles in the mechanisms involved in the athlete’s paradox phenomenon seen in VIE group. Future analysis is clearly required to test this hypothesis.