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Intramyocellular Lipids: Anthropometric Determinants and Relationships with Maximal Aerobic Capacity and Insulin Sensitivity

Department of Endocrinology and Metabolism (C.T., O.B., M.H., D.D., O.T., A.F., S.J., H.-U.H., M.S.) and Section on Experimental Radiology (J.M., B.W., K.B., C.C., F.S.), Department of Diagnostic Radiology, Eberhard-Karls-University, D-72076 Tübingen, Germany; and Medical Clinic (A.N.), Department of Sports Medicine, University of Freiburg, D-79106 Freiburg, Germany

Address all correspondence and requests for reprints to: Dr. Michael Stumvoll, Medizinische Universitätsklinik, Otfried-Müller-Strasse 10, D-72076 Tübingen, Germany. E-mail: michael.stumvoll{at}med.uni-tuebingen.de.

The existence of metabolically relevant intramyocellular lipids (IMCL) as assessed by the noninvasive ¹H-magnetic resonance spectroscopy (MRS) has been established. In the present studies, we analyzed the relationships between IMCL in two muscle types [the predominantly nonoxidative tibialis muscle (tib) and the predominantly oxidative soleus muscle (sol)] and anthropometric data, aerobic capacity (VO₂max, bicycle ergometry, n = 77) and insulin sensitivity (hyperinsulinemic euglycemic clamp, n = 105) using regression analysis.

In univariate regression, IMCL (tib) was weakly but significantly correlated with percentage of body fat (r = 0.28, P = 0.01), whereas IMCL (sol) was better correlated with waist-to-hip ratio (r = 0.41, P < 0.0001). No significant univariate correlation with age or maximal aerobic power was observed. After adjusting for adiposity, IMCL (tib) was positively correlated with measures of aerobic fitness. A significant interaction term between VO₂max and percentage of body fat on IMCL (tib) (P = 0.04) existed (whole model r² = 0.26, P = 0.001). In contrast, aerobic fitness did not influence IMCL (sol). No correlation between insulin sensitivity as such and IMCL (tib) (r = -0.13, P = 0.2) or IMCL (sol) (r = 0.03, P = 0.72) was observed. Nethertheless, a significant interaction term between VO₂max and IMCL on insulin sensitivity existed [P = 0.04 (tib) and P = 0.02 (sol)]; [whole model (sol) r² = 0.61, P < 0.0001, (tib) r² = 0.60, P < 0.0001].

In conclusion, obesity and aerobic fitness are important determinants of IMCL. IMCL and insulin sensitivity are negatively correlated in untrained subjects. The correlation between the two parameters is modified by the extent of aerobic fitness and cannot be found in endurance trained subjects. Thus, measurements of aerobic fitness and body fat are indispensable for the interpretation of IMCL and its relationship with insulin sensitivity.

This work was supported by a grant from the Deutsche Forschungsgemeinschaft (DFG No. JA-1005/1-1, DFG Stu-192/9-1), the Federal Ministry of Education and Research (Fö. 01KS9602), a grant from the European Community (QLRT-1999-00674) and the Interdisciplinary Center of Clinical Research Tübingen (IZKF).

Abbreviations: EMCL, Extramyocellular lipids; IMCL, intramyocellular lipids; ISI, insulin sensitivity index; MRS, magnetic resonance spectroscopy; sol, soleus muscle; tib, tibialis muscle; WHR, waist to hip ratio; VO₂max, maximal aerobic capacity.

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