Response: Association of Thigh Muscle Mass with Insulin Resistance and Incident Type 2 Diabetes Mellitus in Japanese Americans (Diabetes Metab J 2018;42:488-95).

We thank Dr. Kim for expressing interest and giving comments on our article entitled “Association of thigh muscle mass with insulin resistance and incident type 2 diabetes mellitus in Japanese Americans” which was published in Diabetes & Metabolism Journal [1].

We agree with Dr. Kim's suggestion of the potential value of measurement of thigh intramuscular or intermuscular fat content. However, these measurements were not available for this study. Previous studies demonstrated that fat in these areas was significantly associated with insulin resistance and the development of type 2 diabetes mellitus (T2DM) [234]. Although fat infiltrated in muscle accounts for a relatively small portion of the total skeletal muscle, it may be important in the development of T2DM apart from muscle mass [45].

In response to your question about whether we have investigated the relationship between abdominal fat distribution and thigh muscle mass, we have done so and will present the results here. The correlation between thigh muscle mass area and intra-abdominal fat area was -0.007 (P=0.923 in men) and -0.064 (P=0.380 in women) and between thigh muscle mass area and subcutaneous abdominal fat area 0.197 (P=0.005 in men) and 0.123 (P=0.089 in women). As the subcutaneous fat depot represents the largest adipose depot in the body, we believe that its positive correlation with thigh muscle reflects the previously observed correlation between overall adiposity as measured by body mass index (BMI) and muscle mass [6]. Indeed, after adjustment of the association between thigh muscle area and subcutaneous abdominal fat area for BMI, there was a significant negative association apparent in men and women. In the current study, thigh muscle area was inversely associated with insulin resistance independent of total abdominal fat area. An interaction between thigh muscle mass area and BMI in relation to T2DM was also observed after adjusting for total abdominal fat area.

Thank you again for your interest in our research and your thoughtful comments.