Katsunori Jinnouchi

Purpose of Review Insulin resistance is a key component of the metabolic syndrome, type 2 diabetes mellitus (T2DM), and related cardiovascular complications. However, despite its clinical relevance, a simple and reliable surrogate marker of insulin resistance is lacking. The body muscle-to-fat ratio (BMFR) is a novel anthropometric index, and particularly in treatment-naïve patients with T2DM, BMFR correlates more closely with insulin sensitivity, assessed using the hyperinsulinemic–euglycemic clamp, than conventional indices, such as the Matsuda Index and the Quantitative Insulin Sensitivity Check Index. In this review, we critically evaluate the current evidence regarding the BMFR as an index of insulin resistance and metabolic disease risk, highlight its strengths and limitations versus conventional measures, and discuss future directions. Recent Findings Several recent studies have demonstrated the importance of body composition metrics for the evaluation of metabolic health, including those by Guan et al. (Adv Nutr), Oliver et al. (J Cachexia Sarcopenia Muscle), and Vieira et al. (Rev Endocr Metab Disord). The findings have resulted in a growing recognition of the need for reliable, accessible markers of muscle quality and metabolic risk. Summary BMFR offers advantages over the fat-to-muscle ratio and conventional anthropometric indices as an intuitive and clinically relevant marker of insulin resistance. It represents a promising method of screening for obesity-related complications, evaluating treatment responses, and guiding personalized care, including in patients with high-risk phenotypes, such as sarcopenic obesity. With further validation, BMFR could become a standard tool for the evaluation of metabolic health and risk in clinical practice.