MR-based fat fraction changes in subcutaneous adipose tissue in people with obesity undergoing a weight loss intervention: results from the LION study

Background: Subcutaneous adipose tissue (SAT) plays a significant role in metabolic regulation. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) can non-invasively measure proton density fat fraction (PDFF), providing quantitative methods for monitoring adipose tissue composition changes during obesity treatment. However, the comparative reliability of MRI versus MRS for evaluating SAT PDFF changes during weight loss, and the differential responses of superficial versus deep SAT depots, remain unclear. This study aimed to compare MRI and MRS methods in evaluating SAT PDFF variations in people with obesity before and after weight loss, compare PDFF changes between superficial and deep SAT, and examine associations with standard anthropometric markers. Methods: A human intervention study was conducted on adults with obesity [body mass index (BMI) ≥30 kg/m$^{2}$] who underwent an 8-week low-calorie formula diet and completed an MRI scan (3T) before (n=127) and after dietary intervention (n=87). In addition, PDFF was measured in the superficial and deep abdominal SAT depots using MRI and single-voxel MRS. Intermethod and pre-/post-dietary PDFF analyses were conducted in comparison to anthropometric parameters (weight, BMI, and body fat percentage). Conclusions: Both MRI and MRS can reliably quantify PDFF in SAT of people with obesity undergoing weight loss interventions. Both deep and superficial SAT PDFF decrease after weight loss and SAT PDFF reduction is related to body fat changes after weight loss.