Project Summary Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited disorder that leads to end- stage kidney disease. To date, tolvaptan is the only approved intervention to slow kidney disease progression in patients with ADPKD. However, tolvaptan is constrained by high cost and common side effects that limit adherence and is only indicated for rapidly progressing ADPKD. Thus, alternative or concurrent interventions that may slow progression of ADPKD are of considerable clinical importance. Similar to the general population, body-mass index has been increasing in patients with ADPKD, and approximately nearly 70% of adults with ADPKD are overweight or obese. Adipocytes do not simply act as a fat reservoir, but are active endocrine organs, and thus, may be a promising clinical target for ADPKD management. Mounting evidence also suggests that a metabolic defect exists in ADPKD, which likely contributes to cystic epithelial proliferation and subsequent cyst growth. Mild-to-moderate food restriction profoundly slows cyst growth and maintains renal function in numerous rodent models of PKD via mechanisms including activation of AMP-activated kinase and suppression of mammalian target of rapamycin-S6 kinase signaling and insulin-like growth factor-1 levels. Additionally, we have shown that overweight and obesity are strong independent predictors of more rapid kidney growth, measured by total kidney volume (TKV). We recently completed a R03-funded pilot study supporting that a behavioral weight loss intervention via daily caloric restriction (DCR) in adults with ADPKD and overweight or obesity: 1) is feasible and acceptable; 2) slowed kidney growth (annual %∆ in height- adjusted TKV [htTKV]); 3) reduced abdominal adiposity; and 4) altered markers of biological pathways implicated in ADPKD progression and metabolism. However, our pilot and feasibility study was limited by a small sample size, relatively short duration, and lack of a control group. Thus, to translate these promising results of our pilot study towards clinical practice, we propose a parallel-group, randomized, controlled clinical trial in 126 adults with ADPKD and overweight or obesity to directly compare the efficacy of behavioral weight loss intervention based on DCR vs. control (standard clinical advice for ADPKD) for slowing kidney growth over a longer duration. Changes in abdominal adiposity will serve as a secondary outcome. Effects of weight loss on circulating and adipose markers of biological pathways will provide mechanistic insight. Specific Aim 1: Determine the effect of a DCR-based behavioral weight loss intervention on kidney growth (annual %∆ htTKV by MRI over 24 months) vs. control (standard clinical dietary advice for ADPKD). Specific Aim 2: Quantify changes in abdominal adiposity (visceral, subcutaneous, and total) by MRI in each group and their association with changes in htTKV and markers of biological pathways. Specific Aim 3: Measure changes in ...