Knee osteoarthritis (OA) is the most prevalent form of arthritis, a significant cause of disability in the U.S. With an aging population and the rise in obesity rates, the prevalence of knee OA is expected to climb, significantly reducing quality of life (QOL) for those suffering from this debilitating condition. Current national efforts to reduce analgesic utilization highlight the critical need for safe, effective, and accessible alternatives for pain relief. Low-carbohydrate diets (LCDs) reduce inflammation and pain independent of weight loss, indicating that diet interventions offer a non-pharmacological complementary treatment. However, differences exist in metabolism that are rarely addressed in diet intervention studies. Thus, it is important to assess the potential of different diets in a broad population of chronic pain sufferers to determine the potential of diets to reduce knee OA pain. We have shown that a LCD was associated with reduced evoked knee OA pain, daily pain and oxidative stress when compared to either a USDA diet or a diet-as-usual control. Both experimental diets reduced weight to a similar degree, arguing that diet quality was likely the key factor in pain reduction, as opposed to weight loss. However, previous studies comparing diets have utilized diet prescriptions with less control for adherence to the diets. To overcome this obstacle, and in line with our recent work, we will provide all snacks and meals during the diet intervention to increase adherence and retention in the study, allowing for better control over diet interventions and consistency of foods within each study group. We will recruit adults with knee OA (N=200) to complete our two-phase protocol. Phase 1 will involve a 1-week diet run-up that will allow for quantification of pain measures, psychosocial variables (socioeconomic status, nutritional knowledge, proximity to grocery stores, food insecurity), and diet quality to provide a baseline for comparison. Phase 2 will be a 6-week randomized diet intervention (LCD or USDA diet) in which both groups will be provided with all meals at the direction of study personnel and input from participants. Evoked pain tasks, measures of pain disability, severity, catastrophizing, and interference will be assessed every 3 weeks in addition to QOL measures, mood, and depression. Physiological variables will be assessed through blood draws (inflammatory profile) and dual-energy X-ray absorptiometry scans (DXA; body composition, visceral fat) at the end of Phases 1 and 2. This will be the first study to examine the efficacy of these diets to reduce knee OA pain with an emphasis on interactions with biopsychosocial variables. Changes in all pain measures following Phase 2 will be assessed with respect to published measures of clinically-meaningful differences in pain and disability, as well as for statistical significance. The central hypothesis is that the LCD will improve pain and QOL in participants with knee OA more than the USDA diet, but that both will be beneficial. Specific Aim 1: To investigate the efficacy of the diets to reduce pain and improve QOL. Hypothesis 1: The LCD group will show significantly greater reductions in: a) self-reported pain (\>1.7 in pain intensity) and, b) evoked pain (\>30%) when compared to the USDA diet. Hypothesis 2: The LCD group will show greater improvements in: a) QOL, b) mood, and c) self-reported improvement (\>50% participants reporting much improved or very much improved). Hypothesis 3 (secondary): Both diets will result in improved pain disability, severity, catastrophizing and pain related fear; the LCD will outperform the USDA diet. Specific Aim 2: To explore individual differences in diet and baseline measures. Hypothesis 1: Baseline diet quality will be negatively associated with baseline pain sensitivity Hypothesis 2: Those reporting greater a) food insecurity and/ or b) proximity to grocery stores will report poorer-quality diets. Specific Aim 3: To determine whether physiological variables contribute to diet effects or lack thereof. Hypothesis 1: Baseline physiological measures (inflammatory profile) will predict: a) pain sensitivity, and b) reductions in pain. Hypothesis 2: Change in physiological measures (inflammatory profile, adiposity, leptin) will be related to: a) change in pain measures, b) change in QOL, c) self-reported improvement and, d) mood.