ABSTRACT Polycystic ovary syndrome (PCOS) is the most common endocrine disorder for women of reproductive age. Women with PCOS are at a high risk of health complications. In fact, women with PCOS have a 3-7 times higher risk for type 2 diabetes, and obese women with PCOS have an even higher risk of type 2 diabetes. Experts recommend that the first-line treatment for overweight and obese women with PCOS should be diet and lifestyle interventions. Yet, experts disagree about the specific nutritional advice these interventions should encourage, with different groups recommending no particular diet, a high-carbohydrate Dietary Approaches to Stop Hypertension (DASH) diet, or a lower carbohydrate diet. A systematic review of previous dietary trials in PCOS did find a slight benefit of lower carbohydrate diets for glucose control, weight loss, insulin, and insulin resistance. This may be because carbohydrate intake raises glucose levels, which in turn increases insulin secretion. Insulin then stimulates ovarian androgen production and inhibits the release of fatty acids from cells. Previous research in type 2 diabetes suggests that not simply lower carbohydrate intake, but a very low- carbohydrate (VLC) diet, with less than 20% of calories from carbohydrates, designed to reduce body weight and glucose levels more significantly than other types of lower carbohydrate diets, would be even more effective for improving metabolic health in PCOS. Although reviews note that a VLC diet has shown promise for type 2 diabetes, prediabetes, obesity, and cardiovascular risk, evidence for PCOS is only emerging. In our recently completed 4-month trial of 29 overweight or obese women with PCOS following a VLC diet, we found significant improvements in outcomes, including glycated hemoglobin (HbA1c; -0.21%) and body weight (-7.7%). We propose a 12-month randomized, controlled trial for 184 overweight or obese (BMI of 28-55 kg/m2) adults with PCOS comparing a VLC diet and a standard-of-care low-fat, Dietary Approaches to Stop Hypertension (DASH) diet. The program for both groups is delivered online, as in our pilot study. Aim 1: To compare changes in glycemic control. We hypothesize that the VLC version will lead to greater improvements in HbA1c at 12-months. Aim 2: To compare changes in secondary health outcomes tied to type 2 diabetes risk, including glycemic variability, conversion to normoglycemia, body weight, and body fat percentage. We hypothesize that the VLC version will lead to greater improvements in secondary health outcomes at 12- months. Aim 3: To explore intervention satisfaction and acceptability. The guiding objective of this proposal is to explore a novel diet and lifestyle approach for reducing the future risk of type 2 diabetes for overweight or obese women with PCOS. We anticipate that the proposed research will have an important impact on diet and lifestyle recommendations for this high-risk, understudied population.