In a 12-week parallel study, women with a verified diagnosis of endometriosis will be randomly assigned to follow a low-fat plant-based diet or to stay on their usual diets for 12 weeks. Participants in both groups will be asked to make no changes to their exercise patterns for the study period. Changes in pain, quality of life, and inflammatory biomarkers from baseline to final will be the primary outcomes. Secondary outcomes will include changes in body weight, blood lipids, gut microbiome composition, and hormonal changes.

Endometriosis is a chronic, painful condition that affects 5-10% of reproductive aged women. It is characterized by the presence of endometrial cells outside the uterus, causing pain and infertility. Current therapies to temporarily control symptoms include surgery and medical options with significant side effects. Women with endometriosis may have several surgeries by their mid-20s. The medical costs associated with endometriosis in the US alone are about $3.6 billion annually, and the total annual health care cost associated with endometriosis in 2002 was $22 billion, one-third of which is direct health care costs, with two-thirds attributed to loss of productivity.

Diet may play important roles in endometriosis through its effect on circulating estrogen concentrations and inflammatory processes. Reduced fat intake and increased fiber intake have been shown to reduce estrogen levels, a key determinant of endometriosis activity. Some evidence suggests a pathogenic role for the gut microbiome, which influences the mucosal immune system and pelvic inflammation. The microbiome is responsive to short term dietary intervention, although long term changes have not been well characterized.

Therefore, the investigators propose a novel intervention strategy using a plant-based diet to improve pain, inflammation, and quality of life.

The morbidity caused by endometriosis is substantial in terms of pain, reduced quality of life, lost productivity, and effects on relationships. Current medical therapies are limited in efficacy and have major side effects. Typical palliative surgery does not cure endometriosis. Better understanding of diet's ability to influence the disease process has the potential to improve quality of life and daily functioning without the cost and side effects of current therapies.

Inclusion Criteria:

1. Women with a surgical, imaging, or clinical diagnosis of endometriosis
2. Age 18-45 years
3. Stable health condition and medications for past 3 months
4. Modified Biberoglu and Behrman (B&B) pelvic pain score of at least 5/9
5. Able to follow a plant-based diet for 12 weeks
6. Willing to be randomly assigned to either a plant-based group or a control group that will not make any dietary changes for 12 weeks

Exclusion Criteria:

1. Body mass index ≥ 40 kg/m2
2. Smoking or drug abuse during the past six months
3. Alcohol consumption of more than 2 drinks per day or the equivalent, episodic increased drinking (e.g., more than 2 drinks per day on weekends), or a history of alcohol abuse or dependency followed by any current use
4. Unstable medical or psychiatric illness
5. Already following a plant-based diet
6. Pregnant or breastfeeding, or plans of pregnancy within the study period
7. Hysterectomy or ovariectomy
8. Fibroids, ovarian cysts, pelvic inflammatory disease
9. Endocrine inflammatory conditions, such as Cushing's syndrome, Hashimoto's thyroiditis, Graves' disease, type 1 diabetes mellitus, and Addison's disease
10. Lack of English fluency
11. Unable or unwilling to participate in all components of the study
12. Evidence of an eating disorder