Fat quality and quantity appear to be modulating factors for endometriosis. A 2010 prospective study found that the intake of palmitic acid (a saturated fatty acid derived mainly from meat and dairy products) and trans fat was associated with an increased risk of endometriosis, while total fat consumption did not confer the same risk (27). Although artificial trans fats have been banned in the United States, trans fats are naturally present in meat and milk derived from cows and sheep (28).

Animal fats are of particular interest as high consumption has been linked to other gynecological diseases. The NIH-AARP Diet and Health Study found that participants with the highest total fat intake had a 28% greater risk for ovarian cancer compared to those with the lowest intake; only animal-derived fats had a significant positive correlation (29). Whether animal fats similarly influence the risk of developing endometriosis remains unknown.

In contrast, certain fats may play protective roles. A 2010 study following 70,709 premenopausal women reported that those consuming the most omega-3 fatty acids were less likely to be diagnosed with endometriosis, compared with those with the lowest omega-3 consumption (27).

Because estrogens are key in the pathogenesis of endometriosis, dietary factors that modulate estrogen activity may be clinically important. Reducing dietary fat and increasing dietary fiber has been shown to reduce circulating estrogen concentrations by roughly 10 to 25 percent (20, 30, 31). The same phenomenon has been identified in postmenopausal women (32). In addition, some have suggested that estrogens occurring in animal products, particularly dairy and meat products, may have biological effects when ingested (33).

Two prospective studies demonstrated significant correlations between red meat consumption (both unprocessed and processed) and the risk of developing endometriosis (34, 35). The Nurses' Health Study II showed that women consuming more than two servings of red meat per day had a 56% greater risk of endometriosis, compared with those consuming less than one serving of red meat per week (RR 1.56, 95% CI 1.22, 1.99) (35). Increased poultry intake was also associated with higher risk of endometriosis (RR 1.14, 95% CI 0.88, 1.49), although risk estimates for specific intake categories were not significant. Increased risk associated with fish and shellfish intake was no longer apparent after adjustment for race, parity, body mass index, age at menarche, menstrual cycle length, pelvic or breast exam in the past year, and energy intake. An Italian study reported increased odds of developing endometriosis associated with a higher intake of beef, other red meat, and ham (34). Red meat consumption may be associated with higher levels of estradiol and estrone sulfate, and thereby with higher concentrations of steroids, inflammation, and the development of endometriosis (36). Furthermore, red meat consumption may promote expression of pro-inflammatory markers, which appear to be implicated in endometriosis pathogenesis and progression (37). A summary of the association of different kinds of meat with the risk of endometriosis is shown in Table 1.

Because of the role of inflammation in endometriosis, and the fact that reducing dietary fat and increasing dietary fiber reduce circulating estrogen concentrations, the effects of plant-based diets on inflammation have been of clinical interest. An 8-week study of 100 participants with coronary artery disease published by the American Heart Association found that those following a strictly vegan diet experienced less inflammation than those following a diet including animal products (38). A 2021 literature review confirmed the anti-inflammatory properties of a plant-based diet (39).

In a randomized crossover trial in women with dysmenorrhea, a low-fat vegan diet was shown to increase plasma concentrations of sex-hormone binding globulin, which, in turn, would be expected to reduce estrogen activity. The diet also reduced severity and duration of pain and moderated premenstrual symptoms (40).

Observations from the Nurses' Health Study II, including 81,961 premenopausal women, suggested that low-Glycemic Index foods are associated with reduced risk of endometriosis (41). Increased fruit fiber has been associated with a reduced risk of endometriosis in two independent studies (34, 42).

Plant-based foods contain increased amounts of polyphenols when compared to an omnivorous diet, which, when metabolized into bioactive compounds, can reduce inflammation (39). When comparing a plant-based diet to an omnivorous diet, the gut microbiome contains a larger number of anti-inflammatory compounds (43). As noted above, a crossover trial using a low-fat plant-based (vegan) vs. a placebo demonstrated significant benefits of the dietary intervention in dysmenorrhea; application to endometriosis awaits further research.

Seaweed may have estrogen-modulating properties. A double-blind trial testing the effects of seaweed in healthy postmenopausal women found an inverse dose-response relationship between seaweed consumption and serum estradiol concentrations, i.e., the higher the seaweed consumption, the lower the estradiol concentrations (44), which may partly explain its protective effects for breast cancer and lower postmenopausal breast cancer incidence and mortality in Japan (39). Seaweed may also reduce estradiol concentrations in pre-menopausal women. A case study in three pre-menopausal women reported that intake of bladderwrack, an edible brown kelp, significantly increased the length of menstrual cycle and reduced estradiol levels (45). These studies suggest potential benefits of seaweed consumption on estradiol concentrations, but effects on endometriosis await further research.

Vitamin D may play a role in the prevention and treatment of endometriosis. A 2020 meta-analysis found that low vitamin D levels were associated with increased risk of endometriosis diagnosis and increased severity of symptoms (46). In a randomized, placebo-controlled trial in women with endometriosis, 50,000 IU vitamin D every 2 weeks for 12 weeks reduced pelvic pain by 1.12 points on a self-reported scale. It was also associated with reduced high-sensitivity C-reactive protein and increased total antioxidant capacity, suggesting that an antioxidant and anti-inflammatory effect may be involved (47).

Free radicals are generated through physiological processes and exposure to environmental factors and cause oxidative stress, which often results in cell damage or death (48), contributing to a wide range of pathologies (49). Conversely, antioxidants are molecules that combat free radicals and can be endogenously synthesized or exogenously consumed. Because of the role of inflammation in endometriosis, research studies have addressed the potential role of antioxidant intake. A 2009 study from Mexico reported that women with endometriosis had a 30% lower intake of vitamin C and consumed 40% less vitamin E, compared with women who did not have endometriosis. Following 3 months on a high-antioxidant diet, women with endometriosis displayed higher peripheral concentrations of supplemented vitamins (50). A later randomized control trial found that supplementation with vitamins C and E significantly reduced endometriosis symptoms, compared with placebo (51).