Phytoestrogens can alleviate some pathological processes related to nonalcoholic fatty liver disease (NAFLD). However, there are limited and contradictory studies on the relationships between phytoestrogens (especially single phytoestrogen) and NAFLD. The purpose of this study was to explore the relationships between urinary phytoestrogen concentrations and NAFLD in American adults. This cross-sectional study used the data of the National Health and Nutrition Examination Survey from 1999 to 2010, and 2294 adults were finally enrolled in this study. The concentrations of phytoestrogens were measured in urine samples, and urinary phytoestrogens were divided into tertiles according to the concentration distributions. The diagnosis of NAFLD was determined by the United States fatty liver index. The main analysis used a multivariate logistic regression model. The fully adjusted models included gender, age, race, education, marriage, poverty, body mass index, waist circumference, smoking, diabetes, hypertension, total cholesterol, high-density lipoprotein cholesterol, triglycerides, and other five phytoestrogens. In the fully adjusted model, the urinary enterolactone (ENL) concentration was negatively correlated with NAFLD (OR of Tertile 3 : 0.48, 95% CI 0.25–0.94). When stratified by age and gender, the urinary ENL concentration was negatively correlated with NAFLD in males aged 40–59 years (OR of Tertile 3 : 0.08, 95% CI 0.01–0.82), while the urinary equol concentration was positively correlated with NAFLD in such population (OR of Tertile 3 : 4.27, 95% CI 1.02–17.85). In addition, a negative correlation between enterodiol (END) concentration and NAFLD was observed in males aged 60 years or over (OR of Tertile 2 : 0.18, 95% CI 0.05–0.69). Collectively, in middle-aged males, urinary ENL may be associated with a lower risk of NAFLD, while urinary equol may be related to a higher risk. In addition, urinary END has a possible relationship with a reduced risk of NAFLD in elder males. Definitely, clinical randomized controlled trials are needed to further verify the conclusions.

Aims The association between the gut microbiota and non-alcoholic fatty liver disease (NAFLD) has been documented; however, the causal relationship between them remains unclear. The aim of this study was to explore the causal relationship between the gut microbiota and NAFLD using Mendelian randomization. Methods We conducted a Mendelian randomization study, using gut microbiota data (n = 18340) from MiBioGen consortium as the exposure and the NAFLD dataset from FinnGen R9 release data (n = 377,277) as the outcome. Inverse variance weighted was employed as the primary analysis method, and sensitivity analysis was performed. Additionally, a network graph resembling a phylogenetic tree was created to further reveal the evolutionary relationships among gut microbiota. Results Six bacterial features exhibited causal relationships with NAFLD: Actinomycetales (OR = 1.50, 95% CI: 1.01–2.21, p = 0.043), Actinomycetaceae (OR = 1.49, 95% CI: 1.01–2.20, p = 0.043), Actinomyces (OR = 1.36, 95% CI: 1.09–1.71, p = 0.006), Prevotella 7 (OR = 1.19, 95% CI: 1.01–1.39, p = 0.039) exhibited potential detrimental effects on NAFLD, whereas Anaerofilum (OR = 0.85, 95% CI: 0.72–0.99, p = 0.042) and Gordonibacter (OR = 0.84, 95% CI: 0.72–0.98, p = 0.024) exhibited potential protective effects. Furthermore, Actinomycetales, Actinomycetaceae, and Actinomyces are located on the same evolutionary branch. Conclusions Our study revealed a causal relationship between six gut bacteria and NAFLD. These findings shed light on the role of specific gut bacteria in the pathogenesis of NAFLD and offer valuable insights into future therapeutic interventions and preventive strategies.