Background: Non-alcoholic fatty liver disease (NAFLD) had become a growing public health problem worldwide, and dietary interventions had important potential in the prevention and treatment of NAFLD. Moreover, previous animal studies had shown that flaxseed had a good improvement effect in animal NAFLD models. Objectives: Assess whether flaxseed powder could improve liver lipid content in patients with NAFLD. Methods: In this 12-week randomized controlled clinical trial, 50 patients were randomly assigned to the flaxseed group (n = 25) and the control group (n = 25). The flaxseed group received 30 g/d of flaxseed powder orally before lunch or dinner along with health education, while the control group received only health education. The primary outcome was intrahepatic lipid content assessed by proton density fat fraction estimated by magnetic resonance imaging, and secondary outcomes were body composition measurements, liver function, and glucolipid metabolism. Results: Patients in the flaxseed group showed significantly lower liver fat content, body fat percentage, obesity index, visceral fat area, serum total bilirubin (TBIL), direct bilirubin (DBIL), indirect bilirubin (IBIL), aspartate aminotransferase (AST), total cholesterol (TC), and triglyceride (TG) levels after a 12-week intervention compared to pre-intervention levels, while serum apolipoprotein A1 (Apo A1) and high-density lipoprotein cholesterol (HDL-C) levels significantly increased, with all differences being statistically significant (P < 0.05). Analysis of gut microbiota showed that, at the phylum level, flaxseed intervention significantly increased the abundance of Bacteroides and Actinobacteria, while decreasing the ratio of Firmicutes to Bacteroidetes. At the genus level, the relative abundance of Clostridium-sensu-stricto-1, Parasutterella, Lachnospiracea-NK4A136-group, Eubacterium-xylanophilum group, and Bifidobacterium in the gut microbiota of the flaxseed group was significantly higher than that of the control group (P < 0.05), whereas the relative abundance of Coriobacteriaceae_UCG-002 was significantly lower than that of the control group (P < 0.05). Conclusions: Flaxseed powder intervention for 12 weeks had the effect of improving liver lipid deposition, liver function, body composition indicators, and lipid metabolism in patients with NAFLD. It also regulated the gut microbiota in NAFLD patients, increasing the abundance of beneficial bacteria while reducing harmful bacteria. This suggested that flaxseed was one of the natural and effective foods for improving NAFLD.
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