Weight Loss and Omega-3 Supplementation Modulate the Microbiome in Women with Increased Breast Cancer Risk

Obesity is associated with gut dysbiosis, chronic inflammation, and insulin resistance. We assessed the proportional change in fecal microbial populations in a pilot study (n = 34) of peri/postmenopausal women with a body mass index ≥28 kg/m2 who were randomized to receive either 3.25 g/day of omega-3 fatty acids or a placebo during a weight loss intervention. Body composition was assessed using dual X-ray absorptiometry, and fecal and blood samples were collected. The median weight change was -10%. Among participants who lost ≥10% of their weight, those assigned to omega-3 fatty acids showed the greatest decrease in the Firmicutes:Bacteroidetes ratio and displayed favorable changes in systemic biomarkers. Notable increases in the proportional abundance of short-chain fatty acid (SCFA)-producing microbes including Phocaeicola vulgatus and Alistipes putredinis were observed in women receiving omega-3, which correlated with improvements in breast cancer biomarkers such as bioavailable estradiol, adiponectin:leptin ratio, and C-reactive protein levels. Women administered omega-3 fatty acids displayed increased % change in plasma SCFA propionate and decreased butyrate, suggesting that intervention differentially modulated circulating bacteria-derived SCFA metabolites. High-dose omega-3 fatty acids, when added to a behavioral weight loss intervention, promoted beneficial shifts in the gut microbiome and associated with improved breast cancer risk factor biomarkers. PREVENTION RELEVANCE: Obesity is a modifiable risk factor for breast cancer, characterized by chronic inflammation and altered adipokines. This trial addresses the need to enhance weight loss by targeting underlying metabolic and inflammatory drivers. We show that omega-3 polyunsaturated fatty acids (eicosapentaenoic acid/docosahexaenoic acid) shift SCFA-producing microbiota, increase propionate, and correlate with improved breast cancer risk factor biomarkers.