Abstract PO5-14-01: Weight loss and omega-3 polyunsaturated fatty acid intervention in overweight and obese peri- and postmenopausal women with increased breast cancer risk modifies the gut microbiome and is associated with circulating biomarkers

Background: Obesity-associated gut microbiome dysbiosis is thought to increase risk for postmenopausal breast cancer through alteration of estrogen metabolism and an increase in pro-inflammatory microbe abundance. Many studies have reported an increase in the Firmicutes/Bacteroidetes phyla ratio as a biomarker of obesity and is associated with adverse metabolic outcomes. Marine omega-3 eicosapentaenoic (EPA) and docosahexaenoic (DHA) fatty acids are thought to have favorable metabolic and inflammation resolving properties resulting in reduced risk for several epithelial cancers. One possible mechanism is the effect of EPA and DHA on the gut microbiome. We assessed the effect of weight loss and/or omega-3 supplementation on the gut microbiome and circulating molecular breast cancer risk factors. Methods: 46 peri/postmenopausal overweight and obese women at increased risk for breast cancer were enrolled into a weight-loss intervention and after 2 weeks randomized to 3.25 g/day combined EPA and DHA or placebo (PMC8416797). Fecal samples were collected at 0, 2-weeks and 6-months, with matched fecal samples for all timepoints available for 26 patients. Fecal samples at 6-months were available for 34 patients. 12M read depth metagenomic sequencing was performed on DNA isolated from feces to determine shifts in the gut microbiome. Fasting and non-fasting blood was also collected for circulating biomarker analysis. Body composition by DXA scan were also performed at baseline and 6-month time points. Results: Mean baseline BMI was 32 kg/m2. There was no significant difference between arms in age, baseline BMI, % body fat, or baseline Firmicutes/Bacteroidetes ratio. We identified gut microbiota species (A. hadrus and P. faecium) that correlated with metabolic parameters (% body fat, adiponectin/leptin ratio, and fasting insulin), species (P. dorei and B. ovatus) that correlated with estradiol levels, and mucin-degrading species (D. formicigenerans and R. gnavus) whose abundance correlated with inflammation (as measured by circulating monocyte-chemoattractant protein 1 (MCP-1)). Median 6-month weight change for all randomized subjects was -10% from baseline. Favorable change in the Firmicutes/Bacteroidetes ratio was associated both with the degree of weight loss and with omega-3 supplementation, although significant species modification differed between weight loss and omega-3 PUFA groups. Women with both >10% weight loss and omega-3 supplementation had a significantly reduced Firmicutes abundance and lower Firmicute/Bacteroidetes ratio than other groups. Species abundance shifts appear to differ between weight loss and omega-3 supplementation with 3 species modified by weight loss and 12 species shifted by omega-3 PUFA administration. Omega-3 PUFA supplementation significantly reduced proportional abundance of Dorea formicigenerans, which was a microbe associated with inflammation. Conclusions: Omega-3 supplementation at 3.25 g/day compared to placebo had a greater net effect on changing the gut microbiome in overweight and obese women at increased risk for breast cancer undergoing a weight loss intervention. Change in the gut microbiome may serve as a useful response biomarker in Phase II prevention trials of omega-3 fatty acid and weight loss. Citation Format: Katherine L. Cook, Erin Giles, Amy Kreutzjans, Shahid Umar, Christie Befort, Bruce Kimler, Stephen Hursting, Carol Fabian. Weight loss and omega-3 polyunsaturated fatty acid intervention in overweight and obese peri- and postmenopausal women with increased breast cancer risk modifies the gut microbiome and is associated with circulating biomarkers [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-14-01.