Background: Dietary supplements (DS), defined as herbal preparations, vitamins and minerals (DSHEA Act 1994), are used by 70% of cancer survivors. The World Cancer Research Fund/American Institute for Cancer Research 2018 Cancer Prevention Recommendations specify not to use DS to protect against cancer. There is some evidence that DS are linked to harmful outcomes, including increased mortality and breast cancer recurrence. DS have potential pharmacokinetic interactions via cytochrome P450 enzyme and pharmacodynamic interactions via estrogenic or hepatotoxic activity, which may increase or reduce clinical efficacy or potentiate adverse effects of Tamoxifen and Aromatase Inhibitors (AIs). This analysis examines baseline usage of DS and potential interactions with Tamoxifen and AIs among breast cancer survivors enrolled in the Lifestyle, Exercise, and Nutrition (LEAN) study, a randomized healthy eating and exercise lifestyle intervention. Methods: We examined the use of DS in 151 breast cancer survivors enrolled in the LEAN Study. Women with a body mass index (BMI) ≥ 25 kg/m 2 were randomly assigned to a 6-month healthy eating and exercise lifestyle intervention (n=93) or to usual care (n=58). At baseline and 6 months, we asked participants to self-report regular DS use, defined as at least 3 times a week for at least one month. The intervention group received evaluation and counseling of DS use by a Registered Dietitian (RD) with a Certified Specialty in Oncology Nutrition (CSO). Potential interactions of moderate and major ratings between DS with Tamoxifen and AIs (Letrozole, Exemestane and Anastrozole) were identified through the Natural Medicines Database by both a pharmacist specializing in oncology and a RD, CSO. Results: Out of 151 women, at baseline 120 (80%) reported using DS. Fifty-four different formulations were enumerated. The majority of women (72, 60%) were taking ≥ 3 formulations. One quarter of women (29, 24%) were taking ≥ 5 formulations (range = 1 - 20). Of the 54 formulations, 33% had potential interactions with Tamoxifen and/or AIs according to the Natural Medicines Database. Total interactions by medication were: 18 Tamoxifen, 14 Letrozole, 14 Exemestane, 9 Anastrozole. The majority of these interactions (87%) were associated with herbal preparations. Conclusions: Our findings are indicative of the high prevalence of DS use among breast cancer survivors and the potential risk of interactions with prescribed hormonal therapy. More research is needed to test if counseling from a RD, CSO can reduce DS use and ultimately lead to better outcomes due to fewer interactions with breast cancer treatments.

Citation Format: ThaiHien Nguyen, Maura Harrigan, Courtney Mcgowan, Annette Hood, Fangyong Li, Brenda Cartmel, Leah Ferrucci, Melinda L Irwin, Tara Sanft. Dietary supplement use in a healthy eating and exercise lifestyle intervention in breast cancer survivors: The lifestyle exercise and nutrition (LEAN) study [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS8-09.