The purpose of this pilot study is to determine if suppressing estrogen in premenopausal women results in changes in gut microbiota and if dietary flaxseed modulates these changes.

This pilot study will begin to address whether gut microbiota change with estrogen suppression. Specifically, the investigators will test whether gut microbial diversity and abundance change in response to estrogen suppression and consumption of dietary flaxseed. To test this possibility the investigators will recruit premenopausal women (age 20-40 years old)and collect fecal samples before and after 1 month of estrogen suppression with GnRH agonist. The investigators will analyze the gut microbiota in response to estrogen loss and whether this differs with the consumption of flaxseed.

Inclusion Criteria:

• healthy premenopausal women (20-40 years)
• normal to overweight (22-29.9 kg/m2)
• normally menstruating (25-35 day cycles)
• not have used estrogen-based contraception for >6 months.
• sedentary to moderately active (exercise ≤120 min week-1)
• must not be taking phytoestrogenic dietary supplements, or lipid- or glucose- lowering medications.

Exclusion Criteria:

• smoking
• pregnancy or breastfeeding
• Hormonal contraceptive use (past 6 mo.)
• Women with contraindications to GnRHAG:
• History of fragility fracture
• Low BMD (i.e., proximal femur or lumbar spine z scores < -2.0)
• Abnormal vaginal bleeding
• History of breast cancer or other estrogen-dependent neoplasms
• History of venous thromboembolic events
• Hypersensitivity to leuprolide acetate or benzyl alcohol (the vehicle for injection of leuprolide acetate)
• Evidence for depressive symptoms (Score ≥ 18 on the Beck Depression Inventory, BDI)
• Moderate or severe renal impairment defined as a calculated creatinine clearance <50 mL/min based on the equation of Cockcroft and Gault91
• Chronic hepatobiliary disease, conservatively defined as liver function tests (AST, ALT, alkaline phosphatase, Total Bilirubin) >1.5 times the upper limit of normal (if such values are obtained on initial screening and thought to be transient in nature, repeated testing will be allowed)
• antibiotic or probiotic use within 2 months of sample collection

Biological samples will be masked for endpoint analysis