Resistance and Aerobic Exercise for Prevention in Women With Dense Breasts (REP-D)

The goal of this study is to learn more about how exercise might lower the risk of developing breast cancer in women with dense breast tissue by studying changes that occur in breast tissue and blood as a result of participating in an exercise program.

The names of the study groups in this study are:

* Exercise Training Group
* Waitlist Control Group

The goal of this study is to learn more about how exercise might lower the risk of developing breast cancer by studying changes that occur in breast tissue and blood as a result of participating in an exercise program. Participants will be placed into one of two study groups: Exercise Training Group vs. Waitlist Control Group. Randomization means that a participant is put into a group by chance.

The research study procedures include screening for eligibility and baseline and end of study assessments which include: breast biopsies, assessment of physical activity habits, aerobic and strength testing, blood draws and questionnaires.

Participants randomized to the exercise group will be expected to participate in exercise sessions throughout the study. They will also be asked to complete logs of their activity.

Participants will be in this research study for up to 16 weeks.

It is expected that 46 women will take part in this study.

The Breast Cancer Research Foundation is supporting this research study by providing funding.

Eligibility Criteria:
Inclusion Criteria:

* Women aged 18-59
* Heterogeneously dense/extremely dense (BIRADS 3 or 4) breast tissue on mammogram within the past 12 months
* Physically inactive; engaging in \<60 minutes of moderate or vigorous intensity exercise per week, as assessed by Godin Leisure Time screener
* Not pregnant or breastfeeding
* English speaking and able to read English. Investigators are only enrolling English-speaking participants to this study at this time due to the fact that investigators currently only have English-speaking exercise trainers with the proper qualifications to conduct the exercise training
* Written informed consent prior to any study-related procedures

Exclusion Criteria:

* Self-reported inability to walk 2 blocks (at any pace)
* Prior history of breast cancer; prior DCIS is allowable as long as participant is not taking endocrine therapy and has at least 1 breast that has not been irradiated
* On oral or implantable hormonal contraceptives, postmenopausal hormone replacement therapy, a selective estrogen receptor modulator or an aromatase inhibitor; Mirena or other IUD is acceptable
* Consistent strength training in the past 3 months
* Use of weightloss drugs
* Cardiovascular, respiratory or musculoskeletal disease or joint problems that preclude moderate exercise. Examples would include unstable angina, recent myocardial infarction, oxygen-dependent pulmonary disease, and osteoarthritis requiring imminent joint replacement. Moderate arthritis that does not preclude physical activity is not a reason for ineligibility
* Psychiatric disorders or conditions that would preclude participation in the study intervention (e.g. untreated major depression or psychosis, substance abuse, severe personality disorder)