CDK and Body Composition Study

Inclusion Criteria:

* Histologically confirmed adenocarcinoma of the breast that is Estrogen Receptor (ER) and/or Progesterone Receptor (PR) positive based on current American Society of Clinical Oncology-College of American Pathologists (ASCO-CAP) guidelines
* Metastatic or locally advanced/inflammatory, unresectable breast cancer not amenable to potentially curative surgery
* Measurable and/or non-measurable as defined by RECIST 1.1 criteria
* Patients must be a candidate to start an FDA approved CDK 4/6 inhibitor (palbociclib, ribociclib, abemaciclib) as part of standard of care treatment
* Female, or male patients, and age \>=18 years
* Eastern Cooperative Oncology Group (ECOG) performance status 0-2
* Ability to understand and the willingness to sign a written informed consent document
* Concomitant therapy with bisphosphonates, RANKL inhibitors or growth-colony-stimulating factor (G-CSF) is allowed as per physician decision

Exclusion Criteria:

* History of allergic reactions attributed to compounds of similar chemical or biologic composition to CDK 4/6 inhibitors or other agents used in the study (e.g., fulvestrant, letrozole, anastrozole, exemestane)
* BMI \< 18.5
* Prior CDK 4/6 use in any setting
* Inability to undergo anthropometric measurements
* Inability to undergo CT scan imaging
* Women of child-bearing potential must not be pregnant or breast feeding. They must also agree to use adequate contraception (hormonal or barrier method of birth control) and not be breast feeding prior to study entry, for the duration of study participation, and for up to 10 days after completion of all protocol therapy. Should a woman become pregnant or suspect she is pregnant while participating in this study or up to 10 days after completion of protocol therapy, she should inform her treating physician immediately
* Intercurrent illness that would substantially increase the risk of treatment associated complications (e.g., active infection, uncontrolled diabetes mellitus or hypertension)
* Psychiatric illness/social situations that would interfere with the patient's ability to comply with the treatment regimen
* Patients with untreated brain metastasis are excluded. Patients with a prior history of brain metastasis are eligible if they have received prior brain radiation (whole brain or stereotactic radiosurgery) or surgery, have stable intracranial disease for at least 3 months after completion of local therapy, and are not taking corticosteroids for treatment of brain metastasis
* Patients who have not recovered (i.e., CTCAE Grade ≤1 or baseline) from an adverse event due to a previously administered agent, excluding alopecia
* Patients with inability to swallow and retain pills
* Malabsorption syndrome or other gastrointestinal illness that could affect oral absorption
* Patients with active implanted medical devices (cardiac pacemakers, defibrillators or patients connected to electronic life support devices)