Myopenia and Mechanisms of Chemotherapy Toxicity in Older Adults With Colorectal Cancer

Inclusion Criteria:

* Newly diagnosed metastatic CRC or newly recognized metastatic recurrence of CRC \>= to 3 months (12 weeks) from completion of treatment of non-metastatic CRC.
* Planning to or recently started to undergo undergo immunotherapy and/or 5-FU based chemotherapy as a first line of treatment. 5-FU chemotherapy can be 5-FU alone or in combination with oxaliplatin and/or irinotecan +/- immunotherapy. Capecitabine is also acceptable.
* Estimated life expectancy \>= 6 months.
* Patients must be able to comprehend English or Spanish (for questionnaire completion).
* Ability to understand and the willingness to sign a written informed consent document.
* Patient eligibility is not dependent on BMI or weight. Patients with a significant (+- \> 10%) body weight change in the previous 12 months are eligible for this study.

Exclusion Criteria:

* Patients enrolled on hospice.
* Prior systemic chemotherapy for metastatic colorectal cancer (ok if adjuvant chemotherapy completed \>= 3 months (12 weeks) prior to this disease disease recurrence and treatment).
* Patients may not be receiving any other investigational agents.
* No untreated brain metastases. Patients with treated brain metastases are eligible.
* Patients on or planned to undergo radiation therapy in near future.