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.