Strength and Muscle Related Outcomes for Nutrition and Lung Function in CF

Inclusion Criteria: * Cohort 1: Participants are eligible if their percentage of predicated forced expiratory volume in 1 second (ppFEV1) is \<70% during the 12 months prior to enrollment (\>50% of measurements, eliminating periods of exacerbation) * Cohort 2: Participants are eligible if their percentage of predicted forced expiratory volume in 1 second (ppFEV1) is 70% or greater during the 12 months prior to enrollment (\>50% of measurements, eliminating periods of exacerbation). * Both cohorts match by age, gender, race and CFTR genotype severity. Exclusion Criteria: * No prior solid organ transplantation * No initiation of an investigation drug within 28 days prior to and including Baseline Visit. * No initiation of new chronic therapy (e.g., ibuprofen, azithromycin, inhaled tobramycin, Cayston, CFTR modulator, etc.) within 28 days prior to and including Baseline visit. * No acute use of antibiotics (oral, inhaled or IV) or acute use of systemic corticosteroids for respiratory tract symptoms within 14 days prior to and including Baseline visit. * For the BIA sub-study - Individuals with an implanted pacemaker will be excluded. * No initiation of a drug for weight loss (such as a GLP-1 receptor agonist) or bariatric surgery within 6-months prior to and including the Baseline visit. * Patients with continued rapid change or extreme GI symptoms related to weight loss therapy should be excluded at the discretion of the study investigator.