This study evaluates cancer-related weight and muscle mass loss, symptoms, and physical function (cachexia) in patients undergoing treatment for colorectal, lung, or pancreatic cancer that cannot be removed by surgery (unresectable). Patients with these cancer types are at risk for developing cancer cachexia (CC), which is defined as weight loss, muscle loss, and fat loss due to cancer. CC has been associated with reduced physical performance, impaired quality of life, and poorer survival. Many studies that have evaluated treatments for cancer-related weight and muscle loss have aimed to treat all patients with weight loss exactly the same and, unfortunately, have not been successful. Like different cancer types, weight and muscle loss related to cancer may have different causes in different individuals and the best treatment strategy for this condition may not be a one-size-fits-all approach. Information gathered from this study may help researchers develop new diagnostic criteria for CC and design better treatments and clinical trials for cancer-related weight and muscle loss in the future to improve the quality of life in patients with advanced colorectal, lung, or pancreatic cancer.