Cancer develops when normal cells undergo a series of DNA damage, ultimately leading to the transformation of a pre-cancerous lesion to a malignant tumor. In this context, exposure over the life course is what will pave the way for a susceptible organism to exogenous and endogenous agents for cancer development. Current research on the correlation between cancer and nutrition has predominantly concentrated on preventive measures, leaving a significant void in the realm of co-treatment and care. Although some patients will have only one treatment, the majority will have a combination of treatments, such as surgery often followed by chemotherapy and radiation therapy. Malnutrition and a loss of muscle mass commonly occur in cancer patients and can exert a detrimental impact on clinical outcomes. This chapter aims to provide a comprehensive overview of the changes in metabolism and body composition that occur in individuals with cancer.