This opinion paper presents a short review of the potential impact of protein on muscle anabolism in cancer, which is associated with better patient outcomes. Protein source is a topic of interest for patients and clinicians, partly due to recent emphasis on the supposed non-beneficial effect of proteins; therefore, misconceptions involving animal-based (e.g., meat, fish, dairy) and plant-based (e.g., legumes) proteins in cancer are acknowledged and addressed. Although the optimal dietary amino acid composition to support muscle health in cancer is yet to be established, animal-based proteins have a composition that offers superior anabolic potential, compared to plant-derived proteins. Thus, animal-based foods should represent the majority (i.e., ≥65%) of protein intake during active cancer treatment. A diet rich in plant-derived proteins may support muscle anabolism in cancer, albeit requiring a larger quantity of protein to fulfill the optimal amino acid intake. We caution that translating dietary recommendations for cancer prevention to cancer treatment may be inadequate to support the pro-inflammatory and catabolic nature of the disease. We further caution against initiating an exclusively plant-based (i.e., vegan) diet upon a diagnosis of cancer, given the presence of elevated protein requirements and risk of inadequate protein intake to support muscle anabolism. Amino acid combination and the long-term sustainability of a dietary pattern void of animal-based foods requires careful and laborious management of protein intake for patients with cancer. Ultimately, a dietary amino acid composition that promotes muscle anabolism is optimally obtained through combination of animal- and plant-based protein sources.