The proposed study is aimed at examining mitochondrial function as a potential target of action of vitamin D on muscle metabolism, size, and strength in preventing the progression of cachexia. This is the first clinical trial designed to understand the effects of vitamin D on muscle metabolic dynamics driving dysfunction in cachectic muscle. Our preliminary data suggest that vitamin D promotes lipid partitioning and muscle metabolic function, which the investigators hypothesize, will mitigate cachexia via improved muscle health and quality that translates into reduced fatigue, and improved patient resilience to multimodal cancer therapy.