Vitamin D repletion is linked to improved muscle mitochondrial function, lipid deposition and preservation; however, while vitamin D insufficiency is common in cancer, the mechanistic effects of vitamin D on muscle metabolic health in cancer patients have not been studied. This is important to address because cancer cachexia is characterized by marked muscle wasting, anabolic resistance, ectopic fat infiltration, mitochondrial dysfunction and contributes to decreased survival. With novel strategies to address this knowledge gap, the investigators will use a combination of advanced metabolic analytical approaches with complementary model systems in cell culture and human subjects to understand the biochemical and physiological mechanisms underlying cancer cachexia in relation to the role of vitamin D in conjunction with resistance exercise (RE). By combining analyses of muscle size and local tissue hemodynamics in vivo, metabolomics analyses of muscle tissue and isolated mitochondria, and changes in anabolic cell signaling, lipid metabolism and oxidative capacity of primary muscle cells in vitro, the investigators will identify mechanisms underlying muscle response to vitamin D repletion. Our previous findings, together with data that exercise improves muscle vitamin D storage and retrieval, suggest that vitamin D repletion synergizes with RE to improve muscle metabolic function and protein synthesis. Our overall objective is to examine mitochondrial function and anabolic resistance as potential targets of action of vitamin D on muscle metabolism, size and strength in preventing the progression of cachexia. The aims of this study are to: 1) non-invasively quantify lipid redistribution, local muscle tissue metabolism and muscle mass and strength of cancer patients before and after 12 weeks of double blinded vitamin D repletion with exercise and protein supplementation (VitD) compared to exercise and protein supplementation only (Ctl); 2) determine differences in muscle mitochondrial function in live tissue biopsied from human gastrocnemius from VitD compared to Ctl; and 3) identify mechanisms whereby vitamin D and exercise regulate muscle anabolic signaling and mitochondrial activity in primary human myotube cultures. Our central hypothesis is that vitamin D promotes muscle lipid availability for β-oxidation in response to exercise, thereby preventing lipotoxicity in the muscle and potentially improving anabolic sensitivity in muscle during cancer cachexia. The impact of this project, the first nutrition and exercise study designed as an inexpensive intervention, is to understand the effect of vitamin D on the metabolic and anabolic dynamics which underpin dysfunction in cachectic muscle. If vitamin D promotes lipid partitioning, muscle metabolic function and/or anabolic sensitivity, these adaptations will ultimately improve cancer therapy by combating cancer cachexia. Further, diffuse optical spectroscopy techniques have the potential to identify the minimum effective intervention dose for optimizing metabolic health leading to more practical and individualized lifestyle prescriptions to reduce health care costs.