Vitamin D deficiency initiates a loss of combat effectiveness by impairing physical and cognitive functioning of combat Operators. Synthesized in response to sunlight and consumed in the diet, vitamin D functions as a hormone and regulates gene expression for nearly 300 genes throughout the human body. These target genes are involved processes essential to combat operations, such as immune function, response to stress, inflammation, and regulation of calcium movement. Since widespread vitamin D deficiency is observed across the U.S. population, poor vitamin D status is expected in Servicemembers. Physical conditions linked to vitamin D deficiency include increased risk for muscle or bone injury, muscle weakness, and reduced neuromuscular function. Hormonally, vitamin D levels have been positively correlated with testosterone levels. Vitamin D deficiency is also associated with cognitive decline, depression, and may prolong recovery following mild traumatic brain injury (mTBI). Since vitamin D deficiency elevates systemic inflammation, poor vitamin D status at the time of brain injury may prolong the inflammatory response and exacerbate postconcussive symptoms. Furthermore, veterans with mTBI experience chronic endocrine dysfunction. While vitamin D status has not been assessed post-mTBI, it is plausible that vitamin D levels are altered along with testosterone and growth hormone, raising the question of whether vitamin D deficiency results from trauma-related hormonal abnormalities or whether vitamin D deficiency increases the risk for endocrine dysfunction. Through its association with testosterone production, vitamin D deficiency may increase the risk for posttraumatic stress disorder (PTSD) since testosterone levels are altered in veterans with PTSD. Therefore, vitamin D status has a significant impact on Operator health and performance. Supplementing vita-min D to deficient Operators provides a noninvasive and low-cost intervention to maintain combat force.