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Vitamin D is both a nutrient you eat as well as a hormone that's produced by your skin in response to sunlight exposure. It plays a significant role in bone health by helping regulate calcium and phosphorus, which are essential minerals that work together to build strong bones. Vitamin D also has a role in your nerve, muscle and immune systems, as well as your mental health.
This vitamin D blood test evaluates 25-hydroxyvitamin D (25-D): the best measure of how much vitamin D is stored in your body. A prolonged deficiency in vitamin D has been shown to lead to fatigue, bone pain and even depression.
While Vitamin D occurs naturally in very few foods, many are fortified with it, including dairy products and cereals. If vitamin D levels are low, vitamin supplementation may be helpful, especially when access to natural sunlight is limited. Vitamin D rich-foods include vitamin D fortified milk or plant-based alternatives, vitamin D fortified breakfast cereals and fatty fish (trout, salmon, tuna and mackerel and fish liver oils).