You might’ve heard that vitamin D can be crucial for bone health, and that taking supplements may be in order if you’re not getting enough naturally from sunlight or from D-rich foods like eggs, oily fish, or fortified milks. You’ve maybe even heard that vitamin D has been shown to improve cognitive function and to lower the risk of heart attack . It may come as welcome news to learn that vitamin D could also heighten your energy levels .

That makes sense, since, according to the Cleveland Clinic , some of the top symptoms of vitamin D deficiency in adults are fatigue, muscle weakness, and low mood, which can lead to feeling down and exhausted. What is vitamin D? Vitamin D, also referred to as calciferol, is a fat-soluble vitamin available in foods, either naturally or through fortification, and through supplements, according to the National Institutes of Health Office of Dietary Supplements . It is also produced within the body’s cells when ultraviolet (UV) rays from sunlight hit the skin, triggering vitamin D synthesis.

No matter what the source, though, vitamin D must be activated within the body—first through the liver, which converts vitamin D to calcidiol, and then in the kidney, which forms what’s known as calcitriol. Once activated, vitamin D promotes calcium absorption in the gut, prevents muscle cramps and spasms, and leads to healthy bone growth. Vitamin D also plays a role in reducing inflammation, immune function, and glucose metabolism.

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