Everyone needs some D in their life! But don’t assume that your levels of this often-undervalued nutrient are good enough to keep you healthy. Vitamin D is a fat-soluble nutrient that plays a key role in pretty much all your physiological functions. At optimal levels, vitamin D may be responsible for protecting you against many modern-day diseases. Very few foods naturally contain vitamin D but you can eat certain foods and add supplements to give you the best results.