The importance of vitamin D in basically every aspect of health is being stressed everywhere it seems. Especially on this latitude, where the sun feels like some distant dream during 8 months of the year, it's a simple fact that we need something to substitute sunshine with. Not only do we need vitamin D to maintain some sort of energy levels during the dark months, but it also provides some life-saving attributes to our physical bodies. It prevents toxicity and regulates the concentration of calcium and phosphate in the blood stream, and is thus an essential ingredient for healthy bones. Vitamin D also helps to cure internal inflammations and acts as an immune system by defending the body from bad bacteria. Low vitamin D levels has been associated with cancer, increased mortality, osteoporosis, cardiovascular disease, multiple sclerosis, diabetes, alzheimer's, influenza, and tuberculosis.
This super D is basically the only nutrient that needs to be taken as a supplement (unless you eat high amounts of fish daily, or live in Australia). The ridiculous EU recommendation of 5 micrograms per day is nowhere near enough, and should be converted to between 50 and 150 micrograms/day. It's most easily taken in liquid form, free from any other unnecessary ingredients.
Read more about vitamin D here, here and here for example. Or here.