The sunshine vitamin. Vitamin D is a fat-soluble vitamin that has properties of both a vitamin and a hormone and is required for the absorption and utilisation of calcium and phosphorus. Sunlight triggers the body to make its own vitamin D, so supplementation would be needed in the winter months. Crucial for strong bones and healthy teeth, vitamin D also keeps the immune system strong and is now considered one of the most important vitamins because of its numerous protective roles in the mutation of cells, elevation of mood and cardiovascular health. It can help relieve conjunctivitis and helps the body assimilate vitamin A. Signs of vitamin D deficiency: Severe tooth decay, rickets and other bone and dental problems.