Vitamin D, which is produced by the body through exposure to sunshine, helps the body control calcium and phosphate levels, important for healthy bones. It can also be obtained from food sources such as fatty fish and egg yolks but it can be difficult to obtain enough vitamin D from food alone in countries with little sunlight so food is often fortified.
For the benefit of the never-ending supplement fad industry, some papers have linked vitamin D deficiency with a host of health problems including cardiovascular disease, cognitive impairment, autoimmune conditions, and now Vitamin D levels have been linked to an increased risk of bladder cancer, though the source is a systematic review of just seven studies so it may not warrant a panic attack just yet.