Vitamin D is essential for strong immunity and maintaining healthy bones and teeth. Several studies have found that low levels of vitamin D in the blood correlate with a wide range of illnesses, including cancer, cardiovascular disease, diabetes, and osteoporosis.A few studies have even hinted that low vitamin D levels are associated with severe COVID-19, although the research is inconclusive.Despite these associations, evidence for the benefits of widespread vitamin D supplementation to prevent disease in otherwise healthy people has been mixed. For example, a large clinical trial found no evidence that a vitamin D supplement prevented cardiovascular disease and cancer in older adults.