A daily multivitamin, and maybe an extra vitamin D supplement, is a good way to make sure you're getting all the nutrients you need to be healthy. True, a healthy diet should provide nearly all the nutrients you need. But many people don't eat the healthiest of diets. That's why a multivitamin can help fill in the gaps, and may have added health benefits. The folic acid in most multivitamins helps prevent neural tube defects in newborns, if women take it before they become pregnant; folic acid may also lower the risk of heart disease, colon cancer, and breast cancer.
Vitamin D from a multivitamin or single supplement can lower the risk of colon and possibly many other cancers, as well as other chronic diseases.