Most cyclists do not need to take vitamins, however, some cyclists may benefit from vitamin and mineral supplements if they are deficient in a nutrient or fall short of the recommended daily intake.
For example, vitamin D
vitamin Dis found in only a few foods, and although we can produce it from sun exposure, many people have suboptimal vitamin D levels. In this instance, supplemental vitamin D may be helpful for maintaining bone health and normal immune function but it depends on a variety of factors such as dietary intake, sun exposure, genetics, and skin color.
Eating a well-balanced diet with fruits, vegetables, whole grains, and protein, should largely provide the nutrients you need to support cycling performance and general health. If you think you may be deficient in a certain nutrient, talk to your doctor about checking your levels. More information on supplements for biomarkers can be found here