This question is about Running
Most runners do not need to take any Vitamin B12 unless they are deficient. Vitamin B12 is abundant in animal products (such as meat, dairy and eggs); however, since most plant-based foods are low in vitamin B12, vegetarian and vegan runners are at risk for deficiency.
Since vitamin B12 is needed for energy pathways in the body, runners with poor vitamin B12 status may have a decreased ability to perform exercise at high intensities [ 1
1]. If you’re deficient in vitamin B12, talk to your healthcare provider about adding a supplement to your routine.
Woolf, K., & Manore, M. M. (2006). B-Vitamins and Exercise: Does Exercise Alter Requirements?, International Journal of Sport Nutrition and Exercise Metabolism, 16(5), 453-484. Retrieved Jul 20, 2021, from http://journals.humankinetics.com/view/journals/ijsnem/16/5/article-p453.xml