Vitamins are by definition essential nutrients. B vitamins, in particular, are involved in enzyme systems that yield energy from the food we eat. If vitamin status is poor, then these enzyme systems do not operate at full capacity. This may be more noticeable if you are active. Supplementing with additional B vitamins can allow these systems to work at a higher capacity, allowing you to “feel” more energetic. It is important to note, however, that B vitamins do not provide energy on their own. They are not stimulants, like caffeine. If your B vitamin status is fine and you supplement with more, I would NOT expect an increase in energy. Vitamin D is important for normal health and functioning, but should not have any noticeable impact on energy levels.