Riboflavin is a water-soluble B vitamin.

Like the other B vitamins, it plays a key role in energy metabolism, and is required for the metabolism of fats, ketone bodies, carbohydrates, and proteins.

References

http://www.nlm.nih.gov/medlineplus/druginfo/natural/patient-riboflavin.html; Brody, Tom (1999). Nutritional Biochemistry. San Diego: Academic Press. ISBN 0-12-134836-9.