Vitamin D status and physical activity interact to improve bone mass in adolescents. The HELENA Study - Valtuena, J; Gracia-Marco, L; Vicente-Rodriguez, G; Gonzalez-Gross, M; Huybrechts, I; Rey-Lopez, J P; Mouratidou, T; Sioen, I; Mesana, M I; Martinez, A E Diaz; Widhalm, K; Moreno, L A (Osteoporosis International, 2012)