By Noel Randewich SAN FRANCISCO (Reuters) – GNC Holdings, Vitamin Shoppe and other dietary supplement sellers could take a hit to their financial health in 2016 if safety concerns attract new action from U.S. federal and state officials. Shares of GNC and Vitamin Shoppe fell more than 30 percent last year after allegations of impure and dangerous ingredients in dietary supplements hurt already sluggish demand for their products. Last year, a challenge from New York’s attorney general led to an agreement by GNC to adopt new testing standards for ingredients used in its products.