Why GNC filed for bankruptcy protection despite vitamin sales boom

The vitamin and dietary supplement market in the U.S. is worth almost $50 billion, according to the Nutrition Business Journal. Brick-and-mortar retailers like GNC, The Vitamin Shoppe and Vitamin World have faced an onslaught of competition from big-box retailers like Walmart and Target, grocery stores like Kroger and Whole Foods and e-commerce giants like Amazon.