Pittsburgh — The vitamin and health supplement niche of the brick-and-mortar retail market continues to experience intense competitive pressure, observes veteran retail consultant Jeff Green. So it came as no surprise to Green when GNC Holdings Inc. officials revealed in a second-quarter earnings call earlier this week that it will shutter up to 900 stores in North America by the end of 2020. “It used to be that specialty health supplements were only found in specialty stores such as GNC, Vitamin Shoppe and other regional chains,” says Green, a partner …