OVER the course of a year, Alex Feintuch, a 20-year-old sophomore at the University of South Carolina, spent more than $1,000 on fitness supplements. Mr. Feintuch wanted to add size and definition to his muscles, and to “see results as quickly as possible.” He did research and tried dozens of products, with mixed results. “Of the products that I have bought, I’ve found many that work well,” he said. “But some don’t and were a waste of money.”