Acute effects of combined and isolated caffeine and theanine supplementation on physical and cognitive performance in competitive athletes: a randomized, double-blind, placebo-controlled crossover study - Tuncer, S. Y.; Ozdenk, S.; Yildirim, U. C.; Erkan, D.; Sari, C.; Gundem, M. C.; Sar, H.; Yilmaz, B.; Karakulak, I.; Akca, F. (Frontiers in Nutrition, 2026)