Armstrong et al (2008). "Influence of betaine consumption on strenuous running and sprinting in a hot environment." J Strength Cond Res 22(3): 851-60.
Investigators found a trend towards longer sprint time until exhaustion when betaine was present in either a sports beverage or in flavored water compared to placebo.
Papers 2004-7:
Millard-Stafford et al (2005). "Fluid replacement in the heat - effects of betaine." Med Sci Sports Exerc 37(5): S28.