Betaine is a naturally occurring osmolyte found in a handful of foods. Betaine may promote water retention in muscle cells which can increase cell swelling during exercise, AKA: help you get a better pump. This increase in cell swelling may also benefit protein signaling after exercise. Betaine has a decent amount of research behind its effectiveness, however, we’re lacking studies that last longer than 1 month when concerning betaine use. Regardless, from acute and short term data, betaine seems like a solid addition to a pre- or intra-workout supplement.
▪️ Source: Apicella, J. M., Lee, E. C., Bailey, B. L., Saenz, C., Anderson, J. M., Craig, S. A., ... & Maresh, C. M. (2013). Betaine supplementation enhances anabolic endocrine and Akt signaling in response to acute bouts of exercise. European Journal of Applied Physiology, 113(3), 793-802.