Betaine is formed from glycine and exists in  a variety of foods (e.g., wheat, beets, spinach, and shellfish).
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The body can also synthesize its own betaine, which assists in several important physiological functions in our bodies. In the body, betain helps keep cells hydrated, reduces inflammation, maintains intestinal function, protects DNA, and buffers homocystine levels (from meat digestion). Important for muscle function, betain works as a methyl donor to aid in creatine synthesis. Based on the physiological importance of betaine, scientists from the College of New Jersey decided to investigate the ergogenic effects of short-term supplementation. In a double-blind, placebo controlled study, researchers studied the impact of 2 weeks of betaine ingestion on muscular endurance, power performance, and rate of fatigue. Volunteers consisted of 24 active college-aged males who were assigned to either a betaine supplement group or a placebo group. Subjects were tested prior to supplementation, one week into supplementation, and two weeks into supplementation and testing occurred over 2 days. On day one, subjects performed a bench press throw power test, a vertical jump power test, and were asked to complete as many repetitions as possible on bench press and squat exercises at 75% of 1 rep maximum. Peak and average power were calculated for each rep completed. On the second testing day, subjects performed 2 Wingate anaerobic power tests, where each test was separated by a 5-minute active recovery. It was concluded that betaine supplementation significantly improved muscular endurance during squats and increased the total number of reps performed at 90% of peak power.  Remarkably, these results were observed within the first week of supplementation!

Hoffman JR, Ratamess NA, Kang J, Rashti SL, Faigenbaum AD. Effect of betaine supplementation on power performance and fatigue. Int Soc Sports Nutr. 2009 Feb 27;6:7.