Betaine (TMG or trimethylglycine) is an amino acid compound found for instance in spinach and wheat. If you are cutting out carbohydrates from your diet in order to lose fat, you might unknowingly also be cutting out betaine. 100g of whole wheat bread will provide a bit over 1g of betaine. Spinach is then an alternative way to increase your betaine levels, since it is particularly high in this nutrient, with 645 milligrams per 100 grams. However, chewing 100g of whole wheat bread and chewing 100g of spinach is two different things.
As a supplement, betaine is typically derived from beets. Considering that so few people eat beets, it would be easy to draw the conclusion that this supplement will of course stand out in clinical studies. But it is worth mentioning that choline is a precursor to betaine. This means that you don't necessarily have to consume betaine containing foods to get the benefits from betaine. Choline containing foods, such as chicken, beef liver, eggs or soybeans, can also improve your betaine levels.
According to one study betaine increased peak cycling power during repeated high-intensity intervals. The dosage was 2.5g per day [ref 1]. A study from 2010 confirmed that two weeks of betaine supplementing improves the vertical jump [ref 2].
ref 1: J Luke Pryor,corresponding author Stuart AS Craig, and Thomas Swensen - Effect of betaine supplementation on cycling sprint performance. J Int Soc Sports Nutr. 2012; 9: 12.
ref 2: Elaine C Lee, Carl M Maresh,corresponding author William J Kraemer, Linda M Yamamoto, Disa L Hatfield, Brooke L Bailey, Lawrence E Armstrong, Jeff S Volek, Brendon P McDermott, and Stuart AS Craig - Ergogenic effects of betaine supplementation on strength and power performance. J Int Soc Sports Nutr. 2010; 7: 27.