Betaine-13C3 (Trimethylglycine-13C3; Carboxy-N,N,N-trimethylmethanaminium-13C3) is the 13C labeled form of Betaine. This natural compound, also known as Trimethylglycine, is present in numerous foods and acts as an active methyl donor crucial for maintaining normal DNA methylation patterns. It is widely distributed in plants, animals, microorganisms, and rich dietary sources such as seafood, spinach, and wheat bran. Additionally, Betaine serves as an osmolyte, aiding in the preservation of cellular water and ion balance in poultry, thereby enhancing their heat stress tolerance. It supports protective osmotic activity, particularly in birds facing heat stress. Moreover, Betaine can bolster the resistance of various gut microbes to osmotic changes, thereby improving microbial fermentation activity.