Betaine is a functional nutrient commonly used as a feed additive in animal nutrition, mainly as methyl donor and organic osmolyte. Compared to choline, betaine is more efficient as it can directly donate one of its methyl groups, whereas choline needs to be converted to betaine within the mitochondria of the liver cells. Additional betaine molecules in the diet increase the efficiency of osmoregulation. Betaine is absorbed by stressed cells to maintain (intestinal) cell integrity, cellular activity, protein structure and homeostasis. Positive effects of betaine are reported on feed digestibility, carcass quality and in periods of heat stress.