This review focuses on the metabolic and osmoregulatory functions of betaine and its impact on nutrient digestibility and performance in pigs and poultry. Betaine is the trimethyl derivative of the amino acid glycine, and is present in plant and animal tissue. It has been shown to play an important role in osmoregulation of plants, bacteria and marine organisms. Due to its chemical structure, betaine exerts a number of functions both at the gastrointestinal and metabolic level. As a methyl group donor, betaine is involved in transmethylation reactions and donates its labile methyl group for the synthesis of several metabolically active substances such as creatine and carnitine. Therefore, supplementation of betaine may reduce the requirement for other methyl group donors such as methionine and choline. Beneficial effects on intestinal cells and intestinal microbes have been reported following betaine supplementation to diets for pigs and poultry, which have been attributed to the osmotic properties of betaine. Furthermore, betaine potentially enhances the digestibility of specific nutrients, in particular fiber and minerals. Moreover, at the metabolic level, betaine is involved in protein and energy metabolism. Growth trials revealed positive effects of supplemental betaine on growth performance in pigs and poultry, and there is evidence that betaine acts as a carcass modifier by reducing the carcass fat content. In conclusion, due to its various metabolic and osmoregulatory functions, betaine plays an important role in the nutrition of monogastric animals.

This review focuses on the osmoregulatory function of betaine and its effect in terms of alleviating heat stress in poultry. Poultry appear to be particularly sensitive to temperature‐associated environmental challenges, especially heat stress. High ambient temperatures are deleterious to productive performance in poultry, including broilers, laying hens, quails and turkeys, resulting in considerable economic losses. Heat stress impairs overall poultry production by decreasing feed intake and negatively affecting intestinal development, leading to reduced nutrient digestibility. Apart from inducing a high mortality rate, heat stress is known to depress growth rate and reduce meat yield in broilers. In layers, lower feed intake impairs ovarian function, leading to decreased feed efficiency, egg production and egg quality. In addition, reduced immune functions, such as thyroid activity and antibody production, are evident in poultry exposed to heat stress. Heat stress increases the production of oxidants, causing oxidative stress and lipid peroxidation of cell membranes. Poultry respond physiologically and behaviourally when encountering the negative effects of heat stress, attempting to return the body to homeostasis. This requires energy at the expense of weight gain or egg production. Due to its zwitterionic structure, betaine has osmoprotective properties that aid in protecting intestinal cell proteins and enzymes from environmental stress, including high ambient temperature, thereby counteracting performance losses. Betaine also exerts an osmoregulatory role in cells, regulating water balance, and this results in more stable tissue metabolism. Inclusion of betaine in the diet may be beneficial for alleviating physical reactions to heat stress, as indicated by increases in nutrient digestibility. In broilers, betaine supplementation increases weight gain and breast muscle yield, while improving feed conversion. In layers, betaine supplementation improves egg production, egg quality traits and immune indices. In conclusion, due to its osmoregulatory functions, betaine plays an important role in alleviating heat stress in poultry.

The study was conducted to investigate the effects of graded dietary inclusion levels of betaine on ileal and total tract nutrient digestibilities and intestinal bacterial metabolites in piglets. A total of eight barrows with an average initial body weight of 7.9 kg were randomly allocated to one of the four assay diets with two pigs per treatment in four repeated measurement periods. The assay diets included a basal diet based on wheat, barley and soybean meal alone, or supplemented with a liquid betaine product at dietary levels of 1.5, 3.0, or 6.0 g betaine per kilogram diet (as-fed). Ileal digestibilities of dry matter and neutral detergent fibre increased both quadratically and linearly, and ileal digestibility of glycine increased linearly as dietary betaine level increased (p < 0.05). Furthermore, total tract digestibility of crude protein increased quadratically (p < 0.05) and total tract digestibilities of most amino acids tended to increase quadratically (p = 0.06 to p = 0.11) with increasing dietary betaine level. Moreover, there were linear increases in the concentrations of most bacterial metabolites which were significant p < 0.05 for ileal d-lactic acid and for faecal diaminopimelic acid. The results demonstrate that dietary betaine supplementation stimulates bacterial fermentation of fibre in the small intestine and bacterial degradation of crude protein in the large intestine.

This study investigated the effects of dietary protein levels and betaine supplementation on nutrient digestibility and performance of Japanese quails. In total, 765 Japanese quails were randomly assigned to a 3×3 factorial arrangement, with five replicates of 17 quails each. Three basal diets were formulated to contain three crude protein levels (16.5, 18.0, and 19.5%). Each protein level was supplemented with 0, 0.06, and 0.12% betaine. The diet with 16.5% dietary crude protein with no betaine supplementation resulted in the lowest crude fiber digestibility, while the 18.0% CP diet supplemented with 0.12% betaine generated the highest crude fiber digestibility (p<0.05). The diets with 18.0 and 19.5% crude protein increased crude fiber digestibility, but reduced ether extract digestibility (p<0.01). Moreover, betaine supplementation increased dry matter, crude protein, crude fiber, and crude ash (p<0.01) digestibility and tended to increase ether extract digestibility (p=0.09). The increase in egg weight for the 18.0 and 19.5% protein diets was correlated with a decrease in feed conversion ratio (p<0.05). However, feed intake and egg production were not affected by protein levels. Betaine supplementation enhanced all performance variables (p<0.01). The diets with 18.0 and 19.5% crude protein resulted in heavier yolks and eggshells than the 16.5% crude protein diet (p<0.05), whereas betaine supplementation increased yolk, albumen, and eggshell weight (p<0.01). The 18.0 and 19.5% protein diets produced similar responses in most evaluated parameters. Laying Japanese quails can be fed diets with 18.0% crude protein. Moreover, betaine supplementation provided several benefits, and particularly improved nutrient digestibility, performance, and egg quality.