|
|
|
|
|
|
|
Choline requirements for various species are given in Table 1.
Choline is freely soluble in water and ethanol, but insoluble in organic solvents. It is a strong base and decomposes in alkaline solution with the release of trimethylamine.
Used in liquid (70% activity) or dried form (25% to 60% activity), choline chloride is stable in multivitamin premixes but can decrease the stability of other vitamins present, and therefore should be added separately.
|
 |
|
|
|
|
|
|
|
Choline is found naturally in many feedstuffs, which may contribute significant amounts of choline to the diet. Various fish meals were found to contain on average approximately 4,400 ± 1,000 ppm (standard error of 45 samples) (Barlow et al., 1979). Little work has been done to evaluate the availability of choline from various feedstuffs for fish (Zhang and Wilson, 1999). Work with poultry suggests that choline bioavailability is variable among and within feedstuffs (Emmert and Baker, 1997), as has been found for catfish (Zhang and Wilson, 1999).
Dietary factors such as methionine, betaine, myo-inositol, folic acid and vitamin B12, or the combination of different levels and composition of fat, carbohydrate and protein in the diet, as well as the age, caloric intake and growth rate of fish, all influence the lipotropic action of choline and therefore requirement of this nutrient.
Choline may be synthesized in the body if enough methyl groups are available. Methionine may contribute methyl groups to ethanolamine to form choline, but trout have been shown incapable of this conversion (Ketola, 1976; Rumsey, 1991). In addition, synthesis in the body is usually not fast enough to meet the choline requirement for normal growth. The dietary content of methionine or of folic acid and vitamin B12 for de novo synthesis of methyl groups can influence the dietary requirement of choline (Wilson and Poe, 1988).
|
|
|
|
Methionine is involved in phosphatidylcholine synthesis via methylation of phosphatidylethanolamine (Holub et al., 1975). Dietary choline may reduce the need for methionine used to form lecithin. Methionine may contribute methyl groups to ethanolamine to form choline, but trout have been shown incapable of this conversion (Ketola, 1976; Rumsey, 1991). In channel catfish, methionine was shown to spare some of the choline requirement (Wilson and Poe, 1988).
Choline/betaine is involved in methionine metabolism, acting as a methyl donor to regenerate methionine from homocysteine. The importance of this pathway is not known in fish.
|
|
|
|
Work with lecithin (phosphatidylcholine) suggests that synthesis in larval fish is insufficient and that choline does not meet or replace the need for preformed lecithin (Kanazawa et al., 1981, 1983a, b; Poston 1990a, b; Poston, 1991a, b; Kanazawa, 1993, 1995; Geurden et al., 1995; Craig and Gatlin, 1997; Koven et al., 1998). Supplementation of approximately 4% lecithin to the diets results in better growth and survival in salmonids (Poston, 1990a, b; Poston, 1991a, b;) and reduced albinism in flatfish (Kanazawa, 1995). Yet this is not the case in older fish (Stickney and Andrews, 1972; Watanabe and Takeuchi, 1976; Hung et al., 1987; Hung and Lutes, 1988; Hung, 1989; Orthoefer et al., 1995).
The requirement of lecithin by crustaceans seems to be less clear, with various studies supporting or not supporting a requirement (D’Abramo and Baum, 1981; D’Abramo et al., 1981; Kanazawa, 1982; Orthoefer et al., 1995; Teshima, 1997; Shiau, 1998).
|
|
|
|
Free betaine has been shown to act as a feeding stimulant (palatability enhancer) in fish (Carr, 1982; Mackie, 1982; Mackie and Mitchell, 1982; Rumsey, 1986). Studies using betaine hydrochloride have shown virtually no value for this compound as a feeding stimulant (Hughes, 1993). Other studies have shown free betaine not to stimulate appetite (Hughes, 1991, 1993). Betaine has been suggested to act as an osmoprotectant (Virtanen et al., 1989; Clarke et al., 1994; Kidd, 1997) in some studies and not in others (Duston, 1993), and as a methyl donor (Anonymous, 1996a, b).
The addition of betaine in the diet has been shown to spare, but not totally replace, approximately 50% of the choline requirements of trout (Rumsey, 1991).
|
 |
|
|
|
|
|
|
|
view references
|
|
|
|
|
