Bioavailability of Methionine Analogues

In addition to dl-methionine, which is marketed in a 99% pure feed-grade form (Degussa, 2001), other sources of supplemental methionine activity are available to the animal feed industry. In particular, 2-hydroxy-4-(methylthiol)butyric acid (HMB), commonly known as methionine hydroxy analogue, has been manufactured in both solid and liquid forms. The solid consists of two moles of HMB bound to calcium by the two carboxyl carbons and generally contains 86% HMB. The liquid contains 88% HMB, which exists in monomer (77%), dimer (17.2%), trimer (4%), and oligomer (1.8%) forms (Novus International, 2001). All HMB products are 50% d-HMB and 50% l-HMB.

There has been considerable controversy about the biological value of HMB relative to dl-methionine. The controversy has been fuelled by the commercial importance of methionine supplements, especially in poultry. Although there has been a wide range of estimates, poultry seem to utilize HMB with lower molar efficiency than dl-methionine (see review by Lewis and Baker, 1995). In pigs, however, most, but not all, research supports the conclusion that HMB and dl-methionine are equal sources of methionine activity on a molar basis (Becker et ai, 1955; Urbanczyk et ai, 1981; Reifsnyder et ai, 1984; Steinhart and Kirchgessner, 1985; Roth and Kirchgessner, 1986; Chung and Baker, 1992a; Stockland et ai, 1992; Knight et ai, 1998; Römer and Abel, 1999). Although, this conclusion continues to be challenged (Pack and Höhler, 2000), data of Chung and Baker (1992a; Fig. 8.4) and Knight et ai (1998; Fig. 8.5) provide clear illustrations of equal efficacy of dl-methionine and the liquid form of HMB.

Because HMB does not contain nitrogen, supplementation with HMB results in less excretion of urinary nitrogen than supplementation with an equivalent amount of dl-methionine (Römer and Abel, 1999). This would obviously be advantageous when minimization of nitrogen excretion is important.

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