SPECIAL STORY FEED & ADDITIVE MAGAZINE September 2023 67 though omasum is accepted as an absorption site of HMBA, Noftsger et al. (2005) recovered only 2.3 percent of the ingested HMBi as HMBA in omasal digesta from rumen cannulated cows, while having a positive response in milk protein concentration of the same cows following HMBi supplementation during the same study. This data showed HMBi absorption occurred before the omasal site. Dietary HMBi intake in ruminants results in an increase of seven times higher plasma Met within two hours (Kihal et al., 2021). However, because of various testing techniques and limited data on ruminal escape, intestinal absorption and Met conversion site or rate, there is still no agreement on the final Met bioavailability value of HMBi (NRC, 2021), although there is sufficient scientific evidence to understand that it is a reasonable source of metabolizable Met. At the rumen Breves et al. (2010) observed the fate of HMBi via the in vitro using chamber system, which keeps tissues alive in buffer to monitor absorption kinetics. To better understand rumen wall absorption, they separated rumen side and serosal side (outside of the organ) and treated them separately with HMBi. They recovered a minimal amount of total HMBi from serosal buffer while obtaining greater HMBi from the rumen-side buffer. Evidence suggests that HMBi cannot pass the rumen wall intact, is hydrolyzed to HMB at tissue surface and carried over out of epithelium via passive diffusion or possibly at low rates of MCT-1 transport. Remarkably, the concentration gradient between the mucosal and serosal sides of rumen tissue serves as the driving force in both scenarios. This implies that a higher HMBi level in KESSENT® MF (Kemin Europa, Belgium) may accelerate the rate of rumen absorption compared to any counterparts with lower levels of HMBi, even though ultimately available MP is estimated to be identical in the widely used nutritional models. On the other hand, there was also no Met available from HMBi in the artificial tissue system without any esterase enzymes. Earlier in vitro reports showed that HMBi remained intact up to 60% of total amount against microbial degradation (Robert et al., 2000). Although EU Commission (2003) concluded HMBi hydrolysis Category Additive1 Animal species Premixture Final feed Proposed incorporation via Water for drinking DL-Met based DL-Met based DL-Met based DL-Met based DL-Met based DL-Met sodium salt based DL-Met analog-based DL-Met analog-based DL-Met analog-based DL-Met2 DL-Met-fat3 DL-Met-min3 DL-Met-cop2 DL-Met-ec2 DL-Met-Na2 HMBa2 HMBa-Ca2 HMBi2,3 All Ruminants Ruminants Dairy cows Ruminants All All All Ruminants Yes Yes Yes Yes Yes No No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No No No No Yes Yes No No Table 1. A summary of the sources of Met additives in the feed industry 1DL-Met = DL-methionine; DL-Met-fat = encapsulated DL-Met by using fat, and saturated fatty acids; DL-Met-min = encapsulated DL-Met by using mineral matrices; DL-Met-cop = encapsulated DL-Met by using copolymer vinylpyridine/styrene; DL-Met-ec = encapsulated DL-Met by using ethyl cellulose; HMBa = the hydroxy analog of Met; HMBa-Ca = the calcium salt of the Met hydroxy analog; HMBi = the isopropyl ester of methionine hydroxy analog; 2EFSA, 2012; 3Graulet et al., 2005
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