WATT PoultryUSA - December 2017 - 57
Endogenous Amino Acid Loss and
Bacterial Biomass Have Consequences
A.J. Cowieson1 and N.E. Ward2 | DSM Nutritional Products | Kaiseraugst, Switzerland1 and Parsippany, NJ2
Modern poultry breeds require diets that provide for rapid
muscle development. Because of the extraordinary growth rates
of todayʼs birds, even small deﬁcits in amino acids can have
Increasingly, we recognize that endogenous protein loss, or protein
loss that is non-dietary in origin, plays an important role in this
process. Endogenous protein loss refers to any protein, peptide, or
amino acid of non-dietary origin that exits the terminal ileum.
Source and Signiﬁcance
A considerable amount of endogenous protein is secreted into the
gut during the digestive process. These proteins vary in amino acid
composition, origin and in their ability to be broken down. Important
endogenous sources include mucin, bile, sloughed epithelial cells and
digestive enzymes. Amino acid losses attributed to these sources can
be as much as 10-15 gram/kg of dry matter intake, or 1-4 grams per
gram of ingested protein.
The dominant amino acids in endogenous proteins include glycine,
threonine, and glutamic acid. While 75-90% of these amino acids
are recovered before they leave the terminal ileum, a portion exits
the ileum at a signiﬁcant metabolic cost in terms of both amino acids
per se and net energy. Experiments show that losses can range from
2892 kcal/kg for aspartic acid to 6740 kcal/kg for phenylalanine.
Therefore, factors that affect the endogenous protein ﬂow will also
have an important bearing on digestible energy.
Microbial Biomass and
Endogenous Protein Loss
Microbial biomass or protein is a peculiarity because it is neither
dietary nor endogenous. Microbial protein represents a confounding
"sink" of amino acids that can undergo substantial alteration in
composition caused by bacterial metabolic processes, including
conversion of non-protein N to protein-N.
Some researchers advocate a separation of microbial protein from
other endogenous protein. This is because over 60% of protein in
the ileum comes from bacterial biomass, the remainder being mucin,
sloughed animal cells, and digestive enzymes. Furthermore, the
amino acid composition of bacterial protein differs markedly from
that of mucin or bile. Both mucin and bile proteins are dominated
by threonine, serine, glycine, proline, and cystine. Amino acids
prominent in bacterial sources include glutamic acid, aspartic acid,
and leucine (Table 1).
Table 1. Amino acid composition (g/100 g amino acids) of various
endogenous proteins, including bacterial sources.*
Glutamic acid beneﬁts gut energy partitioning and nutrient absorption,
while leucine has positive ramiﬁcations on protein accretion. Changes
in the recovery of these different sources of endogenous protein have
important implications in animal nutrition.
Peptidoglycans Can Be Relevant Biomass
Dead cells and cell wall fragments, also known as intestinal
"rubbish,"constitute the main source of bacterial biomass. Nearly
60% of fecal mass is bacterial and approximately 30-35% of that is
dead or non-viable. The great majority of these bacteria (almost
75%) are Gram-positive. As much as 90% of the cell wall in
Gram-positive bacteria is comprised of peptidoglycan (PGN). PGN
is a massive polymer of amino acids (eg, peptido-) and sugars
(eg,-glycan) unique to bacteria.
The protein from PGN differs considerably across bacterial species,
and contains a mixture of D and L forms of amino acids. Not all D
isomers can be utilized nutritionally. That said, the quantity of this
rubbish may have a greater negative impact on performance than
the presence of the D isomers. This is because fragments of dead
bacteria could physically impair normal enzyme-substrate link-ups.
Implications and Conclusions
Excess endogenous protein loss can be lowered with additives
like feed enzymes, and with ingredient treatments that blunt
antinutritional effects. Reducing these secretory losses means
more efﬁcient N cycling. And less intestinal rubbish could lead to
improvements in N and energy recovery because factors that interfere
with enzyme-substrate couplings hamper digestive efﬁciency.
Lowering endogenous protein losses and bacterial biomass offers
unique opportunities for more efﬁciency in rapidly growing birds.
*Ravindran (2016) and Miner-Williams et al. (2009).
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