Faecal Amino Acid Digestibilities

The digestion of amino acids in the large intestine is the result of microbial activity. The disappearance of amino acids from this part of the digestive tract would not be necessarily a problem if disappearance represented absorption of amino acids. However, several studies have clearly demonstrated that protein digestion through microbial activity does not contribute to maintenance or tissue accretion since the absorbed end products of microbial fermentation are ultimately excreted in the urine (Zebrowska, 1973; W√ľnsche et al., 1982; Mosenthin et al., 1992). In conclusion, due to microbial metabolism of nitrogenous material both from exogenous (dietary) and endogenous sources, only a relatively small proportion of the amino acid excretion in faeces is directly related to the amino acids recovered at the distal ileum. According to Low (1982), amino acids of dietary origin appear to account for less than 10% of the total amino acids at the faecal level, the main part consisting of microbial and endogenous sources. Depending on diet composition, between 50 and 90% of total nitrogen in faeces can be attributed to bacterial nitrogen assimilation (Poppe et al., 1983; Kreuzer et al., 1989; Sauer et al., 1991).

There is general agreement that in most cases apparent ileal digestibility values of most indispensable amino acids are lower than corresponding digestibilities determined at the faecal level. For example, cystine, threonine and tryptophan usually disappear to a considerable extent in the large intestine (Zebrowska et al., 1978; Sauer et al., 1982; Mosenthin et al., 1994). On the other hand, microbial net synthesis for methionine and sometimes for lysine has been reported in some studies resulting in lower faecal than ileal digestibility values (Sauer et al., 1982; Tanksley and Knabe, 1982; Sauer et al., 1991). Thus, depending on the amino acid and on the feedstuff, digestibility values obtained by the faecal analysis method overestimate (which is usually the case) or underestimate those obtained by the ileal analysis method. Therefore, it is now recognized that the ileal analysis method should be considered as an improvement over the faecal analysis method which was originally developed by Kuiken and Lyman (1948) for rats and which thereafter has been used extensively in studies with pigs (Dammers, 1964; Eggum, 1973).

A comparison of apparent faecal and ileal amino acid digestibilities in raw and heated soy flakes illustrates the inadequacy of the faecal analysis method for measuring amino acid digestibility values (Table 10.1). The difference between faecal and ileal digestibility values of amino acids in raw soy flakes averaged 30% whereas the corresponding difference in heated soy flakes was approximately 7%. Although faecal digestibilities indicated that heated soy flakes were a better nutrient source than raw soy flakes, the magnitude of this difference was substantially underestimated (Vandergrift et al., 1983).

It is obvious that apparent ileal amino acid digestibilities are a more sensitive approach to describe the nutritive value of feedstuffs than faecal digestibilities. The poorer the protein quality of feed, the more important ileal digestibility values are compared to faecal digestibility values. Convincing evidence that ileal rather than faecal digestibility values should be used in practical diet formulation for growing pigs was provided by Dierick et al. (1988). In this study the performance of pigs was related to digestibility measurements. There was a higher correlation between average daily gain and ileal rather than faecal protein digestibility (r = 0.76 vs. r = 0.34). In the same order, for feed efficiency (kg feed kg-1 carcass gain) the correlation coefficients were -0.87 and -0.65, respectively. In agreement with these findings, apparent ileal lysine digestibility coefficients were found to accurately indicate the amount of dietary lysine available for growth (Moughan and Smith, 1985; Schulz and Bohme, 1994; Rademacher et al., 1995). These results provide sufficient evidence that nitrogen absorbed in the large intestine does not contribute significantly to protein synthesis in growing pigs.

There is, however, a need to focus on some potential drawbacks in the interpretation and the use of apparent ileal amino acid digestibilities in diet formulation for pigs. It has to be emphasized that the impact of microbial fermentation in the small intestine on protein digestion and amino acid absorption is probably underestimated (Bergner et al., 1986; Torrallardona et al., 1994; De Lange and Fuller, 2000). It can be assumed that in particular threonine that accounts for approximately 25% of mucus protein (Lien et a I., 1997) is the preferential source of endogenous nitrogen for microbes attached to the intestinal surface. Furthermore, it has to be stressed that apparent ileal digestibility estimates are not corrected for endogenous nitrogen and amino acid losses that contribute at different levels to the total flow of nitrogen and amino acids at the distal ileum. As a result, apparent ileal protein and amino acid digestibility values may vary considerably, depending on the relative contribution of endogenous nitrogen and amino acids to non-digested dietary (exogenous) sources of nitrogen and amino acids. Approaches for adjust-

Table 10.1. Apparent ileal and faecal amino acid digestibility values of the indispensable amino acids in raw and heated soy flakes. (From Vandergrift et al., 1983.)

Digestibility (%)

Faecal Heal Difference (%)a

Amino acid

Raw

Heated

Raw

Heated

Raw

Heated

His

80

90

48

82

32

8

He

68

84

43

78

25

6

Leu

68

87

37

80

31

7

Lys

72

87

44

85

28

2

Met

61

83

47

82

14

1

Thr

65

83

32

72

33

11

Trp

75

87

25

72

50

15

Vai

64

85

35

78

29

7

difference between faecal and ileal digestibility values.

difference between faecal and ileal digestibility values.

merits of endogenous nitrogen and amino acid recoveries that allow for the determination of true ileal protein and amino acid digestibilities are discussed here.

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