Effect of ET Ratio on Protein and Amino Acid Utilization

In studies with isonitrogenous diets, the estimates of optimum E:T ratios required for protein deposition did not differ from those for protein utilization (Sugahara and Ariyoshi, 1968; Wang and Fuller, 1989; Heger et al, 1987, 1998; Gotterbarm et al, 1998; Lenis et al., 1999). However, when essential N was held constant and E:T ratio was altered by changing total dietary N, a considerably lower E:T ratio was needed for maximum N reten tion than for optimum N utilization. Thus in rats fed on diets meeting the requirements for essential amino acids, maximum N retention was attained at an E:T ratio of 0.38, whereas an E:T ratio of 0.51 was needed for maximum biological value of protein (Heger et al, 1987). In a similarly designed experiment with pigs (Heger et al, 1998) the highest N retentions were observed at E:T ratios not exceeding 0.48 whereas maximum total N utilization, measured as N retention relative to N ingested was found at an E:T ratio of 0.66 (Fig. 6.5). A similar response was observed by Roth et al (1993) who estimated the minimum amount of total N required for maximum N retention in growing pigs. They used practical-type diets with constant concentrations of essential amino acids in which crude protein level was gradually reduced in seven steps from 170 g kg-1 to 100 g kg-1. Maximum N retention was attained at an E:T ratio of 0.59 whereas an E:T ratio of 0.71 was needed for maximum biological value of protein. N excretion at maximum N retention

  1. 2 r n 0.8
  2. 2 r n 0.8

- 0.4

  1. 5 0.6 E:T ratio
  2. 6.5. Relationship between E:T ratio and N retention (•) or N utilization (o) in growing pigs at constant concentration of essential dietary N. (From Heger et al., 1998. Reproduced with permission of The Nutrition Society.)
  3. 5 0.6 E:T ratio c o co n

Fig. 6.5. Relationship between E:T ratio and N retention (•) or N utilization (o) in growing pigs at constant concentration of essential dietary N. (From Heger et al., 1998. Reproduced with permission of The Nutrition Society.)

was significantly higher than that at maximum N utilization. Further studies by Roth and his associates (Roth et al., 1994b, 1999) as well as those of other authors (Mitchell et al., 1968; Lenis et al., 1999) also demonstrated that, at essential amino acid intakes corresponding to the requirement, a lower E:T ratio and therefore higher total N intake was needed for maximum N retention than for maximum protein utilization.

The response of N retention to changes in E:T ratio at constant concentration of essential N (Fig. 6.5) indicates that protein deposition is maintained at the maximum level until either non-essential or total N becomes limiting. Then there is a linear decrease, apparently arising from the conversion of a proportion of essential amino acids into nonessential amino acids required for the synthesis of body protein. The low utilization of total N at low E:T ratios is presumably due to the surplus of non-essential amino acids, which are partly degraded under these conditions. On the other hand, when the optimum E:T ratio is exceeded, N utilization decreases as a result of non-essential amino acid deficiency. A signifi cantly lower E:T value corresponding to the breakpoint in N retention as compared to E:T value required for maximum N utilization suggests that non-essential N has a 'sparing effect' on the utilization of essential amino acids.

To clarify this phenomenon, it is useful to explore the relationships between the utilizations of essential, non-essential and total N. As shown in Fig. 6.6, the utilization of essential N gradually decreases as the E:T ratio increases whereas the utilization of non-essential N exhibits a sharp increase. At an E:T ratio of 0.66 corresponding to maximum total N utilization, less than 0.5 of essential amino acid N is utilized, whereas the non-essential N utilization exceeds 1.0. This implies that a proportion of the essential amino acids is used for the synthesis of non-essential amino acids under these conditions. Similar conclusions were drawn by Lenis et al. (1999) who studied the effects of three E:T ratios on total, essential and non-essential N utilization in growing pigs at three levels of total dietary N. This procedure allowed a comparison of the changes in N utilization at constant concentrations of both total and essential N. They

  1. 6.6. Relationship between E:T ratio and the utilization of essential (•) or non-essential (o) N in growing pigs at constant concentration of essential dietary N. (From Heger et al., 1998. Reproduced with permission of The Nutrition Society.)
  2. 2 0.4 0.6 0.8

E:T ratio

Fig. 6.6. Relationship between E:T ratio and the utilization of essential (•) or non-essential (o) N in growing pigs at constant concentration of essential dietary N. (From Heger et al., 1998. Reproduced with permission of The Nutrition Society.)

found that the efficiency of essential N utilization decreased with increasing E:T ratio in both cases, but was compensated by an increased utilization of non-essential N, so that the total N utilization remained unchanged. A similar relationship between the essential and non-essential N utilization at constant concentrations of total or essential dietary N was found in experiments with rats (Heger and Frydrych, 1989) but the information regarding other species is lacking.

The low utilization of essential amino acids at the optimum E:T ratio for the utilization of total dietary N is rather surprising. It is generally accepted that the efficiency of utilization of essential amino acids is higher than that of non-essential amino acids (McFarlane and von Holt, 1969; Aguilar et al., 1972). It has been shown, however, that a proportion of the essential amino acids is inevitably degraded even at suboptimal levels of intake (Kang-Lee and Harper, 1978; Harper and Benjamin, 1984; Tanaka et al., 1995). These losses are supposed to be an inevitable consequence of the presence in tissues of amino acid-degrading enzymes that are part of the mechanisms regulating protein metabolism in response to quantitative and qualitative changes in protein supply (Heger and Frydrych, 1989). It seems that these inevitable losses can be partly or fully counterbalanced by the synthesis of non-essential amino acids from nitrogen released during essential amino acid catabolism. Thus the organism is able to conserve nitrogen that would otherwise be lost, thereby maximizing the overall efficiency of utilization of the ingested protein (Heger and Frydrych, 1989; Roth et al., 1993). Differences between optimum E:T ratios in diets and those calculated from amino acid composition of body protein may serve as an indirect evidence of this hypothesis. The E:T ratios of body protein (Table 6.2) tend to be lower than those required for maximum growth or protein deposition. This supports an assumption that a proportion of ingested essential amino acids is converted into the non-essential amino acids that are incorporated into body protein.

Another factor that may be related to the 'sparing effect' of non-essential amino acids is endogenous protein loss. The major part of endogenous protein originates from deconju-gated bile salts and mucin glycoproteins that are resistant to enzymatic hydrolysis. While glycine accounts for more than 90% of the total amino acid content of bile (Souffrant, 1991), the mucin proteins contain high proportions of proline and serine (Roberton et ai, 1991). The E:T ratio calculated from the amino acid composition of endogenous protein is therefore lower than that of body protein and ranges from 0.43 to 0.52 (Leterme et ai, 1996; Hess and Sève, 1999; Stein et al., 1999). It is possible that non-essential N is one of the factors limiting the utilization of amino acids released by protein breakdown. Thus an increased supply of non-essential N may increase N retention by optimizing the amino acid balance of the precursor pool available for the resynthesis of body protein. Even though this mechanism is more likely to operate at near-maintenance conditions, its effect at higher protein intakes cannot be excluded.

Table 6.2. E:T ratios of body and endogenous proteins.

E:T ratio

Reference

Rat carcass

0.55

Pellet and Kaba (1972)

Pig whole body

0.54

Kyriazakis and Emmans (1993)

Pig empty body gain

0.54

Bikker et al. (1994)

Chick whole body

0.56

Mo ran (1995)

Turkey whole body

0.56

Mo ran (1995)

Pig endogenous protein3

0.44

Stein etal. (1999)

Pig endogenous proteinb

0.47

Hodgkinson etal. (2000)

aFed on protein-free diet.

bFed on enzyme-hydrolysed casein 100 g kg 1.

aFed on protein-free diet.

bFed on enzyme-hydrolysed casein 100 g kg 1.

The higher N retention at low E:T ratios and constant concentration of essential N might also result from the adaptive response of the organism to the excessive intake of nonessential amino acids. It has been demonstrated that the size and protein content of internal organs increase with increasing supply of dietary protein (Noblet et al., 1987; Kerr et al., 1995). An increased N retention was observed in chicks (Shapiro and Fisher, 1962) and pigs (Kerr and Easter, 1995) when nonessential amino acids were added to the diet. The accumulation of 'labile protein reserves' occurs over several days after a change in diet (Munro, 1964), and could affect N retention, particularly in short-term balance experiments. However, a 14-day adaptation period followed by a 7-day collection period, as used in the experiment of Lenis et al. (1999) seems to be long enough to counter the effect of 'labile protein' accumulation.

The current ideal protein concept is supposed to be equally applicable both to maximum N retention (or growth rate) and maximum N utilization (or minimum N excretion). This is undoubtedly the case of essential amino acid pattern. However, the optimum proportions of essential and non-essential amino acids for these functions are apparently different and the definition of ideal protein including the non-essential N requirement should reflect this fact. This could be of help in formulating practical diets aimed at achieving maximum growth rate or minimum N excretion.

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