Optimum ET Ratio for Growth and Protein Deposition

Soon after experiments with purified diets had been started in the early 1940s, it became apparent that use of essential amino acids as a sole source of dietary N did not promote normal growth of rats (Kinsey and Grant, 1944) or chicks (Hegsted, 1944; Luckey et al., 1947) and that, to attain maximum growth rate, it was necessary to supplement the mixture of essential amino acids with one or several non-essential amino acids or another source of non-specific nitrogen (Rose et al., 1948; Frost and Sandy, 1951; Rechcigl et al., 1957; Greenstein et al., 1957; Klain et al., 1959). Favourable effects of the inclusion of non-essential amino acid supplements were observed even in diets containing an excess of essential amino acids sufficient for the synthesis of non-essential amino acids (Adkins et al., 1966). This observation led to the proposal that the rate of synthesis of one or more non-essential amino acids is not sufficient to support maximum growth rate (Adkins et al., 1966). In the following years the importance of a correct balance between the essential and non-essential amino acids was established in other species such as pigs (Mitchell et al., 1968), turkeys (Bedford and Summers, 1988), preruminating lambs (Dove et al., 1977a,b), kittens (Anderson et al., 1980), fish (Mambrini and Kaushik, 1994; Schuhmacher et al., 1995) and humans (Kies and Linkswiler, 1965; Daniel et al., 1970).

Stucki and Harper (1961, 1962) were among the first researchers to make quantitative estimates of the optimum E:T ratio within a broad range of the ratios and at several levels of dietary N. In their experiments with weanling rats (Stucki and Harper, 1962), they used purified diets containing 75-150 g CP kg-1 in which the E:T ratios ranged from 0.33 to 1.0. Lower performance was observed in diets with extreme E:T ratios whereas E:T values ranging from 0.53 to 0.81 had no significant effect on growth rate, feed conversion or protein efficiency ratio. A marked decrease in growth rate and protein utilization was found in the diet with the highest N concentration containing only essential amino acids (E:T = 1.0); this might be attributable to an adverse effect of the excess of one or more essential amino acids. In contrast, the negative effect of low E:T ratios on growth rate was more pronounced in diets with the lower total N concentration, suggesting a lack of essential amino acids.

A further extensive series of experiments on growing rats was carried out by Young and Zamora (1968) who evaluated the effect of 12 essential to non-essential amino acid ratios on weight gain and protein efficiency ratio at two dietary crude protein levels. Their findings support those of Stucki and Harper (1962)

indicating that both too low and too high E:T ratios are inhibitory to growth regardless of nitrogen intake and that the dietary crude protein level affects both the shape of the response curve and the optimum E:T ratio (Fig. 6.1). The adverse effects of high E:T ratios were exaggerated by high dietary crude protein level whereas the lowest weight gains at low E:T ratios were associated with low crude protein (CP) level. The optimum E:T ratio decreased as the CP level of the diet increased. At 139 g CP kg-1, maximum growth was attained with E:T ratio of 0.61 whereas the E:T value required for maximum growth rate at 181 g CP kg-1 was 0.58. In another experiment with growing rats, the optimum E:T ratio required for maximum N retention at dietary CP concentration of 100 g kg-1 was estimated to be 0.68 (Heger et al., 1987).

There is relatively little information available on optimum E:T ratios in pigs and poultry estimated within a sufficiently broad range of values. Mitchell et al. (1968) studied the effect of adding four levels of glutamate to a semipurified diet with a constant concentration

120 r

E:T ratio

Fig. 6.1. Relationship between E:T ratio and weight gains of rats fed on diets containing 22.3 (•) or 29 (o) g N kg 1 . (From Young and Zamora, 1968. Reproduced with permission of the American Society for Nutritional Sciences.)

E:T ratio

Fig. 6.1. Relationship between E:T ratio and weight gains of rats fed on diets containing 22.3 (•) or 29 (o) g N kg 1 . (From Young and Zamora, 1968. Reproduced with permission of the American Society for Nutritional Sciences.)

of essential amino acids on N retention in 10kg piglets. They found that the young pigs utilized dietary N most efficiently when the E:T ratio was 0.40. In their experiment, however, only three animals per treatment were used. Wang and Fuller (1989) estimated the optimum E:T ratio in growing pigs fed isonitroge-nous diets at four E:T values ranging from 0.34 to 0.58, using a rectilinear model to fit the data. Based on their results it can be concluded that the minimum E:T ratio required for maximum protein deposition is approximately 0.46. Lenis et al. (1999) measured N retention in pigs at three E:T ratios within three dietary N concentrations and concluded that, to attain maximum N retention and N utilization, the E:T ratio should be about 0.5. In their experiments, optimum E:T ratio tended to decrease with increasing dietary N concentration.

A more detailed study with pigs comprising six E:T ratios ranging from 0.25 to 0.86 was conducted by Heger et al. (1998). At a constant concentration of total dietary N

(24.5 g kg-1), the response of N retention to varying E:T ratios was curvilinear with a maximum corresponding to an E:T value of 0.61. A similar relationship was found for N utilization (Fig. 6.2). In the region around the optimum, however, the dose-response curves were flat and within the E:T range of 0.49-0.74 no significant difference between treatments was found. There are other experiments with pigs in which the optimum E:T ratio required for maximum N retention was estimated to be near 0.6 (Roth et al., 1993; 1994b; Gotterbarm et al., 1998).

Estimates of optimum E:T ratios for poultry were comparable to those found in rats and pigs. Stucki and Harper (1961) studied the effect of altering the E:T ratio on growth of chicks fed on purified diets containing 153-302 g CP kg They concluded that the growth rate was maximized when the E:T ratio was about 0.66. Dietary CP level had no significant effect on the response measured. However, the results of this experiment were confounded by using racemic forms of some

0.65

0.60

0.50

0.45

0.65

0.60

0.50

0.45

0.40

0.35

E:T ratio

Fig. 6.2. Relationship between E:T ratio and N retention (•) or N utilization (o) in growing pigs at constant concentration of total dietary N. (From Heger et al., 1998. Reproduced with permission of The Nutrition Society.)

essential and non-essential amino acids, d-iso-mers of essential amino acids being considered fully available for the chick. Later, Sugahara and Ariyoshi (1968) reassessed the results of Stucki and Harper (1961) taking into account the nutritional value of d-amino acids and concluded that the optimum E:T ratio was 0.57. In their own experiments, Sugahara and Ariyoshi (1968) studied growth rate, N retention and the concentration of amino acids in serum of chicks fed on isoni-trogenous purified diets with E:T ratios ranging from 0.38 to 0.71. Weight gain and N retention responses to changing E:T values were similar (Fig. 6.3) and both reached their maximum at an E:T ratio of 0.56. The concentration of free amino acids in serum was considerably influenced by the diet composition. The sum of non-essential amino acids gradually decreased with the increasing E:T ratio, and the ratio of essential to non-essential amino acids broadly reflected their concentrations in the diet. At the highest E:T values, however, there was a decrease of the serum essential amino acid level, which indi cated that a proportion of the essential amino acids was converted into non-essential amino acids under these conditions. In turkeys, Bedford and Summers (1988) observed maximum growth rate and carcass protein deposition at an E:T ratio of about 0.6. Part of the glycine, however, was included into the essential amino acid group in this study.

A number of other experiments with poultry have been carried out to study the effect of adding non-essential amino acids to low-protein diets. Unlike rats and pigs, birds responded to a higher non-essential N supply (mostly as glutamate and/or glycine) by decreased feed intake and a moderate reduction in growth rate. Feed conversion, however, was improved and was comparable to that of a positive control (Parr and Summers, 1991; Han et al., 1992; Huyghebaert and Pack, 1996; Aletor et al., 2000). In most experiments, the addition of glutamate or a mixture of non-essential amino acids to low-protein diets decreased body fat content (Fancher and Jensen, 1989; Han et al., 1992; Kerr and Kidd, 1999). However, no

T3 3

T3 3

E:T ratio

Fig. 6.3. Relationship between E:T ratio and weight gains (•) or N retention (o) in chicks. (Plotted from data by Sugahara and Ariyoshi, 1968.)

E:T ratio

Fig. 6.3. Relationship between E:T ratio and weight gains (•) or N retention (o) in chicks. (Plotted from data by Sugahara and Ariyoshi, 1968.)

such effect was observed in the study by Aletor et al. (2000). It seems that the response of chicks to the supplements of non-esssential amino acids may depend on genotype (Leclercq et al., 1994; Alleman et al., 2000) or diet composition (Deschepper and de Groote, 1995).

Several conclusions can be drawn from studies aimed at estimating optimum E:T ratios in rats, pigs and poultry.

  • Optimum E:T ratios for growth or protein deposition estimated at a constant level of total dietary N and using the same amino acid classification do not differ substantially between species, the mean value being 0.55-0.6.
  • Optimum E:T ratio depends on dietary N concentration, being lower at higher dietary N and vice versa.
  • There is a substantial range near the optimum within which the response to varying E:T ratio does not change appreciably.
  • Both too low and too high E:T ratios have an adverse effect on performance.

Although the poor growth or N retention at low E:T ratios is clearly due to the deficiency of essential amino acids, the effect of high E:T ratios is not fully understood. It has been suggested that the synthesis of one or more amino acids commonly classified as non-essential might not be sufficient to maintain maximum growth rate (Adkins et al., 1966), thus causing a reduction in protein deposition. However, there is no direct experimental evidence supporting this premise. Another possibility is that the efficiency of conversion of some essential amino acids into non-essential amino acids is low, and limits the amount of non-essential N available at a given concentration of total dietary N. Even though there is compelling evidence that all essential amino acids can be converted into non-essential amino acids (Aqvist, 1951), experiments evaluating the process from the quantitative point of view suggest that the conversion efficiency of some essential amino acids is lower than that of non-essential amino acids. Comparing the efficacy of various sources of non-essential N in chicks using the slope-ratio assay, Allen and Baker (1974) found that arginine and lysine were poorly converted into non-essential N and none of the essential amino acids tested was as efficient as glutamate as a source of non-specific N (Table 6.1). Heger (1990) studied the effects of excessive amounts of essential amino acids on N retention and the biological value of protein in rats. The basal diet containing essential amino acids in proportions corresponding to an ideal protein pattern and supplemented with a small amount of non-essential N (E:T ratio 0.96) was inferior to a diet with E:T ratio of 0.65. However, when the levels of arginine, lysine or sulphur amino acids were reduced while maintaining total N concentration constant by supplements of a non-esssential amino acid mixture, there was a significant increase in N retention and biological

Table 6.1. Utilization of various amino acids as sources of non-specific nitrogen for growing chicks relative to L-giutamate. (From Allen and Baker, 1974. Reproduced with permission of the Poultry Science Association.)

Amino acid

Gain per g N consumed (g)a

Ratio

L-Giutamate

38.16

1.00

L-Proline

37.69

0.99

Glycine

36.62

0.96

L-Valine

27.87

0.73

L-lsoleucine

26.11

0.68

L-Leucine

23.72

0.62

L-Lysine.HCI

19.07

0.50

L-Arginine.HCI

13.89

0.36

degression coefficient relating weight gain to N intake of amino acid tested.

degression coefficient relating weight gain to N intake of amino acid tested.

value. This suggests that arginine, lysine and sulphur amino acids are either a poor source of non-essential N or that their excess has an adverse effect on protein deposition.

To elucidate this problem in a greater detail, Taylor and his associates (Taylor et al., 1996, 1997, 1998; Rogers et al., 1998) conducted a series of experiments with kittens in which they studied the effect of E:T ratios ranging from 0.11 to 1.0 on weight gains and plasma amino acid pattern. In plasma of kittens fed on diets containing essential amino acids as a sole source of N, a much higher concentration of methionine and an increased concentration of arginine was found as compared to a control. This led Taylor et a I. (1996) to conclude that the poor growth associated with diets having high E:T ratios was the result of an adverse effect of excess methionine and arginine and not an inability to synthesize non-essential amino acids. The subsequent experiments demonstrated that 'near maximal' weight gain could be achieved without any non-essential amino acids in the diet if excesses of methionine and arginine were avoided (Taylor et al., 1996, 1998). As shown in Fig. 6.4 which is a compilation of data from six experiments by Taylor et al. (1997, 1998) adjusted relative to the performance of control groups, there is a broad range of E:T values and dietary crude protein concentrations within which a high growth rate of kittens can be attained as long as an excess of amino acids causing growth depression is prevented.

To date, it is not clear whether these findings can be generalized or if there are some interspecific differences. It has been demonstrated that the negative effect of amino acid excess on growth rate is almost entirely due to the reduction of voluntary feed intake (Fisher et al., 1960; Cieslak and Benevenga, 1984) as was also the case in experiments by Taylor et al. (1996). On the other hand, an increased intake of arginine, lysine or sulphur amino acids had no adverse effect on feed intake in rats or chicks (Allen and Baker, 1974; Heger, 1990). It seems therefore that poor growth and N retentions observed when feeding diets with high E:T ratios result at least partly from the low availabilities of these amino acids as sources of non-essential N.

The low utilization of arginine as a source of non-essential N is not surprising since this amino acid is closely associated with the synthesis of urea. Only about half the arginine supplied in the diet is available for the synthesis of non-essential amino acids, the rest being obligatorily converted into urea (Stein et al., 1986). Urea itself is known to be a poor

Weight gain (g day-"1)

Crude protein (g kg-1 diet)

E:T ratio

Fig. 6.4. Effect of E:T ratio and dietary crude protein on weight gains of kittens. (From Rogers et al., 1998. Reproduced with permission of the American Society for Nutritional Sciences.)

Weight gain (g day-"1)

Weight gain (g day-1)

Crude protein (g kg-1 diet)

E:T ratio

Fig. 6.4. Effect of E:T ratio and dietary crude protein on weight gains of kittens. (From Rogers et al., 1998. Reproduced with permission of the American Society for Nutritional Sciences.)

source of N for the synthesis of non-essential amino acids (Allen and Baker, 1974). The nitrogen of sulphur amino acids is either incorporated into urea after it is released as ammonia or is used for the synthesis of polyamines and taurine. None of the catabolic pathways of methionine or cystine appears to be quantitatively important for amino acid synthesis in mammals (Stipanuk, 1986).

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