Gluconeogenesis also is increased, and hepatic extraction of alanine, a key amino acid gluconeogenic precursor, is accelerated.19 Excessive rates of hepatic glucose production, proteolysis, and amino acid oxidation in type 1 diabetes are all reduced by insulin administration,82 but proteolysis and amino acid oxidation are more resistant to the suppressive effects of insulin.83 Normalization of protein metabolic rates may, therefore, require long-term tight metabolic control.84 Today, with improved glycemic management in type 1 diabetes, a more normal protein synthesis, breakdown, and oxidation should occur.
Mild heating in the presence of reducing sugars or aldehydes results in loss of available lysine with little change in digestibility. Mild to moderate heating causes loss of sulphydryl groups, formation of disulphide cross links, racemisation of L to D-aspartic acid and reduced digestibility of all amino acids. Moisture content during heating is critical in both losses of available lysine and of sulphydryl groups. Mild processing gives best digestibility for monogastric animals and is especially important for young mammals and fish. Ruminant feeds benefit from more severe heat treatment and special processing to reduce protein degradability when amino acid composition is well balanced. Growth under commercial conditions is often less than under good experimental conditions, reflecting challenges to the immune system. Dietary proteins can both cause and affect an immune response. Dietary proteins may need special processing to reduce antigenic factors. The presence of dietary fibre, phytic acid or tannins in protein feeds reduce amino acid digestibility, increase endogenous N loss and the energetic cost of intestinal protein synthesis with consequent reduction in growth rate.
The majority of protein is digested, and the amino acids not used for gut fuel are metabolized in the intestinal mucosal cells and transported by the portal vein to the liver for protein synthesis or gluconeogenesis.12 In the liver, nonessential amino acids are largely deaminated, and the amino group (nitrogen) removed is converted into urea for excretion in the urine.13 It has been shown that in subjects without and with mild type 2 diabetes, ~5070% of a 50-g protein meal is accounted for over an 8-hour period by deamination in the liver and intestine and synthesis to urea.14 It has been assumed that the remaining carbon skeletons from the nonessential amino acids are available for glucose synthesis, which would then enter into the general circulation.
Moughan . (1991) compared the determined growth of pigs fed a barley based grower diet with the response to lysine supplementation of a lysine deficient synthetic diet based on casein. The observed growth was 0.925 of the expected growth based on intake of apparently absorbed lysine. To achieve an equal ME intake, 15.3 percent more dry matter was fed of the test diet than the synthetic diets. Thus the endogenous losses would be less on the synthetic diet leaving more of the supplementary lysine available to support growth. The series of experiments by Batterham . (1990 - 1994) and Beech . (1991) all had the same form; a comparison of cottonseed meal, meat and bone meal and soybean meal as examples of feeds with low, medium and high ileal digestibility. The growth and N retention of pigs fed three diets formulated to supply the same limiting level of ileal lysine, methionine, threonine, tryptophan or isoleucine were measured. The main difference was observed between cottonseed meal and the other two meals, with smaller (lysine, threonine) and non significant differences (methionine, tryptophan), in N retention between meat and bone meal and soybean meal. Diets were fed on a scale to provide the same DE/W 0.75 but the cottonseed diets had 8.3 percent less DE/kg than the soybean diets, with the meat and bone meal diets intermediate. Consequently, the amount of dry matter fed differed and basal endogenous loss would be less for soybean than meat and bone meal or for cottonseed meal allowing more of the absorbed limiting amino acid to be used for growth. The presence of gossypol and raffinose in cottonseed makes this protein particularly susceptible to heat damage by binding, specifically with the epsilon-amino group of lysine (Martinez ., 1961). This may make it unavailable without any major change in digestibility of the protein (see below). Cottonseed meal and products such as dried milk powders where reducing sugars are potentially present, may be special cases where ileal digestibility fails to reflect the full loss of available lysine through early Maillard reactions. For the majority of protein concentrates this is unlikely to be a major factor. Indeed, the Batterham group in a study of isoleucine, where the meals used were cottonseed, lupin seed meal and soya bean meal, ileal digestibility correctly predicted growth performance (Batterham and Andersen, 1994). Correction for the known differences in ileal true digestibility must be an improvement over the use of chemically determined total amino acid content.
It is not necessary to meet the ideal balance for all amino acids. If one or more amino acids are limiting in the diet, it is possible to increase the amount of protein to meet the needs of the limiting amino acids (Carpenter and De Muelenaere, 1965; Boorman, 1992). This can be important for areas where abundant cheap supplies of a poor quality protein are available. If complementary proteins and synthetic amino acids are not economically available, then quantity can make up for quality. The disadvantage is the excess of the other amino acids is increased further and these need to be deaminated and excreted, with consequent reduction in the energy value of the diet and increased pollution.
When these two are mixed the surplus amino acids of one protein complement the deficiencies of the other. When the two are combined in a ratio to achieve the minimum crude protein needed by young chicks (currently estimated as 22.4 percent of a corn-soya diet), only the sulphur amino acids remain limiting. Supplementation with methionine will correct the deficiency. In this example the supply of lysine (CS 103) and threonine (CS 107) are also just met. The next amino acid in surplus is estimated as valine (CS 112), followed by isoleucine and arginine. This sequence of limiting amino acids has been demonstrated with growth trials in chicks (Fernandez ., 1994). In theory it should be possible to decrease the diet crude protein by between 10 and 20.2 percent, through using less soya, but supplementing with methionine (0.221 percent) plus lysine (0.136 percent) and threonine (0.043 percent) (all of which are now commercially available), to create the ideal protein balance with valine. Further reduction in the crude protein and soya should be possible, but only with more supplementation with methionine, lysine and threonine and also with valine, isoleucine and arginine, all of which are closely similar in CS.
The amino acid with the lowest score below 100 is the limiting amino acid. Amino acids present in a greater amount relative to the ideal protein than the limiting amino acid, i.e. having a higher score, can only be used in protein synthesis up to the level sustained by the limiting amino acid. The amount in excess will be deaminated and the carbon skeleton used as a source of energy. Consequently, the score for the limiting amino acid becomes the chemical score for the protein. An example of the use of the ideal protein pattern to calculate chemical score of feeds is given in Table 3. For maize, lysine is the first limiting amino acid. For soybean meal, methionine +cystine (M+C) is the first limiting.
In the absence of reducing sugars much higher temperatures, above 100 °C for several hours, are required to bring about loss of FDNB-available lysine (Carpenter and Booth, 1973). Under these conditions, cross links form between the epsilon-amino group of lysine and of the carboxyl group of aspartic acid and glutamic acid (or their amides) to form new peptide-like cross links (Hurrell ., 1976). In addition, cystine loses hydrogen sulphide to form a dehydroalanine residue plus a cysteine residue; the dehydroalanine and cysteine then recombine to form lanthionine creating a new C-S-C cross link between peptide chains. Dehydroalanine may also be formed by dehydration of serine. Under certain conditions, especially alkaline pH, the epsilon-amino group of lysine reacts with dehydroalanine to form a lysinoalanine cross link. These new cross links reduce the digestibility of the protein and hence the availability of all amino acids, not just those directly involved. These conditions are not experienced during normal processing but have occurred when destabilized fish meals have overheated through lipid oxidation during storage and transport. Even autoclaving at 133 °C for 20 minutes at 3 bars, as required for the treatment of meat and bone meal, is estimated from these studies to cause only a 2 to 3 percent loss of FDNB-reactive lysine.
Figure 5. The effect on protein synthesis, degradation and net retention in pigs when fed a basal diet supplemented with either fat or carbohydrate at a constant protein level, or with protein supplied at constant energy. Source: Reeds ., 1981
In the ruminant feed is fermented in the rumen, volatile fatty acids are absorbed from the rumen and omasum and provide the major part of the metabolizable energy taken up by the animal. The fermented digesta leave the rumen along with the microbial biomass and are subjected to further digestion in the abomasum (true stomach) and intestines, much as in the monogastric animal. Microbial protein is digested and absorbed in the small intestine and supplies the major part of the absorbed amino acids. The amino acid balance of microbial protein is good, with methionine determined as the first and lysine as the second limiting amino acid for growing sheep (Storm and Ørskov, 1983, 1984). The amino acid needs of the animal can be met at maintenance level by microbial protein alone. With increase in energy supply above maintenance, extra microbial protein is produced and a low level of production can be sustained.
Protein synthesis in the body involves a considerable expenditure of energy to create the activated amino acids to be linked together. In addition, protein tissues are constantly being turned over. For every one unit of net accretion of protein about 5 units of protein are synthesised. Some tissues are turning over faster than others. Indeed some of the fastest tissue replacement, such as in the intestinal epithelium and liver, lead to little or no net accretion. The energy cost of protein synthesis in protein turnover, just to maintain the existing protein, has been estimated to account for 15 to 33 percent of energy needed for maintenance. When additional energy is provided, there is an increase in protein synthesis and a decrease in protein degradation and these two effects combine to enhance net protein retention. When additional protein is supplied at constant energy, there is an increase in both protein synthesis and in protein degradation, resulting in a smaller net increment in protein retention. This is illustrated in Figure 5, which gives the determined synthesis and degradation contributions to the net N retention. With increasing protein in the diet there are frequently small improvements in carcase quality, measured as increased protein and decreased fat content. These changes arise from the decreased net energy value of protein compared with carbohydrate and the increased energy required for increased protein turnover driven by higher dietary protein intake, resulting in reduced energy available for fat synthesis.