Herbert G. Botting

Digestibility of protein and amino acids in selected foods as determined by a rat balance method

Values (%) for true digestibility of crude protein and individual amino acids in 20 selected foods were determined by the rat balance (fecal) method. The products were fed as the sole source of protein in diets containing 8% crude protein (N × 6.25). Lowest true protein digestibility values (79–84) were obtained for pinto beans, kidney beans and lentils; intermediate values (89–92) were obtained for chick peas, beef stew, skim milk (over heated), rolled oats, whole wheat cereal, and pea protein concentrate; and highest values (94–100) were obtained for sausage, macaroni-cheese, rice-wheat gluten cereal, skim milk, tuna, soy isolate, peanut butter, chicken frankfurters, beef salami, casein and casein + methionine. In animal foods, peanut butter and soy isolate, the differences between true digestibility of crude protein and most individual amino acids were less than 5%. However, the values for true digestibility of methionine and cystine were up to 44% lower than those of crude protein in pinto beans, kidney beans, lentils, chick peas and pea concentrate. In these legumes, digestibility of crude protein was not a good predictor of digestibility of the limiting amino acids.

Relationship between amino acid scores and protein quality indices based on rat growth

Protein efficiency ratio (PER), relative PER (RPER), net protein ratio (NPR) and relative NPR (RNPR) values, and amino acid scores were calculated for 20 food products (casein, casein + Met, beef salami, skim milk, tuna, chicken frankfurters, sausage, heated skim milk, peanut butter, rolled oats, soy isolate, chick peas, pea concentrate, kidney beans, wheat cereal, pinto bean, lentils, rice-wheat gluten cereal, macaroni-cheese, and beef stew). In most cases, PER, RPER, NPR or RNPR ranked the products in the same order and positive correlations among the protein quality methods were highly significant (r=0.98−0.99). Amino acid scores (based on the first limiting amino acid, Lys-Met-Cys, Lys-Met-Cys-Trp or lys-Met-Cys-Trp-Thr) were positively correlated to the PER, RPER, NPR or RNPR data (r=0.61−0.75). Inclusion of the correction for true digestibility of protein improved the correlations between amino acid scores and the indices based on rat growth. The correlations were especially high between Lys-Met-Cys scores (corrected for true digestibility of protein) and PER, RPER, NPR or RNPR (r=0.86−0.91). Inclusion of the correction for true digestibility of individual amino acids did not result in further improvements of the correlations in most cases. It is concluded that adjusting amino acid scores for true digestibility of protein would be sufficient and further correction for digestibility of amino acids would be unnecessary in mixed diets.

Effect of amino acid supplementation on protein quality of soy-based infant formulas fed to rats

The powder forms of soy-based infant formulas obtained from four manufacturers were fed to weanling rats for two weeks, as the sole source of protein in diets containing 8% protein, 20% fat, and adequate amounts of minerals and vitamins. The relative protein efficiency ratio (RPER) and the relative net protein ratio (RNPR) values (casein + methionine=100) of diets containing unsupplemented formulas were 71–81 and 78–85, respectively. Supplementation of the formula diets with lysine (0.2%), methionine (0.2%), threonine (0.1%) or tryptophan (0.05%) increased the level of the supplemental amino acid in rat serum but generally failed to improve the RPER or RNPR values. Addition of all four essential amino acids to the formula diets, however, caused a marked improvement in their protein quality (RPER or RNPR values=100). The data suggested that proteins in soy-based formulas could be marginally co-limiting in several indispensable amino acids.

Influence of high dietary threonine on growth and amino acids in blood and tissues of rats.

SummaryDiets containing 8 or 15% protein from casein plus limiting amino acids, 25% fat and adequate levels of other nutrients for rat growth were supplemented with 0, 0.5, 1.0, 2.0 or 4% of excess L-threonine. Addition of up to 1% excess threonine had little effect on weight gains or food intakes of weanling rats, but addition of 2 and 4% threonine caused a drastic reduction in weight gains or food intakes (up to 41%); the adverse effect being more severe in rats fed lower protein diets. Addition of graded levels of excess threonine resulted in (5 to 47-fold and 4 to 20-fold) increase in concentration of free threonine in rat plasma and brain, respectively. Addition of excess threonine also caused up to 5-fold increase in plasma level of 3-methylhistidine, suggesting increased muscle protein breakdown.

Sulfur amino acid requirements of the growing rat fed eight percent dietary protein

Abstract Methionine and methionine plus cystine requirements of the growing rat fed equivalent to 8% dietary protein were determined. Diets formulated to be adequate in all nutrients except sulfur amino acids (SAA) and providing 4% protein from ANRC casein and equivalent to 4% casein protein from amino acids (excluding methionine and cystine) were supplemented with graded levels of L-methionine. The diet providing 0.34% total SAA (0.32% L-methionine + 0.02% L-cystine) gave the best feed/gain ratio, relative net protein ratio (RNPR), liver protein utilization (LPU) and plasma amino acid acid parameters. When similar ANRC casein + crystalline amino acid diets providing 0.44% total SAA but varying in ratio (by weight) by methionine/cystine were fed to rats, optimal feed/gain ratio, RNPR and plasma amino acid parameters were obtained when cystine replaced 33 to 60% of dietary L-methionine. Inclusion of L-cystine at the expense of L-methionine in these 8% protein diets improved overall rat performance and utilization of dietary methionine. Supplementation of the casein + amino acid basal diet with methionine sulfoxide, methionine sulfone (oxidized forms of methionine) or cysteic acid (oxidized form of cysteine/cystine) to provide 0.34% total SAA, indicated that relative to methionine (100), methionine sulfoxide was completely available and methionine sulfone (68) and cysteic acid (55) were less available. Correction for methionine and cystine digestibilities in ANRC casein suggests that the SAA requirements for the growing rat are 0.33% of diet or 4.1% of dietary protein when 8% protein diets are fed.

Effects of soybean trypsin inhibitors and DL-ethionine on growth and serum parameters in young rats

Abstract Two three week feeding trials tested the effects of dietary soybean trypsin inhibitors (SBTI) and/or DL-ethionine (methionine antagonist) on growth, protein digestibility, and clinical parameters in young male and female Sprague Dawley and Wistar rats. SBTI-fed rats had decreased weight gain, feed efficiency, protein digestibility and increased serum urea nitrogen relative to casein-fed control animals. Ethionine without SBTI also had negative effects. Combination of the two factors resulted in greatest growth deficits and increased serum levels of urea nitrogen, triglycerides, alkaline phosphatase and glutamate pyruvate transaminase. Dietary inadequacy or unavailability of sulfur amino acids in rats fed SBTI and ethionine was suggested by poor growth and serum accumulation of total essential amino acids, methionine sulfoxide, and several amino acid precursors of the methionine transsulfuration pathway. DL-methionine supplementation of diets containing SBTI or SBTI + ethionine or use of a soy protein isolate + ethionine diet (low in SBTI) improved growth and feed efficiency and normalized clinical chemistry parameters and serum free amino acid levels further suggesting that a deficit in supply or availability of sulfur amino acids was a factor in the observed changes.

Effects of folate, vitamin B-12 and vitamin B-6 supplementation on methionine-induced hyperhomocysteinemia in rats

An excessive intake of dietary methionine increases plasma total homocysteine (tHcy, an independent risk factor for premature cardiovascular disease) by enhancing the synthesis of homocysteine. Information on the influence of excess dietary vitamins involved in homocysteine metabolism on the methionine-induced hyperhomocysteinemia is, however, limited. Thus, a six-week study was conducted to determine the influence of excess folic acid, vitamin B-12 and vitamin B-6 on the methionine-induced hyperhomocysteinemia in rats. Supplementation of the casein control diet with 10 and 20 g/kg L-Met increased plasma tHcy to 2.0 and 8.0 times control, respectively. The hyperhomocysteinemia caused by the addition of 10 g/kg L-Met to the control diet, was completely counteracted by extra folic acid or three vitamins combined (folic acid, 2 mg/kg; vitamin B-12, 25 μg/kg; plus vitamin B-6, 6 mg/kg) but the addition of extra vitamin B-12 or vitamin B-6 alone had no effect on plasma tHcy. Similarly, extra dietary folic acid or the three vitamins combined caused substantial reduction in plasma tHcy of rats fed the control diet supplemented with 20 g/kg L-Met but addition of vitamin B-12 or vitamin B-6 alone exacerbated plasma tHcy.

Influence of dietary soybean trypsin inhibitors and DL-ethionine on sulfur amino acid adequacy of diets for young rats

Weanling male Wistar rats were fed 20% protein diets based on casein or either of two combinations of soy protein isolate and ground raw soy providing three levels of soybean trypsin inhibitors (SBTI; 0,448 and 808 mg of trypsin inhibited per 100g of diet respectively). DL-ethionine was included at three levels (0,0.05% and 0.10%) with each level of SBTI. After 4, 8 and 12 weeks ofad libitum feeding, diets containing SBTI without DL-ethionine were associated with decreases in weight gain, feed efficiency, serum cholesterol and serum urea nitrogen. Higher levels of triglycerides, glutamate pyruvate transaminase (SGPT) and altered serum free amino acid levels were also found. Increased dietary levels of DL-ethionine also resulted in deficits in growth and feed efficiency, decreased serum cholesterol, increased SGPT and similar alterations in serum free amino acids. Combination of dietary SBTI with DL-ethionine resulted in even greater growth deficits and serum cholesterol decreases as well as increases in SGPT and serum triglycerides and changes in serum free amino acid levels. Methionine deficiency in the young rats fed SBTI and DL-ethionine was indicated by the changes in serum amino acids and growth deficits. Moderation of some effects over the 12 week test period suggested decreased methionine requirements in the older rats.

Dietary cysteine/methionine ratios and taurine supplementation: effects on rat growth, amino acids and bile acids

Abstract Diets containing 15% protein (casein plus arginine, threonine and tryptophan), 20% fat (soybean-coconut oil) and adequate amounts of minerals and vitamins were supplemented with methionine and/or cysteine to provide cysteine/methionine ratios of 0.2, 1.0 and 2.0 simulating those in various dietary proteins, human milk and infant formulas. The dietary cysteine/methionine ratios had significant (P