The protein quality of some enteral products is inferior to that of casein as assessed by rat growth methods and digestibility-corrected amino acid scores

Protein digestibility and quality of six enteral nutrition products sold in Canada were studied by rat balance and growth methods. Casein+L-methionine, 0.2 g/100 g diet (control) and six enteral products (freeze-dried) were fed as the sole source of protein in diets containing 8.61-9.12 g/100 g protein (N x 6.25) to weanling and 18-mo-old rats for a period of 2 and 1 wk, respectively. A protein-free diet was also included in the feeding studies to permit calculations of true protein digestibility and net protein ratio values. Values for true digestibility of protein as determined in old rats for the control diet and the test products were 95 and 89-93%, respectively. Compared with old rats, protein digestibility values were 5-7 percentage units higher in young rats. The 2-wk relative protein efficiency ratio values (42-56%) or the relative net protein ratio values (61-74%) of the enteral products were considerably lower compared to those of the control (100). Supplementation of an enteral product with cysteine, cysteine + tryptophan, cysteine + threonine or cysteine + tryptophan + threonine caused significant improvement in protein quality; suggesting that the product was limiting in these three amino acids. The protein digestibility-corrected amino acid scores for the enteral products were 43-46, 69-75 and 86-93% by using whole egg, casein and the FAO-WHO (1991) pattern as reference proteins, respectively. The results indicate that these enteral products are inferior to casein in protein quality.

Ion distribution in relation to leaf age in Leptochloa fusca (L.) Kunth* (Kallar grass) II. Anions*†

Using Leptochloa fusca (L.) Kunth (Kallar grass) plants, the distribution of Cl^- , NO₃ ^- , H₃ PO₄ ^- , SO₄ ^2- and malate between leaves of various ages has been studied. Plants grown in a reclaimed, salt-affected field, in solution culture and in soil at 10, 100 and 125 mM NaCl have been analyzed. Apparently due to excretion by salt secreting glands on L. fusca leaves and to phloem export, Cl^- concentrations did not increase strongly with leaf age. On a leaf f. wt basis, chloride secretion was constant over the series of increasingly aged mature leaves. If it was related to the chloride increments in the leaves, chloride secretion increased strongly from younger to mature leaves and reached between ISO and 200% of the concurrent Cl^- deposition in the lamina. Changes in the tissue concentrations of nitrate and phosphate with leafage showed a maximum in recently matured leaves. Decreases in older leaves were attributed to nitrate reduction and export of reduced nitrogen and to retranslocation of phosphate. In leaves of field-grown L. fusca nitrate was non-detectable. Sulphate and malate concentrations in laminae continued to increase from the youngest to the oldest leaves. The increasing negative charge resulting from these increases in divalent anions can be accounted for by the loss of charge occurring in connection with reduction of nitrate and export of phosphate. Higher external salinity led, apart from increases in tissue Cl^- , to noticeable decreases in tissue nitrate and phosphate but not in sulphate and malate concentrations, the latter being even increased at higher external NaCl. The observed changes in anion concentrations are compared with and discussed in relation to changes found in Ricinus communis and in Atriplex hortensis.

Evaluation of liquid chromatographic analysis of nutritionally important amino acids in food and physiological samples

Significant advances have been made in standardizing methods for amino acid analysis of foods. The methods included standardized hydrolysis of the food proteins followed by separation and quantitation of the released amino acids by ion-exchange chromatography (IEC). IEC is still the main method in use. Its use is, however, being replaced by the faster higher-performance liquid chromatographic (HPLC) methods of derivatized amino acids. The HPLC separation of precolumn phenylisothiocyanate (PITC) derivatives has been adapted for rapid analysis of all amino acids in protein hydrolysates (12 min) and nutritionally important amino acids in deproteinized physiological samples (20 min). The inter-laboratory variability of the PITC derivatization method has not been determined although the intra-laboratory variation of the HPLC method was found to be similar to that of IEC. When similar hydrolytic conditions were used in preparing protein hydrolysates, amino acid results obtained with the PITC derivatization method were generally in close agreement with those obtained by IEC. There is, however, room for improvement in the HPLC analysis of amino acids in physiological samples.

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-limited in several indispensable amino acids.

Ion distribution in relation to leaf age in Leptochloa fusca* (L.) Kunth (Kallar grass): I. K, Na, Ca and Mg

Using Leptochloa fusca (Kallar grass) plants, the distribution of K, Na, Mg and Ca between leaves of various ages has been studied. Plants grown in a salt-affected, reclaimed field, in a solution culture and in soil in pots at 10, 100 and 125 mM NaCl have been analyzed. Despite the presence of salt-secreting glands on Leptochloa fusca leaves, Na concentrations increased strongly with leaf age, while K concentrations were highest in young leaves and decreased with increasing leaf age. This was due to K retranslocation, which was clearly intensified at higher external NaCl concentration. The data point to the importance of K recycling in this halophyte and also to the use of Na by this salt-secreting species for turgor maintenance in mature and old leaves. Both Mg and particularly Ca concentrations increased with leaf age, indicating that xylem import surpassed phloem export even for the phloem-mobile ion, Mg. In young leaves, Mg concentrations exceeded those of Ca.

A double-blind placebo-controlled glucose challenge in bulimia nervosa: psychological effects

Nineteen bulimic women and 22 age-matched controls were randomly assigned to receive 25 g of glucose or a placebo injection under double-blind conditions. Blood samples of glucose, insulin, and glucagon, and psychometric assessments of mood and food cravings were obtained 10 min before, and 0, 5, 10, 20, 30, 45, and 60 min after injection. Blood levels of the large neutral amino acids (LNAAs) tryptophan, tyrosine, leucine, valine, phenylalanine, and leucine were determined at 10 min before and 60 min after the injection. Bulimic subjects were found to report more symptoms of distressed mood throughout the entire monitoring period than controls. Five minutes following glucose ingestion the self-reports of depression, fatigue, anxiety, and bewilderment rose to a level among the bulimic subjects that was above that at baseline, and was higher than that of bulimia nervosa (BN) subjects receiving placebo. No comparable change in mood was observed among controls. Blood glucose levels were correlated with mood in the bulimic group, but not in controls. In addition, the glucose injection induced a heightened urge to binge in the bulimic group (compared to placebo at 10 and 60 min), whereas reducing food cravings (for sweets) in the controls (at 5 min). When collapsed across time and injection condition, the blood glucose level of bulimics was lower than that of controls. There were no differences in insulin response between the groups. The bulimic group was found to have lower baseline levels of blood tryptophan, whereas no differences in the tryptophan/LNAA ratio were observed either at baseline or following glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

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 100 g 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 of ad 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.

Changes in serum and urinary uric acid levels in normal human subjects fed purine-rich foods containing different amounts of adenine and hypoxanthine

The effect of ingesting some purine-rich foods (beef liver, haddock fillets and soybeans) on uric acid metabolism was investigated in 18 male subjects with no history of gout or kidney disorder. In a crossover design, three isoenergetic and isonitrogenous meals were fed to volunteers during a 3-week period. Only the content of uricogenic bases (adenine and hypoxanthine) varied among the test meals. Ingestion of all experimental meals caused an increase in serum uric acid levels at 120 minutes and this increase was more marked (about twofold) with haddock and soybean ingestion. In all groups, the postprandial serum uric acid levels at 240 minutes were lower than those obtained at 120 minutes, but still remained elevated in comparison to the fasting level. The test foods had little or no effect on serum and urinary creatinine values. As expected, 24-hour urinary uric acid excretion was similar for the three test meals due to the isonitrogenous load of proteins and purines. Assessment of each purine base content rather than the total purine content of foods should be considered in future recommendations for hyperuricemic individuals.

A comparison of fasting serum amino acid profiles of young and elderly subjects

The fasting serum amino acid profile in 37 healthy young women and men (30-35 years) was compared with the fasting profile in 30 institutionalized elderly women and men (80-89 years), an ambulatory, self-fed senior-residence group. Levels of serum lysine, leucine, methionine, valine, and total essential amino acids were significantly lower in the older group than in the younger group; however, citrulline and hydroxyproline were significantly higher in the older group compared to the younger group. Histidine, threonine, tryptophan, and the ratio of tryptophan to large neutral amino acids (isoleucine, leucine, phenylalanine, tyrosine, and valine) were also generally lower in the older group than in the younger group, while the difference due to age was more pronounced in the females compared to males. The essential/nonessential amino acid ratio was lower in females compared to males.

Amino acid ratings of different forms of infant formulas based on varying degrees of processing

Amino acid profiles, protein digestibility and/or amino acid bioavailability for the various forms (powder, liquid concentrate, ready-to-use, etc.) of infant formulas (involving varying degrees of heat processing during preparation) have been determined. Amino acid scores (based on the single most limiting amino acid) were calculated by comparing the essential amino acid data with that of human milk. Amino acid scores were multiplied by total protein (g/100 kcal) to obtain amino acid ratings, which take into account both quality and quantity of protein. Amino acid scores for milk- and soy-based formulas ranged from 49 to 90 and 59 to 81%, respectively, due to deficiencies in methionine plus cystine and/or tryptophan. The deficiency in the limiting amino acids was more marked in liquid concentrate than powder prepared by the same manufacturer. Because of significantly higher total protein contents (g/100 kcal) of soy- (2.65-3.68) and milk-based (2.20-2.95) formulas compared to human milk (1.5), the relative amino acid ratings (human milk = 100) of all formulas except two milk-based liquid concentrates and one ready-to-feed (with values of 77-87%) were greater than 100%. When corrected for protein digestibility, the relative amino acid ratings for all four liquid concentrates were less than 100%. Lower levels of digestible protein and bioavailable amino acids in liquid concentrate compared with powder (prepared by the same manufacturer) would suggest that inferior protein quality of liquid concentrates may be due to more severe heat treatment involved in their preparation.

In vivo rat assay for true protein digestibility: collaborative study

Eight laboratories participated in a collaborative study to estimate precision of a standardized rat assay for determining true protein digestibility in selected animal, fish, and cereal products. Each of 7 test protein sources (casein, tuna fish, macaroni/cheese, pea protein concentrate, rolled oats, pinto beans, and nonfat dried milk) was fed as the sole source of protein at a 10% protein level in mixed diets. Each diet was fed to 2 replicate groups of 4 rats each for a 4-day acclimation period and a 5-day balance period. Mean digestibilities ranged from 98.6% for casein to 72.6% for pinto beans. Repeatability standard deviations ranged from 0.5 to 2.0%; the mean relative standard deviation for repeatability was 0.9% (range 0.5-2.8%). Reproducibility standard deviations ranged from 1.2 to 3.2%, and the mean relative standard deviation for reproducibility was 2.4% (range 1.3-4.4%). The method has been approved interim official first action for determining true protein digestibility in foods and ingredients.

In vitro assay for protein digestibility: interlaboratory study

True protein digestibilities of 17 protein sources were estimated by 6 laboratories using an in vitro, 3-enzyme digestion system in a pH stat. Samples from animal, vegetable, and mixed food sources were freeze-dried (if not already dried), ground, mixed, and shipped to each collaborator along with a sodium caseinate standard and trypsin, chymotrypsin, and peptidase. The uptake of titrant during enzymatic digestion was used to calculate estimates of digestibility. Digestibilities ranged from 100% for casein to 89.9% for whole wheat cereal. Mean relative standard deviations for repeatability were 1.4% for rolled oats and less than 1% for the remaining 16 samples. Mean relative standard deviations for reproducibility ranged from 5.0 to 0.8%; values were less than 2.5% for 13 of the 17 samples.

Uricogenic potential of selected cooked foods in rats

Samples of beefsteak, beef liver, haddock, and mushrooms were cooked by two methods: boiling and broiling. Growth and purine metabolism in rats fed diets containing 20% protein (N x 6.25) were studied for a period of 26 days. The cooked meats and fish were used as the sole source of dietary protein, while cooked mushrooms provided 5% protein in mushroom-casein diets. Body weight, and food and water intakes were recorded weekly while total urine was collected during the last 6 days of the feeding period. Levels of purine metabolites (uric acid and allantoin) in blood and urine were determined by enzymatic methods and reverse-phase HPLC techniques. Dietary treatments had no significant effects on weight gain and food consumption. Compared with animals fed the casein control diet, rats fed the broiled liver, broiled haddock, and boiled mushroom diets had significantly (p less than 0.05) higher concentrations of serum allantoin, suggesting that uricogenicity of these foods was influenced by the method of cooking. The renal excretion of purine metabolites was significantly correlated with purine intake, the renal excretion of allantoin and uric acid being the highest in rats fed the liver diets.

Rapid analysis of nutritionally important free amino acids in serum and organs (liver, brain, and heart) by liquid chromatography of precolumn phenylisothiocyanate derivatives

An amino acid analysis method for protein hydrolysates, using precolumn phenylisothiocyanate (PITC) derivatization and liquid chromatography, was modified for its application in rapid analysis of commonly occurring free amino acids in serum and other physiological samples. The modifications included changes in column temperature (47.5 degrees C compared to 25-35 degrees C used in analyzing protein hydrolysates), method of preparing standard and test samples, and gradient conditions. By using a Waters Pico-Tag amino acid analysis 15 cm long column (which is also used for analyzing protein hydrolysates), separation of 27 PTC-amino acids in human serum and rat liver, brain, or heart was completed in 20 min by the modified method. The total time for analysis and equilibration was 30 min. The modified method was much faster than the traditional ion-exchange methods (2-3 h) or the existing liquid chromatographic methods using PITC derivatization (66-80 min) for determining nutritionally important free amino acids in physiological fluids and tissues. Variability of the method (expressed as coefficients of variation) for the determination (including deproteinization, derivatization, and liquid chromatography) of all amino acids was less than 5%, which compared favorably with the reproducibility of ion-exchange methods.

Evaluation of protein digestibility-corrected amino acid score method for assessing protein quality of foods

The current concepts of protein quality evaluation were reviewed. A detailed examination of existing animal assays and more promising amino acid scoring methods has been carried out by an Ad Hoc Working Group on Protein Quality Measurement for the Codex Committee on Vegetable Proteins during the last 5 years. Several factors such as inadequacies of protein efficiency ratio (PER, the poorest test) and other animal assays, advancements made in standardizing methods for amino acid analysis and protein digestibility, availability of data on digestibility of protein and individual amino acids in a variety of foods, and reliability of human amino acid requirements and scoring patterns were evaluated. On the basis of this evaluation, amino acid score, corrected for true digestibility of protein, was recommended to be the most suitable routine method for predicting protein quality of foods for humans. Amino acid scores corrected for true digestibility of protein (as determined by rat balance method) were termed "protein digestibility-corrected amino acid scores." A detailed method for the determination of the protein digestibility-corrected amino acid score was proposed, and information about the range of scores to be expected in foods or food products was provided in the present investigation. The protein digestibility-corrected amino acid score method is a simple and scientifically sound approach for routine evaluation of protein quality of foods. Accuracy of the method would, however, be confirmed after validation with growth or metabolic balance studies in humans.

Potentiality of protamine sulfate as mutagen

The mutagenicity of protamine sulfate has been clarified based on chromosomal aberration of cultured chinese hamster lung cells (cell line) in direct and metabolic activation methods and microbial mutagenesis (Ames test). In chromosomal aberration test, protamine sulfate caused cytotoxicity in the high doses (2500 and 5000 micrograms/ml) in the presence of rat liver homogenates (S-9). But very negligible or no cytotoxicity occurred in direct method at high dose (5000 micrograms/ml). Structural aberration of chromosome was not occurred in either of the methods. In microbial mutagenesis study, protamine sulfate did not show any cytotoxicity to microbes up to the dose of 5000 micrograms per plate. Furthermore, it did not have any effect in microbes like mutagens or like some toxic agent. The study reveals that protamine sulfate is not mutagen.

Amino acid rating method for evaluating protein adequacy of infant formulas

Amino acid profiles and/or protein digestibility (by the rat balance method) were determined for various forms (powder, ready-to-use, liquid concentrate, etc.) of cow's milk- and soy-based infant formulas obtained from 4 manufacturers. The essential amino acid data of the formulas were compared with that of human milk for the calculation of amino acid scores (based on the single most limiting amino acid). The product of amino acid score and total protein (g/100 kcal) was then termed "amino acid rating." Amino acid scores for the milk- and soy-based formulas ranged from 59 to 90 and from 59 to 81%, respectively, due to deficiencies in sulfur amino acids and/or tryptophan. Because of significantly higher total protein contents (g/100 kcal) of soy- (2.65-3.68) and milk-based (2.20-2.95) infant formulas compared to human milk (1.5), the relative amino acid ratings (human milk = 100) for all infant formulas except 2 liquid concentrates (having values of 87%) were above 100%. Values for true digestibility of protein in milk- and soy-based formulas ranged from 87 to 97 and from 92 to 95%, respectively. When corrected for protein digestibility, the relative amino acid ratings for all the milk-based liquid concentrates were below 100% (77-98%).