Plasma amino acids and cholesterol following consumption of dietary casein or soy protein in minipigs

Numerous investigators have claimed that protein-induced differences in plasma cholesterol are mediated by differences in amino acid composition. We have explored whether the venous postprandial amino acid profile reflects differences in the amino acid composition of the protein consumed. Six adult G?ttingen miniature pigs were fed a semisynthetic diet based on either casein or soy protein isolate. Frequent blood sampling was performed over a whole day after consumption of each diet for 6 wk. Postprandial plasma amino acid concentrations reached their maxima within the first 3 h. A group of eight protein amino acids (Met, Arg, Tyr, Val, Trp, Leu, Lys and Cys) exhibited the most marked and significant protein-dependent differences during this early postprandial phase, whereas Thr and His showed less marked differences. With one exception (Ser) all protein amino acids exhibited venous plasma concentration changes in qualitative accordance with their content in the dietary protein consumed. In quantitative terms, however, venous plasma amino acid changes were less marked than expected from the amino acid composition of the dietary proteins. We conclude that neither the considerable number of amino acids showing differences as reported herein nor the multitude of contradictory reported by others concerning single amino acids affecting serum cholesterol favor the hypothesis that one or several amino acid(s) cause protein-induced hypercholesterolemia.     

Correlation between the cholesterolemic responses produced by dietary cholesterol and casein in rabbits

Rabbits were fed three semipurified diets, namely a cholesterol-free soy protein diet, a cholesterol-free casein diet or a diet containing soy protein plus cholesterol (2 g/kg). One group (Chol-Cas) of 24 rabbits was fed successively the diet containing soy protein plus cholesterol (25 days), the soy protein diet (36 days) and the casein diet (20 days). Another group (Cas-Chol) consisting of 25 animals received consecutively the diet containing casein, soy protein and soy protein supplemented with cholesterol. Hypercholesterolemia was induced by the diets containing cholesterol or casein, whereas regression was seen on the soy protein diet. Within the Chol-Cas or Cas-Chol group the cholesterolemic response to casein and that to cholesterol was significantly correlated, the correlation coefficient being as high as 0.66. This correlation was still observed when calculated for animals either hypo- or hyperresponsive to dietary casein or cholesterol. When all animals were pooled, the correlation coefficient for the cholesterolemic response to casein and to cholesterol was only 0.33. The mean response to dietary cholesterol and to casein was significantly higher in the last dietary period than in the first period.     

Effect of dietary fiber on hypercholesterolemia induced by dietary PCB or cholesterol in rats

Effects of 0.03% polychlorinated biphenyls (PCB) in the diet and various dietary fibers [konjac mannan (KM), pectin, sodium carboxymethyl cellulose (CMC) and cellulose] at a 5% level in the diet on serum and liver lipid metabolism and urinary ascorbic acid were studied. A comparison between dietary PCB and 1% cholesterol in the diet was also made. Serum albumin, protein, total and high density lipoprotein (HDL)-cholesterol, triglyceride, urinary and liver ascorbic acid, liver cholesterol and total lipids were increased in rats fed PCB. Pectin or KM depressed the elevation in serum protein, total and HDL-cholesterol, triglyceride and liver lipids due to PCB intake. Cellulose or CMC had no significant effect on these indices. Urinary ascorbic acid was not decreased by these dietary changes. Serum total cholesterol and low density lipoprotein (LDL) plus very low density lipoprotein (VLDL)-cholesterol, and liver total lipids, and cholesterol were significantly higher, and serum HDL-cholesterol and triglyceride were significantly lower in the cholesterol-fed group as compared to PCB-fed rats. Addition of KM to a cholesterol diet significantly depressed serum total cholesterol and LDL plus VLDL-cholesterol, liver cholesterol and total lipids. It seems likely that cholesterol metabolism is quite different during dietary PCB and cholesterol feeding.     

Fecal steroid excretion in relation to the development of casein-induced hypercholesterolemia in rabbits

Two groups of nine rabbits fed a purified diet containing soy protein were injected intravenously with [4-14C]cholesterol. One group was fed the same diet for 78 d while the other group was fed a diet containing casein. The feces of the rabbits were collected and analyzed for radioactivity present in the neutral steroid and bile acid fractions. The excretion of neutral steroids and bile acids was lowered within 3 d after soy protein was replaced by casein and before the serum cholesterol concentration was increased. However, in the casein-fed rabbits the specific radioactivity of the bile acids was greater, whereas the amount of excreted bile acids was lower than in the soy protein-fed rabbits. After the serum cholesterol level in the casein-fed animals became constant (d 35), a further injection of [4-14C]cholesterol was given to all animals. Kinetic parameters of cholesterol metabolism according to a two-pool model were estimated from the die-away curve of [4-14C]cholesterol in serum. The size of the rapidly exchangeable pool (pool A) was greater in the casein-fed rabbits than in the soy protein-fed rabbits, whereas the size of pool B was similar in the two groups. The partial turnover rate of pool A was lower, as was the partial turnover of the slowly exchangeable pool (pool B), and the production rate of cholesterol was significantly lower on the casein-fed rabbits than in the soy protein-fed rabbits. This study suggests that the reduced excretion of steroids in casein-fed rabbits could be the cause of the hypercholesterolemia and is not the result of it.     

Digestibility of casein, formaldehyde-treated casein and soya-bean protein in relation to their effects on serum cholesterol in rabbits

Adult male rabbits were fed on semi-purified diets containing soya-bean protein isolate, casein or formaldehyde-treated casein as the protein source and 1 g cholesterol and 5 g of the non-absorbable marker chronic oxide/kg diet. The concentration of cholesterol in serum and in liver was increased on both the casein and formaldehyde-treated-casein diets. Excretion of bile acids and their concentration in faeces were lower in rabbits fed on casein or formaldehyde-treated casein when compared with rabbits fed on soya-bean protein. Apparent digestibility of nitrogen was lowest when formaldehyde-treated casein was fed, and highest on the casein diet. In rabbits fed on casein treated with formaldehyde, higher proportions of N were found in the water-soluble and trichloroacetic acid-insoluble protein fractions of the gastrointestinal tract contents compared with rabbits on the other two diets. Absorption of phosphate from the gastrointestinal tract was higher in rabbits fed on casein than in rabbits fed on soya-bean protein or formaldehyde-treated casein. The results indicate that, in rabbits, protein digestibility may not be an important determinant of serum cholesterol.     

Treatment of dietary casein with formaldehyde reduces its hypercholesterolemic effect in rabbits

Rabbits were fed cholesterol-free, semipurified diets containing 42% (wt/wt) casein or 21% casein plus one of the following nitrogen sources: soy isolate, amino acid mixture simulating casein, amino acid mixture simulating soy isolate, formaldehyde-treated casein or formaldehyde-treated soy isolate. Two additional groups of rabbits were fed the 42% casein diet and the diet containing casein plus soy isolate to which 0.4% (wt/wt) pure formaldehyde was added, this amount being identical to the amount of formaldehyde present in the diets with formaldehyde-treated proteins. Growth was somewhat reduced on the three diets containing 42% casein. The diet containing 42% casein to which no formaldehyde had been added induced severe hypercholesterolemia, the level of serum cholesterol after 8 weeks being about 10 mmol/L. The hypercholesterolemia was markedly reduced by the replacement of half of the casein by soy isolate, formaldehyde-treated soy isolate or formaldehyde-treated casein. No significant reduction of the concentration of serum cholesterol was seen when half of the 42% casein was replaced by an amino acid mixture imitating either casein or soy isolate. Formaldehyde per se did not significantly influence the level of serum cholesterol. We conclude that the differential tertiary structure of intact casein and soy isolate is an important factor in determining the cholesterolemic responses in rabbits to these proteins.     

Digestibilities of casein and soya-bean protein in relation to their effects on serum cholesterol in rabbits

Diets containing isolated soya-bean protein induce lower levels of serum cholesterol in animals than diets containing casein. Experiments were conducted to investigate whether differences in digestibility of the proteins might explain this effect. At pH 8 with pancreatic enzymes or intestinal peptidase, soya-bean protein was hydrolysed in vitro much less rapidly than casein. However, with pepsin (EC 3.4.23.1) at acid pH, soya-bean protein was hydrolysed more rapidly than casein. These differences in digestibility may be due to pH-dependent changes in solubility of the proteins. Casein and soya-bean protein were most soluble at alkaline and acid pH respectively. Heat treatment of the proteins resulted in lower solubilities and digestibilities. Sonication of soya-bean protein at pH 7.8 increased solubility but only slightly raised digestibility. When fed to rabbits, enzymically hydrolysed soya-bean protein induced a 2.3-fold higher concentration of serum cholesterol than did intact soya-bean protein. The hypocholesterolaemic effect of soya-bean protein may be partly attributable to its low solubility and digestibility at alkaline pH.     

Essential dietary amino acids for growth of larvae of the yellow mealworm, Tenebrio molitor L.

Larvae of the yellow mealworm, Tenebrio molitor L., have been used to evaluate nutritional quality of proteins and protein isolates. However, such investigations have been complicated by lack of knowledge of dietary requirements of the larvae. To determine essential dietary amino acids for growth of Tenebrio molitor, single amino acids were deleted from the amino acid mixture of the diet. Diets were maintained isonitrogenous with supplementary glycine and, in the case of deleted glycine, with glutamic acid. Growth, as measured by gain in weight, and survival were observed over a 4-week period at 27 plus or minus 0.25 degrees and 65 plus or minus 5% relative humidity. The results indicate that larvae of Tenebrio molitor require a dietary source of the same 10 amino acids essential for growth in rats, other vertebrates, and some protozoa. They also showed that serine, tyrosine, glutamic acid, and possibly glycine were dispensable for growth in this insect. Alanine, cystine, proline, and aspartic acid appeared semidispensable. Survival over the 4-week experimental period was unaffected by deleting amino acids from the diet. The results are discussed in relation to amino acid requirements of other insects and to suggested improvement of the diet of the present investigation.     

Effect of dietary amino acids on behavior and serum levels of amino acids in stress loaded rats

Interrelationships between behavior and concentrations of serum amino acids in stressed rats with immobilization and water immersion were investigated. Rats were subjected to 7 h of immobilization with water-immersion stress (IWS) in each sequential day, and serum amino acids were then determined. On the first day, serum taurine, threonine, valine, leucine, isoleucine, phenylalanine, lysine, and histidine (increased-type of amino acids) were significantly increased, but alanine (decreased-type) was significantly decreased. On days 3 and 7, the increase was retained, except for threonine, histidine, and lysine. Spontaneous activities (locomotion, rearing behavior, hole-poking) under loading water-immersion stress were significantly decreased, but a supplementation of branched-chain amino acids (BCAA) led to recovery. We suggest that pretreatment with some kind of increased type of amino acid, such as BCAA, might effectively prevent decline in spontaneous activities evoked by water-immersion stress.     

Induction of threonine imbalance by dispensable amino acids: relation to competition for amino acid transport into brain.

The ability of low protein diets containing small neutral, dispensable amino acids to induce threonine imbalance has been examined. Diets containing amino acids which compete for threonine transport in vitro (serine, alanine, alpha-amino-n-butyrate) caused depressions of growth and food intake which could be corrected to varying degrees by adding threonine to the diet. Large neutral, indispensable amino acids, moderately inhibitory of threonine transport, also induced the imbalance. Some amino acids that had little or no effect on threonine transport in vitro (acidic amino acids and proline) did not cause growth and food intake depressions. Other non-inhibitory amino acids (arginine and lysine) caused growth depressions which were not satisfactorily corrected by additional threonine alone, but were prevented by supplements of all the indispensable amino acids including threonine. Ornithine which was also not inhibitory of threonine transport was an exception. It induced a moderate growth depression which was corrected by additional threonine. Similar studies showed that histidine or tryptophan imbalance could be induced by feeding diets containing only those large neutral amino acids which compete for histidine or tryptophan transport in vitro. These experiments show that, based on the results of transport competition experiments, it is generally possible to devise amino acid supplements which can induce a dietary imbalance of a given amino acid.     

Suppression of glycogen consumption during acute exercise by dietary branched-chain amino acids in rats

The effects of a diet supplemented with branched-chain amino acids (BCAA; 4.8% or 6.2%) on BCAA catabolism and glycogen metabolism in rats were examined. Rats were fed a BCAA diet or control diet for 4 wk and part of the rats were subjected to exercise training during the experimental period. Feeding the BCAA diet increased serum BCAA concentrations and activity of the hepatic branched-chain alpha-keto acid dehydrogenase complex, the rate-limiting enzyme in the catabolism of BCAA, suggesting that dietary BCAA promotes BCAA catabolism. Although the serum glucose concentration and glycogen contents in the liver and gastrocnemius muscle of rested rats were not significantly affected by feeding of the BCAA diet, those in rats exhausted by acute exercise were 2-4-fold higher in rats fed the BCAA diet than in rats fed the control diet. The activity of pyruvate dehydrogenase complex in the liver and gastrocnemius muscle after acute exercise showed reverse trends; the complex activities (especially in liver) tended to be less in the BCAA diet group than in the control diet group. These results suggest that dietary BCAA spares glycogen stores in liver and skeletal muscle during exercise and that the decrease in pyruvate dehydrogenase complex activity in these tissues by dietary BCAA is involved in the mechanisms.     

The influence of dietary amino acids on lead absorption

Dietary supplements of about 5 g/kg of a number of amino acids increased tissue lead concentrations in newly weaned rats but decreased them in older rats. The retention of both oral and intraperitoneal lead was affected. The uptake of ²⁰³Pb by tissues was reduced when methionine was given in the diet over a period of 5 weeks or when it or ethionine was given by mouth 24 hr before the activity was measured. In the liver the fraction of the total activity found in the nuclei and mitochondria was increased by methionine, but in the kidney only the fraction found in nuclei was increased.