Regression of casein and cholesterol-induced hypercholesterolaemia in rabbits

Animal Proteins as Important Contributors to a Healthy Human Diet

Adequate protein intake is critical for health and development. Generally, protein of animal origin is of higher quality for humans owing to its amino acid pattern and good digestibility. When administered in mixtures it can enhance the quality of plant proteins, but its availability is often low in low-income communities, especially in young children, the elderly, and pregnant and lactating women, who have increased requirements and in whom high-quality protein also stimulates (bone) growth and maintenance. Although high protein intake was associated with increased type 2 diabetes mellitus risk, milk and seafood are good sources of branched chain amino acids and taurine, which act beneficially on glucose metabolism and blood pressure. However, high consumption of protein-rich animal food is also associated with adverse health effects and higher risk for noncommunicable diseases, partly related to other components of these foods, like saturated fatty acids and potential carcinogens in processed meat but also the atherogenic methionine metabolite homocysteine. In moderation, however, animal proteins are especially important for health maintenance in vulnerable persons.

Metabolic effects of arginine in a healthy elderly population

Recently there has been much interest in the use of arginine to stimulate immune responses and to promote wound healing. In the present study, the effect of an oral supplementation with arginine on the metabolism of 45 healthy, nonsmoking, elderly volunteers was investigated. Subjects were divided into two groups that received either arginine aspartate (17 g free arginine) (n = 30) or a placebo (n = 15). The supplements were taken for a period of 14 days. Dietary intake of food was not controlled. Blood chemistry, lipid profiles, and as an index of nutritional status, serum insulin-like growth factor-1 levels and nitrogen balance were compared before and after supplementation. Two weeks of arginine supplementation led to a significant elevation of serum insulin-like growth factor concentrations and an improved and positive nitrogen balance (2.0 +/- 0.41 g N) when compared with controls (0.11 +/- 0.47 g N; p = 0.0114). In addition the arginine-supplemented group demonstrated a decreased total serum cholesterol with a reduction in the low-density lipoprotein but not the high-density lipoprotein fraction resulting in a increase in the ratio of low- to high-density lipoprotein fraction. No adverse effects were observed at this dosage of arginine. The data suggest that oral arginine supplementation may be used safely in elderly humans.

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 Göttingen 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.

The influence of protein and carbohydrate type on serum and liver lipids and lipoprotein cholesterol in rabbits

The non-lipid portions of semi-synthetic diets appear to be important determinants of hypercholesterolemia and atherosclerosis in the rabbit. Serum and liver lipid concentrations were determined in rabbits which had been pair-fed various protein (casein or soy protein isolate) and carbohydrate (sucrose or dextrose) sources as part of low fat, low cholesterol, semi-synthetic diets. It was verified that casein-containing diets render rabbits hypercholesterolemic, while soy protein caused a degree of hypocholesterolemia. Additionally, sucrose, when fed in conjunction with casein, appears to augment this hypercholesterolemic effect. The distribution of total cholesterol among lipoprotein subclasses was increased in both the intermediate density lipoprotein (IDL) (1.006-1.019 g/ml) and low density lipoprotein (LDL) (1.019-1.063 g/ml) fractions and decreased in the high density lipoprotein (HDL) (1.063-1.21 g/ml) fraction when casein is fed. Soy protein feeding caused relatively more cholesterol to appear only in the IDL fraction when compared with commercial chow fed rabbits. Reasons for these differences may involve the saturation or suppression of endogenous lipoprotein hepatic receptors.

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.