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Various components of food and feed such as fat or carbohydrates can affect the cholesterol level in human or animal blood. Proteins can also change it and according to the most experiments the animal proteins show the hyper- while the plant proteins the hypocholesterolemic activity. The way proteins exert their effect on cholesterol metabolism is still unknown. There is a possibility that the amino acid composition of proteins, especially their sulphur amino acid content is the primary cause of such changes. According to another view the undigested fraction of protein absorbs bile acids and makes impossible reabsorption of cholesterol from the intestinal tract. The liver is probably the first site of changes of cholesterol metabolism but, according to some authors, the proteins can affect cholesterol level by changing thyroid activity.
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Phenolic compounds wide spread in the plants are characterized by antinutritive and health-related features. Phenolic acids, e.g. gallic acid or caffeic acid, reduce the activity of proteolitic and amylolitic enzymes. Their health-related features resulted from their antioxidazing activity. Phenolic acids bind with the carbohydrates giving hydrolysable tannins. Other tannins, the condensed ones, more wide spread in plants consist of flavonoid monomers. Tannins bind with proteins forming undigestible complexes. Flavonoids show the structural similarities to the female hormone thus they can be responsible for the infertility in animals. On the other hand they are strong antioxidants, they lower the cholesterol and triacylglycerol levels in blood, and they can be anticancerogenic. Summing up, the health-related features of phenolics seem to be more important than their antinutritive activity.
Dietary fatty acids (FA) may affect in different ways the cholesterol level in animal organism. It is generally accepted that the saturated fatty acids of medium long chain (lauric, miristic, palmitic acids) are hyper-, whereas the polyunsaturated fatty acids (linoleic, linolenic) are hypocholesterolemic. There is no conclusive evidence regarding the effect of stearic (saturated, long-chain FA) and monounsaturated FA (mainly oleic acid). A vast body of experimental evidence indicates that the stearic acid is neutral or hypocholesterolemic at least when compared to the palmitic acid. Oleic acid is rather hypocholesterolemic but its activity depends on the source of fat. It seems to be more hypocholesterolemic in rapeseed oil than in the olive oil.
In spite of documented hypocholesterolemic activity of polyunsaturated fatty acids the high amount of n-6 acids and low amount of n-3 acids in food can be a reason of coronary heart disease (CHD). Thus the safe food with very low n-6 to n-3 ratio is recommended. On the other hand polyunsaturated fatty acid reduce the body fat retention as they inhibit the synthesis of fatty acids in liver, increase fatty acids oxidation and induce the thermogenesis. Similar activity show the medium-chain fatty acids (C₆-C₁₂). When compared with longer chain fatty acids they are readily absorbed, they limit fat deposition and display antibacterial activity. An independent risk factor for CHD is also serum trigliceride concentration and the increased risk starts at their relatively low concentration.
Dietary carbohydrates may affect in different extent the cholesterol level in animal tissues. There is no conclusive evidence regarding the effect of mono- and disaccharides on cholesterol metabolism. The effect of starch depends on the type of plant of origin. A vast body of experimental evidence indicates that the potato starch is hypowhile the wheat starch is hypercholesterolemic. The cholesterolemic effect of starches is probably connected with the changes in rates of food passage through the alimentary tract. Soluble fibres (gums, pectins) are more hypocholesterolemic than the cellulose. Cholesterolemic effect of fibres is probably related to increased excretion of bile acids and reduced cholesterol synthesis by short chain fatty acids originated as a result of fibre fermentation in large intestine.
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