Meat Protein Elevates Cholesterol



America's obsession with dietary fat and cholesterol may be misguided. A study in the Journal of Nutrition implicates meat protein -- not fat -- as a major cause of high cholesterol. Although it is too early to tell whether this finding will hold up in humans, animals on a cholesterol-free diet can end up with high cholesterol if they consume a high casein (milk protein) diet. Researchers in Japan discovered that cholesterol levels skyrocketed when animals were fed casein protein versus soy protein. The difference? Casein is an animal-based protein high in the amino acid methionine. Soy is a plant-based protein deficient in methionine. According to this research and studies done in rabbits, methionine raises cholesterol.

In contrast, when rats are fed a soy diet, their cholesterol falls. Adding methionine to the soy diet sends their cholesterol soaring again. But although the beneficial effect of soy on cholesterol is reduced by methionine, it is not completely reversed. Soy has an additional protective effect that keeps cholesterol somewhat lower than when the animals consume casein only.

Several theories about how soy reduces cholesterol have been suggested. Japanese researchers believe that the high glycine in soy may lower cholesterol. Glycine has been shown to have cholesterol-lowering effects in rodents. If this scenario is correct, soy packs a one-two punch: its lack of methionine keeps cholesterol low, and its glycine content helps lower cholesterol.

Effect Of Estrogen

Estrogen enhances the cholesterol-lowering effect of soy. When female monkeys who had had their ovaries removed were given beta-estradiol with a soy-based diet, their aortic cholesterol fell significantly. Soy has many benefits beyond just lowering cholesterol. In the monkey study, soy improved lipid profiles, insulin sensitivity and glucose effectiveness. It also decreased lipid peroxidation in the arteries. Animals given estradiol showed a reduction in abdominal fat, fasting insulin and body weight, smaller LDL particles, and less aortic cholesterol. Combining soy and estrogen was the most beneficial of all, giving all the benefits of each.

In other studies, high cholesterol was produced in rabbits on a low-fat, zero-cholesterol diet within five weeks if the protein fed the rabbits was casein. Adding butter had no effect. The same rabbits fed a soy-based diet had lower cholesterol than those fed either meat protein or regular chow. According to one research group, cholesterol turns over faster on a soy protein diet, but is absorbed more on a meat diet.

Meat protein has now been associated with two of the major risk factors for heart attack and stroke -- cholesterol and homocysteine.

For Further Reading

Sugiyama K, et al. 1997. Methionine content of dietary proteins affects the molecular species composition of plasma phosphatidylcholine in rats fed a cholesterol-free diet. J Nutr 127: 600-7.

Wagner JD, et al. 1997. Dietary soy protein and estrogen replacement therapy improve cardiovascular risk factors and decrease aortic cholesteryl ester content in ovariectomized cynomolgus monkeys. Metabolism 46: 698-705.

Huff MW and KK Carroll. 1980. Effects of dietary protein in turnover, oxidation and bsorption of cholesterol, and on steroid excretion in rabbits. J Lipid Res 21: 546-48.

Life Extension Foundation.

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