The theory that cholesterol and saturated fat are the cause of cardiovascular disease because they detrimentally alter lipoprotein metabolism has been a disaster for health. That this is still a widely held belief is surprising given the weight of evidence against such a contention. In fact, although dietary saturated fat and cholesterol are not able to influence plasma levels of cholesterol carrying lipoproteins, many other foods are able to cause changes. The nutritional literature is replete with investigations into the cholesterol modulating abilities of various nutrients, yet little is made of this by the mainstream medical establishment. That cholesterol is a key component of bile is also interesting because changes to bile acid secretions have been shown to be associated with changes to plasma lipoprotein levels. The non-digestible carbohydrate components of plants for example, is thought to alter plasma cholesterol levels through its ability to alter bile acid flow. Carbohydrates therefore can influence plasma cholesterol levels because they contain different fibre types.
The lipoprotein altering activity of different carbohydrate foods has been investigated by research. For example, researchers have compared the lipoprotein and bile acid modulating effects of rice to potato1. Standard diets containing Western food were fed to type 2 diabetic subjects in a metabolic ward of a hospital. The meals contained around 30 % of their energy as carbohydrate in the form of either white refined long grain rice or mashed potato made without milk. When the subjects were fed the rice meals for a period of days, the very low density lipoprotein (VLDL; dietary triglycerides) decreased significantly in 6 of the 8 patients. In addition, the formation of bile acids, particularly that of cholic acid declined. The authors reported that there were no changes to the plasma levels of fasting glucose, low density lipoprotein (LDL) cholesterol or high density lipoprotein (HDL) cholesterol in any of the subjects between the two meal types. Therefore rice and potatoes differently affect the metabolism of lipoproteins.
The fibre content of the meal was unlikely to have been the reason that the rice was able to lower plasma levels of VLDL and decrease secretion of cholic acid because the rice contained less fibre than the potato meal (6.7 to 9.9 versus 9.2 13.2 grams in the rice and potato meals, respectively). The most likely explanation for the changes was the starch type within the carbohydrates. Amylose and amylopectin are digested at different rates because one (amylose) is a straight chain of glucose units whereas the other (amylopectin) is branched. This affects the surface area of the molecule and as a result the digestion and absorption rate of the glucose units contained within. As amylopectin is digested more quickly, it can increase postprandial plasma glucose levels and increase glucose delivery to the liver. This can in turn stimulate the de novo lipogenesis pathway and ultimately lead to fatty acid synthesis and increased VLDL formation. Rice tends to have a lower glycaemic index than potato because of its high amylose content.
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