During the 1980s and 1990s, a number of controlled intervention studies in healthy individuals who maintained their body weight showed that high-carbohydrate diets often resulted in higher blood TG levels and lower HDL-cholesterol levels - changes that are atherogenic and increase the risk of coronary heart disease - despite improved total and LDL-cholesterol levels (29). These findings sparked particular concern for people with diabetes because their lipid abnormalities tended to be higher TG and lower HDL-cholesterol level rather than the high total and LDL-cholesterol typically observed in non-diabetic individuals (18). Hence the magnified risk of atherosclerosis in people with diabetes might be related to blood lipid risk factors that are specifically worsened by high-carbohydrate diets.
The biochemical mechanisms responsible for increased plasma TG levels following low-fat, high-carbohydrate diets remain uncertain but are clearly different to those responsible for elevated TG levels following increased fat intakes. Parks et al. (30) demonstrated that high-carbohydrate diets reduce the clearance of VLDL-TG from the plasma, but do not increase VLDL-TG secretion or de novo lipogenesis in the liver as had been postulated.
The mechanisms by which high-carbohydrate diets decrease HDL-cholesterol are also unknown and should be a priority in future research. In two recent cross-sectional studies of healthy adults, a significant inverse association was found between serum HDL-cholesterol concentration and dietary GI for both men and women (the higher the GI rating of the diet, the lower the HDL concentration) (31,32). In fact, the glycaemic index of the diet was the only dietary variable significantly related to serum HDL-cholesterol. These findings suggest that post-prandial glucose and insulin responses may directly influence HDL levels.
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