Low fat diets have been shown to decrease levels of low density lipoprotein (LDL) cholesterol, whilst low carbohydrates have been shown to increase LDL cholesterol. Elevated levels of LDL have traditionally been seens as a risk factor for cardiovascular disease and so elevated levels of LDL with low carbohydrates diets have been highlighted by some researchers as a detrimental physiological change. However, the LDL plasma levels in isolation is less important from a cardiovascular disease risk perspective than the ratio between the low density lipoprotein and high density lipoprotein (HDL) particles. For while elevated levels of LDL increase the risk of cardiovascular disease, elevated levels of HDL protect from cardiovascular disease. Therefore it is not the absolute values of each, but their ratio that determines the cardiovascular risk. Elevated levels of LDL in the presence of elevated levels of HDL and depressed levels of LDL in the presence of depressed levels of HDL may therefore confer similar cardiovascular risk.
The LDL to HDL ratio is interesting when considering low fat and low carbohydrate diets. Low fat diets have been shown consistently to lower plasma levels of LDL, although the effect is small. However, low fat diets also lower plasma levels of HDL, although again the effect is small. Therefore overall the low fat diets will lower total cholesterol levels slightly because the sum of the LDL and HDL particles, increases. In contrast, low carbohydrate diets increase plasma levels of LDL slightly, but at the same time cause a relatively large increase in HDL. Therefore low carbohydrate diets tend to increase total cholesterol. Total cholesterol however, is not a good predictor of cardiovascular disease, a much better predictor being the LDL to HDL ratio. In this respect, the low fat diet does not change the LDL to HDL ratio as both biomarkers move upwards slightly such that their ratio remains constant. However, the LDL to HDL ratio falls with low carbohydrate diets, suggesting that low carbohydrate diets are cardioprotective.
It is unclear whether cardiovascular risk is lowered more through an increase in HDL or a decrease in LDL. However, there are many emerging factors that can further influence the cardiovascular risk, and unless these are considered then it is not possible to accurately predict cardiovascular risk from the LDL to HDL ratio in isolation. The problem is compounded by the fact that the changes are relatively small and in some cases they fail to reach levels of statistical significance. However, when comparing the changes in LDL and HDL on low carbohydrate diets with the change to triglyceride levels, we see that the latter is a much more significant and consistent effect of the diet. The ability of low carbohydrate diets to lower plasma triglycerides is independent of weight loss. In comparison, low fat diets actually increase plasma triglycerides when they are not accompanied by either exercise or weight loss. Carbohydrates in the diet therefore appear to drive plasma triglyceride levels, most notably when refined.
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