Plasma cholesterol, which can be obtained from the diet or produced within the body, is transported along with proteins and phospholipids as lipoproteins. The level of cholesterol in the plasma is not precisely regulated and varies in response to dietary factors. Higher levels of total cholesterol, known as hypercholesterolemia, increase the risk of coronary heart disease (CHD). The correlation between CHD and the level of cholesterol in low-density lipoproteins (LDL-C) is much stronger. As LDL-C levels increase, the risk of CHD also increases. On the other hand, elevated levels of high-density lipoprotein cholesterol (HDL-C) have been linked to a decreased risk of heart disease. Abnormal levels of plasma lipids, known as dyslipidemias, in combination with factors such as smoking, obesity, sedentary lifestyle, insulin resistance, and others, further increase the risk of CHD. Lowering plasma cholesterol through dietary or drug treatment has been proven effective in reducing the risk of cardiovascular events. While dietary changes typically result in modest reductions of 10% to 20% in plasma cholesterol, treatment with statin drugs can decrease cholesterol levels by 30% to 60%. The influence of dietary fats on blood lipids is primarily determined by the chemical composition of the constituent fatty acids in triacylglycerols (TAG). The presence or absence of double bonds, the number and location of double bonds, and the configuration of unsaturated fatty acids all play important roles in determining the impact of TAG on blood lipids. Saturated fats, which consist of fatty acids without any double bonds, are associated with higher levels of total plasma cholesterol, LDL-C, and an increased risk of CHD. Major sources of saturated fats include dairy and meat products, as well as some vegetable oils like coconut and palm oils. It is recommended to limit saturated fat intake to less than 10% of total caloric intake and replace them with unsaturated fats and whole grains. Among saturated fatty acids, myristic acid (14 carbons) and palmitic acid (16 carbons) have the greatest effect on increasing plasma cholesterol levels, while stearic acid (18 carbons) has a minimal impact on blood cholesterol.
Plasma cholesterol may arise from the diet or from endogenous biosynthesis. In either case, cholesterol is transported between the tissues in combination with protein and phospholipids as lipoproteins.
1. Low-density and high-density lipoproteins: The level of plasma cholesterol is not precisely regulated but, rather, varies in response to diet. Elevated levels of total cholesterol (hypercholesterolemia) result in an increased risk for CHD (Fig. 27.10). A much stronger correlation exists between CHD and the level of cholesterol in low-density lipoproteins ([LDL-C] see p. 234). As LDL-C increases, CHD increases. In contrast, elevated levels of high-density lipoprotein cholesterol (HDL
C) have been associated with a decreased risk for heart disease (see p. 235). [Note: Elevated plasma triacylglycerol (TAG) is associated with CHD, but a causative relationship has yet to be demonstrated.] Abnormal levels of plasma lipids (dyslipidemias) act in combination with smoking, obesity, sedentary lifestyle, insulin resistance, and other risk factors to increase the risk of CHD.
2. Benefits of lowering plasma cholesterol: Dietary or drug treatment of hypercholesterolemia has been shown to be effective in decreasing LDLC, increasing HDL-C, and reducing the risk for cardiovascular events. The diet-induced changes in plasma cholesterol concentrations are modest, typically 10%–20%, whereas treatment with statin drugs decreases plasma cholesterol by 30%–60% (see p. 224). [Note: Dietary and drug treatment can also lower TAG.]
B. Dietary fats and plasma lipids
TAG are quantitatively the most important class of dietary fats. The influence of TAG on blood lipids is determined by the chemical nature of their constituent fatty acids. The absence or presence and number of double bonds (saturated versus mono-and polyunsaturated), the location of the double bonds (ω-6 versus ω-3), and the cis versus trans configuration of the unsaturated fatty acids are the most important structural features that influence blood lipids.
1. Saturated fats: TAG composed primarily of fatty acids whose hydrocarbon chains do not contain any double bonds are referred to as saturated fats. Consumption of saturated fats is positively associated with high levels of total plasma cholesterol and LDL-C and an increased risk of CHD. The main sources of saturated fatty acids are dairy and meat products and some vegetable oils, such as coconut and palm oils (a major source of fat in Latin America and Asia, although not in the United States). Many experts strongly advise limiting intake of saturated fats to <10% of total caloric intake and replacing them with unsaturated fats (and whole grains).
Saturated fatty acids with carbon chain lengths of 14 (myristic) and 16 (palmitic) are most potent in increasing the plasma cholesterol level. Stearic acid (18 carbons, found in many foods including chocolate) has little effect on blood cholesterol.