Niacin is a member of vitamin B complex group and also called as vitamin B3. It occurs in two forms: nicotinic acid and niacinamide. The body is able to convert nicotinic acid to niacinamide, which is the form present in blood. As coenzyme, niacin participates in energy transfer reactions of glucose fat and alcohol. Numerous clinical trials have also demonstrated niacin as an effective drug in raising HDL cholesterol. In addition to its HDL cholesterol raising properties, niacin decreases concentrations of plasma triglycerides and apo B-containing lipoproteins. As a drug, niacin is used in larger doses, which are associated with a number of side effects. It is generally accepted that niacin decreases hepatic apo B secretion, which in turn decreases concentration of plasma apo B-containing lipoproteins and increases plasma HDL cholesterol concentration. Since liver is the major organ that regulates apo B secretion and catabolism, liver plays an important role in niacin-mediated changes in lipoprotein metabolism. However, the specific mechanism of action of niacin is not well understood. Therefore, we the present report will investigate any recent discoveries in its mechanism of action.
REVIEW OF LITERATURE
Plasma lipoproteins are important intervening variables in the etiology of atherosclerosis, the lesion underlying coronary heart disease (CHD), stroke, and peripheral arterial disease. Elevated levels of low density lipoproteins (LDL) and reduced levels of HDL are associated with more severe atherosclerosis and with increased risk of coronary heart disease (CHD) in humans. The ratio of HDL to LDL cholesterol is often used to accurately predict the risk of atherosclerosis and CHD. Statins, which are strong inhibitors of 3-hydroxy-3-methyl-CoA (HMG-CoA) reductase, have been used effectively to lower LDL cholesterol, improve HDL/LDL cholesterol ratio and reduce the risk of CHD in subjects with high LDL cholesterol. However, subjects with low HDL also present a greater risk of atherosclerosis and CHD and therefore, there is a greater need for drugs that raise HDL cholesterol.
HDL cholesterol can be raised by a number of ways that include both non-pharmacological and pharmacological treatments. Non-pharmacological ways that raise HDL cholesterol include lifestyle changes such as vigorous exercise, alcohol consumption, weight loss and diet modifications (1-4). However, lifestyle-associated changes do not increase HDL cholesterol to a great extent and depend upon the baseline cholesterol values. Therefore, human subjects with low plasma HDL cholesterol with or without increased LDL cholesterol levels will require drug treatment to prevent premature atherosclerosis and CHD. Most commonly used drugs that raise HDL cholesterol are fibrates and niacin (5-8). Recently Hamoud and his associates studied the effect of treatment of niacin on plasma lipid profile and oxidative stress in seventeen subjects with hypercholesterolemia and low HDL. After the treatment for 12 weeks, the subjects improved their lipid profile and reduced the oxidative stress. These studies suggest an additional effect of niacin in addition to its ability to increase HDL cholesterol.
CHD is the major cause of deaths in US and in other Western countries. The CHD is mainly caused by atherosclerosis in coronary arteries that supply blood to heart. Atherosclerosis is positively associated with increased levels of LDL cholesterol. On the other hand increased levels of HDL cholesterol are negatively associated with atherosclerosis. A number of drugs are available to reduce LDL cholesterol but no drugs are yet available in the market that will increase HDL. Niacin is a vitamin but when used in pharmacological dose it increases plasma HDL cholesterol and decreases triglycerides. There are some side effects of the drug that may prevent its use. However, this is the only drug that can effectively raise HDL cholesterol in subjects with low HDL cholesterol. The recent studies suggest that this drug may reduce the oxidative stress, which is a major risk of atherosclerosis and CHD. CHD is the number one killer in the country and costs billions of dollars Thus, niacin functions as an important vitamin and a drug that can reduce the risk for CHD.