Niacin, or vitamin B-3, is a member of the B-complex family. The term "niacin" refers to two chemically different compounds -- nicotinic acid and nicotinamide -- which are converted in your body to nicotinamide adenine dinucleotide, or NAD, the biologically active form of the vitamin. High doses of nicotinic acid have been used for decades to treat lipid disorders, primarily high triglyceride and low HDL levels. Due to its mode of metabolism, nicotinic acid can sometimes damage your liver. Consult your physician before you take niacin for treating a lipid abnormality.
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Nicotinic acid's effects on your lipid levels are independent of its nutritional activities in your cells. Scientists at the University of Pennsylvania report that nicotinic acid attaches to a cellular receptor in your adipose tissue called GPR109A, leading to a reduction in the release of fatty acids from fat cells. Since fatty acids are the building blocks for triglycerides, fewer fatty acids in your circulation results in reduced triglyceride synthesis. A falling triglyceride level evokes changes in the way your liver processes other lipids, which prompts a fall in your LDL and an increase in your HDL level. These benefits require relatively high doses of nicotinic acid, which can stress your liver.
Two Pathways
According to an August 2007 review in "Clinics in Liver Disease," your liver utilizes two pathways to metabolize nicotinic acid. The first pathway has a low capacity for handling nicotinic acid and produces potentially toxic byproducts. The second pathway is capable of handling a large amount of nicotinic acid, but triggers the production of substances that lead to the unpleasant "flush" -- skin redness, warmth and itching -- associated with niacin therapy. Your liver preferentially uses the first pathway until it is overwhelmed and then shunts additional nicotinic acid into the second pathway.
Various Formulations
Several forms of nicotinic acid are available for therapeutic use. Immediate release forms quickly overwhelm your liver's first metabolic pathway, with the majority of the dose being directed into the second pathway. This produces fewer toxic metabolites, but also triggers a prominent flush. In contrast, sustained-release formulations are gradually and almost entirely metabolized through the first pathway, which reduces flushing but increases the production of toxins and your risk for liver injury. So-called extended-release niacin offers "intermediate" metabolism, with only slightly less liver toxicity than sustained-release forms but with less flushing than immediate-release preparations.
Considerations
Niacin is a useful agent for treating lipid disorders, but its use is associated with side effects. Flushing and liver toxicity are the most prominent problems arising from niacin therapy. Immediate-release formulations cause the most flushing, while sustained-release preparations are most likely to cause liver injury. However, according to experts at Indiana University School of Medicine, almost any niacin formulation can cause some liver stress in doses above 2 to 3 g daily. Ask your doctor which formulation and dosage is best for you.
