Welcome to the world of what some call neutraceuticals. I.e. nutrients being used as if they were drugs.

It is true that the daily requirement for niacin is in the vicinity of 20 mg, but it's also important to understand what that definition means. The RDA (and similar intake guidelines) are defined as the amount of a nutrient that prevents *overt symptoms of deficiency* in 97.5% of *normal healthy* subjects. I've emphasized two bits because the definition is in terms of deficiency symptoms (niacin deficiency is pellagra), and 1 in 40 normal healthy people could still have it at that intake level. And what about people who are neither normal nor healthy? What is the optimal intake, as opposed to the bare minimum?

At much higher doses, you start running into signs of toxicity. Toxicity itself is a really complicated subject, as increasing the intake of any individual nutrient simultaneously increases the demand on other nutrients. It's more than a semantic concern whether or not what we see as toxicity of high intake is really due to toxic processes, or instead, indicates poor intake of other nutrients which work along with that individual one.

Anyway, the toxic threshold for niacin (called the UL, or upper limit) is usually thought to lie at around 3,000 or 4,000 mg/day. That's quite a spread from the minimal intake of 20 mg. The UL is 20,000% more than the RDA.

N.B. Individuals may show signs of toxicity at lower doses. We're talking statistics here, and individuals are not statistics.

So, what about that 500 mg dose of niacin? Chances are, in my opinion, it's much closer to the optimal intake than is the RDA/DRI of 20 mg. In fact, as we find that higher doses of niacin lower trigycerides and promote healthier levels of cholesterol, I would argue that the RDA is far too low. High cholesterol may be a sign of niacin deficiency, and defining intake in terms of pellagra overlooks other benefits of this nutrient.

Niacin supplements come in three common forms. It makes a difference which one you have, but I'd bet you've got a bottle of plain old niacin.

Niacin (once called nicotinic acid) is an excitatory compound. It promotes histamine release. Many people taking doses of plain niacin get a very uncomfortable flushed feeling, particularly in the face (called, not surprisingly, the niacin flush). If dosing continues for a period of time, the flush tends to decrease in intensity. Alternatively, you can avoid it by gradually increasing the dose, to build tolerance.

Signs of liver inflammation (seen as increases in liver enzymes in blood panels) can occur at higher doses of niacin, when taken for extended periods of time. This is the primary sign of what is thought of as niacin toxicity. Simply discontinuing the niacin (or lowering the dose) is all that need be done. Ignoring the effect, though, could lead to liver failure.

Another form, sometimes called "slow release niacin", is inositol hexanicotinate. It's also called inositol polynicotinate. It's simply a sugar molecule (inositol) with six bonds to six individual molecules of niacin. The extra bonds are slowly broken down in the body, giving rise to free niacin at more of a trickle than the burst if you just took pure niacin. For some reason, this form of niacin is most likely to give liver problems.

Finally, you have niacinamide. It was once known as nicotinamide. Niacinamide, quite contrary to the activation seen with niacin, is an inhibitory molecule. It inhibits histamine release, and enhances the activity of GABA at the GABA-A receptor. It is therefore useful as a mild anxiolytic, or a partial treatment for e.g. hayfever. It is also more closely associated than is niacin with the energy-promoting mitochondrial molecule NADH. I know that sounds kind of like opposites, but niacinamide can be both calming and energizing. Some people don't notice anything, though.

Niacinamide doses might be better to keep a little lower than niacin. I generally recommend no more than 2000 mg/day, in divided doses of no more than 500 mg. Niacinamide can potentially make you feel very sedated, so be cautious when you first take it.

It is also a general recommendation to take a B-complex whenever you take any single B-vitamin. As I mentioned earlier, increasing the intake of any single B-vitamin increases the stress/demand on other nutrients. The B-vitamins work together as a team. That's, in fact, why they are sold as B-complex in the first place.

Zinc, magnesium, selenium and chromium demand is also likely to increase with niacin therapy.

Lar