14.5: Niacin

As recounted in Chapter 11, Goldberger’s medical detective work in relating pellagra to a dietary deficiency was a splendid clinical achievement—a feat of personal courage and determination. But in recalling the story, we must not lose sight of the fact that Goldberger made little headway in learning the actual nature of the “antipellagra factor” and how it serves in the normal processes of life.

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Niacin in large doses is used as a drug; the form of niacin makes a difference. Massive doses of nicotinic acid—but not nicotinamide—are used to improve blood cholesterol (see Chap. 9).

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Niacin was isolated in the laboratory long before anyone understood that it had a role in life chemistry. In 1867, a German scientist made it by treating nicotine taken from tobacco (giving it its first scientific name, nicotinic acid). But, tragically, the cure for pellagra sat unrecognized on a shelf while tens of thousands died of the disease. It was 1937 before nicotinic acid was refined from food and clearly identified as a B vitamin.

Nutrition scientists were concerned that the name nicotinic acid would be confused by some with the nicotine found in tobacco, although the two compounds are very different. Nicotine has no vitamin activity. So niacin became the official name of the vitamin.

Niacin occurs in the form of nicotinic acid, and there’s also nicotinamide. Both forms are called niacin and, in practical terms, are considered to have equal vitamin significance. Like thiamin and riboflavin, niacin is mainly used in body chemistry as an indispensable part of coenzymes needed to extract energy from nutrients and is needed by every cell in the body.

Niacin—and Tryptophan—Intake

The body can convert the amino acid tryptophan into niacin, so it’s a bit tricky to assess the niacin adequacy of a diet. In stating the niacin values of foods, niacin equivalents are often used, to include the niacin which tryptophan can ultimately provide. Essentially, one niacin equivalent is the equal of one milligram of niacin.

This doesn’t mean that all of our tryptophan is used to make niacin. Only part is used this way. Niacin conversion doesn’t deplete the body of the tryptophan needed for protein synthesis and other functions. What’s meant is that one doesn’t necessarily have to eat niacin-containing foods to meet one’s niacin requirement. In fact, the typical American diet provides enough excess tryptophan to meet niacin needs.

The way in which nursing infants get their niacin is strong testimony to the effectiveness with which tryptophan can serve as a source of niacin. Mother’s milk has less than half of the daily need of niacin. The rest comes from the ample tryptophan content of the milk.

Generally, niacin is most plentiful in those foods which are good sources of protein. So in eating foods rich in tryptophan, we usually are also eating foods rich in niacin. Add to this the fact that niacin is added to a lot of foods, such as bread and cereals, and there’s little need for concern about niacin in the typical American diet.

Niacin deficiency mainly concerns nations where the protein supply is very limited, especially if much of the protein comes from corn (low in tryptophan). In these nations, multiple nutritional deficiencies are common, and, again, attention must be paid to the whole diet.

If animal foods (meat, milk, eggs) are a regular part of the diet, one consumes not only a good bit of protein but also a good bit of both niacin and “potential niacin” as tryptophan. These foods are also good sources of thiamin and riboflavin. In cultures that prohibit meat, the well-to-do usually avoid nutritional deficiencies by consuming varied plant foods in ample amounts—not just corn, let’s say, but wheat, peas, beans, millet, and a variety of vegetables.

Niacin ferries hydrogen atoms between chemical reactions

There might have been much less pellagra in the American Southeast had plant sources of protein other than corn been widely available—more wheat, peas, beans, nuts, soy products, and the like. In general, one can expect nutritional deficiencies of several kinds when the diet is narrowed to only a few foods.

Some cultures whose diet centered around corn didn’t have as much pellagra. Coffee has niacin—was coffee a common beverage? Was the corn soaked in lime water (water with calcium hydroxide), used to make the masa harina in traditional corn tortillas?

Some of the niacin in corn isn’t easily absorbed; soaking the corn in lime water increases the amount of niacin we can absorb from the corn. Where intake is marginal, seemingly small aspects of one’s diet can make the difference between deficiency and adequacy.