Introduction to Human Nutrition

The Vitamins 159

Vitamers and niacin equivalents

Two compounds, nicotinic acid and nicotinamide,
have the biological activity of niacin. When nicotinic
acid was discovered as the curative and preventive
factor for pellagra, it was already known as a chemical
compound, and was therefore never assigned a
number among the B vitamins. The name niacin was
coined in the USA when it was decided to enrich
maize meal with the vitamin to prevent pellagra; it
was considered that the name nicotinic acid was not
desirable because of its similarity to nicotine. In the
USA the term niacin is commonly used to mean spe-
cifi cally nicotinic acid, and nicotinamide is known as
niacinamide; elsewhere “niacin” is used as a generic
descriptor for both vitamers. Figure 8.10 shows the
structures of nicotinic acid and niacin, as well as the
nicotinamide nucleotide coenzymes, NAD and
NADP.
The nicotinamide ring of NAD can be synthesized
in the body from the essential amino acid tryptophan.
In adults almost all of the dietary intake of trypto-
phan, apart from the small amount that is used for
net new protein synthesis, and synthesis of the neu-
rotransmitter serotonin, is metabolized by this
pathway, and hence is potentially available for NAD
synthesis.

Several studies have investigated the equivalence of

dietary tryptophan and preformed niacin as precur-

sors of the nicotinamide nucleotides, generally by

determining the excretion of niacin metabolites in

response to test doses of the precursors, in subjects

maintained on defi cient diets. The most extensive

such study was that of Horwitt et al. in 1956. They

found that there was a considerable variation between

subjects in the response to tryptophan and niacin,

and in order to allow for this individual variation they

proposed the ratio of 60 mg of tryptophan equivalent

to 1 mg of preformed niacin. Changes in hormonal

status may result in considerable changes in this ratio,

with between 7 and 30 mg of dietary tryptophan

being equivalent to 1 mg of preformed niacin in late

pregnancy.

The niacin content of foods is generally expressed

as mg niacin equivalents; 1 mg niacin equivalent = mg

preformed niacin + 1/60 × mg tryptophan. Because

most of the niacin in cereals is biologically unavail-

able (see below), it is conventional to ignore pre-

formed niacin in cereal products.

Because endogenous synthesis from tryptophan is

more important than preformed dietary niacin, the

main dietary sources of niacin are generally those that

are also rich sources of protein. It is only when the

dietary staple is a cereal such as maize, which is

CH 2 O P POCH 2

N

N N

N

NH 2

O O

OH OH

OH OH

O

OH OH

N

CONH 2

Phosphorylated

in NADP

Nicotinamide adenine dinucleotide

N

CONH 2

O O

N

COOH

Nicotinic acid Nicotinamide

Figure 8.10 The niacin vitamers, nicotinic acid and

nicotinamide, and the coenzyme nicotinamide

adenine dinucleotide.