Introduction to Human Nutrition

128 Introduction to Human Nutrition

The lowest defi ned intake level [lowest threshold
intake (LTI), lower reference nutrient intake (LRNI),
i.e., the average requirement minus a notional 2 SDs]
is not regarded as being useful in the context of assess-
ing the adequacy of population nutrient intakes. This
is because it would identify only those individuals
who were almost certainly not meeting their require-
ment, and by the same token would omit to include
many in the population who would be at appreciable
risk of nutrient inadequacy (in other words, those
whose intake was below the average requirement).
Finally, the tolerable upper levels of intake defi ned
for certain nutrients can also be used as cut-off points
to identify those individuals at risk of consuming
toxic levels of a nutrient.

7.5 Methods used to determine
requirements and set dietary
recommendations

In order to derive the most accurate and appropriate
dietary recommendations, committees of experts are
established that look at the scientifi c evidence and use
their judgment to decide which nutrients to consider
and then, for each nutrient, make decisions in respect
of the:

● criterion by which to defi ne adequacy
● estimation of the average amount required to meet
that criterion of adequacy
● estimated standard deviation of requirement in the
population under consideration (i.e., the shape of
the frequency distribution over the range of require-
ments: broad, narrow, skewed, etc.).
The problem of different committees identifying
different criteria of adequacy is illustrated by vitamin
C (ascorbic acid). Experimental evidence (the Shef-
fi eld and Iowa studies) has shown that an intake of
approximately 10 mg/day is required to prevent the
defi ciency disease scurvy in adult men. At intakes
below 30 mg/day, serum levels are negligible, rising
steeply with intakes of between 30 and 70 mg/day,
after which they begin to plateau (and urinary excre-
tion of the unmetabolized vitamin increases). The
question facing the committees drafting dietary refer-
ence values is whether to choose a level of intake that
allows some storage of the vitamin in the body pool
(e.g., EU AR 30 mg/day for adults), or one that more
nearly maximizes plasma and body pool levels (e.g.,

US EAR 60 and 75 mg/day for women and men,

respectively). Similarly, variations in calcium recom-

mendations exist because some committees choose to

use zero calcium balance as the criterion of adequacy,

while others use maximum skeletal calcium reserves.

In some cases, one recommending body will include

a nutrient among its dietary recommendations while

others will not; for example, vitamin E, the require-

ment for which depends directly on the dietary intake

and tissue levels of PUFAs, which are highly skewed.

The vitamin E requirement corresponding to the

highest levels of PUFA intake would be much higher

than that needed by those with much lower (but ade-

quate) intakes. To set the high value as the recom-

mendation might suggest to those with lower

polyunsaturate intakes that they should increase their

intake of vitamin E (unnecessarily). Thus, in Britain

and Europe, only “safe and adequate” intakes have

been set, based on actual intakes in healthy popula-

tions, which should be at least 3 mg/day for women

and 4 mg/day for men. In contrast, the US RDA (DRI)

has been raised to 15 mg/day as α-tocopherol, based

on induced vitamin E defi ciency studies in humans

and measures of lipid peroxidation.

There are even some examples of dietary compo-

nents that have not traditionally been regarded as

essential nutrients having recommendations set for

them, as in the case of choline. The US DRI defi nes

an adequate intake for choline (of 450 and 550 mg/

day for women and men, respectively), on the basis

that endogenous synthesis of this compound is not

always adequate to meet the demand for it (for

the synthesis of acetylcholine, phospholipids, and

betaine). Dietary intake data for choline and the sci-

entifi c evidence for inadequacy are limited; thus,

dose–response studies would need to be done before

an average requirement could be derived. It is proba-

ble that further dietary components will be included

in dietary recommendations as research data accu-

mulate. Potential candidates include the fl avonoids

and some other antioxidant compounds.

7.6 Methods used to

determine requirements

Deprivation studies

This is the most direct method and involves removing

the nutrient from the diet, observing the symptoms