268 Introduction to Human Nutrition
measures of agreement is generally similar to that
obtained in repeatability studies. Agreement at the
individual level is also not high, with coeffi cients of
variation for differences in individuals ranging from
17% to 33% in these studies and less than 50% of
respondents classifi ed in the same quintile of intake.
Note that even good agreement between two dietary
methods does not necessarily indicate validity, but
may merely indicate similar errors.
Biological measures to validate energy and
nutrient intake
It is now clearly recognized that to assess the validity
of any dietary method, including weighed records, it
is necessary to compare the dietary data with one or
more objective measures that refl ect but are indepen-
dent of food intake. At the group level such measures
include food supply or food expenditure data, and at
the individual level biochemical or physiological mea-
sures that refl ect energy and nutrient intake. The
latter are often referred to as biological or biochemical
markers and include energy expenditure, urinary
breakdown products of protein, sodium, and potas-
sium, plasma levels of vitamins, tissue levels of miner-
als, and the fatty acid composition of subcutaneous
adipose tissue.
Biological marker – any biochemical index in an
easily accessible biological sample that in health
gives a predictive response to a given dietary
component.
(Bingham, 1987).
Biological markers are assumed to be objective,
i.e., they do not rely on memory, or the respondents’
ability to express themselves, and are free of biases
introduced by the presence of the interviewers. These
measures are also subject to errors of measurement
and classifi cation, but these errors are not related
to the errors inherent in dietary intake assessment
methodologies.
The three most widely used measures to assess the
validity of dietary intake data are urinary nitrogen to
validate protein intake, energy expenditure as mea-
sured by the doubly labeled water (DLW) method to
validate energy intake in weight-stable individuals,
and the ratio of energy intake to basal metabolic rate
to identify “plausible” records of food intake.Urinary nitrogen
One of the fi rst to suggest an external measure as a
means of validating dietary intake data was Isaakson
(1980), who proposed urinary nitrogen as an inde-
pendent measure of protein intake according to the
equation:
Reported protein intake (g) =
(24 hour urinary N + 2) × 6.25 (g)
Like the 24 hour recall, a single 24 hour urine collec-
tion does not necessarily refl ect what is “usual.”
However, it appears that urinary nitrogen excretion is
less variable from day to day than dietary protein
intake, and that while 16 days of food intake are
needed to assess habitual protein intake only eight 24
hour urine collections are needed to assess nitrogen
excretion with the same level of confi dence.
Although fewer 24 hour urine collections may
be needed they are, in general, no more acceptable
to respondents than 24 hour food records and also
require access to laboratory facilities. Nevertheless,
they can provide a practical independent assessment,
not only of protein but also of potassium and sodium
intake.Table 10.6 Measures of relative validity for energy intake obtained from a food frequency questionnaire (FFQ) compared with multiple days of
records in three different studies
Measure of validity
7 days of weighed
records12 days of weighed
records15 days of records in
household measuresMean difference (kJ/day) (FFQ – record) 800 926 351
Mean difference (% of overall mean intake) 6 11 4
Coeffi cient of variation of the differences within individuals (%) 32.7 26.6 16.9
Coeffi cient of repeatability (kJ) ±8248 ±4542 ±2950
Correlation coeffi cient – 0.69 0.71
Individuals classifi ed in the same quintile or tertile* on
both occasions (%)
43* 42 44