Introduction to Human Nutrition

Measuring Food Intake 263

Estimation of portion size

Estimation of portion size has long been recognized
as an important source of error in dietary studies
(Young et al., 1953) with coeffi cients of variation of
the differences between estimates and weights of food
portions regularly reported to be around 50% for
foods and 20% for nutrients (Nelson and Bingham,
1997). However, despite the fact that individuals
are known to vary widely in their ability to estimate
portion size, relatively few studies have attempted to
quantify the size of this error or to “calibrate” their
respondents in this respect. The infl uence of some
factors on the determination of portion size is
summarized in Box 10.4.
In attempts to assist respondents to describe
portion sizes, a number of visual aids, known as
portion size measurement aids (PSMAs), have been
developed. These include:

● weighed portions of actual foods
● allowing respondents to serve out portions of food
and direct weighing or measuring of the serving

● commercial or home-made food models:

● food pictures or drawings of different portion

sizes

● photographs of foods in different portion sizes

● abstract shapes of cardboard, wooden or plastic

blocks, wedges, circles, balls, and mounds in

various sizes

● household utensils and containers such as cups,

spoons, jugs, glasses, bowls, and plates in various

sizes

● containers and packets of bought foods, e.g.,

sweet wrappers, potato crisp packets, cold drink

cans and bottles, yoghurt and ice cream cups,

milk cartons.

Each PSMA has advantages and disadvantages. The

type of PSMA chosen will depend, among others,

on the type of study, the target population, whether

interviewers go from house to house or respondents

go to a research centre, available resources, and the

availability of appropriate PSMAs. Probably the most

effective method is a combination of PSMAs such as

food photographs and household utensils. Irrespec-

tive of the type of PSMA used, it is essential that

respondents are able to identify and relate to the

PSMA, that PSMAs be tested in the target population

prior to their use, and that PSMAs are used con-

sistently throughout the study.

Recall errors

Factors that have been studied in relation to the accu-

racy of dietary recall include food consumption

patterns, weight status, gender, and age. Many other

characteristics, such as intelligence, mood, attention,

and salience of the information, however, have also

been demonstrated to infl uence performance tests of

general recall, but have not been studied in the context

of dietary recall.

Short-term memory

Like the ability to estimate portion size, the ability to

remember what was eaten varies with the individual.

Studies that have compared the abilities of different

groups to remember what they have eaten conclude

that women are generally better than men and that

younger adults are better than older adults. In short-

term recalls of intake (e.g., 24 hour recalls) indivi-

duals more often tend to omit an item or items that

they have consumed than to include ones that they

Box 10.4

Food characteristics

• No consistent associations observed with type of food, although
  foods of indeterminate shape are more often associated with
  larger and liquid foods with smaller errors.


• Large portion sizes appear to be more diffi cult to estimate accu-
  rately than small portion sizes.
  Visual aids


• Household measures may be associated with considerable
  errors.


• Food models produce more reliable results than household mea-
  sures, but because only one size is usually available they may
  “bias” respondents to report portion sizes similar to those
  shown.


• Graduated food models and two-dimensional pictures may be
  as helpful as three-dimensional models for estimating portion
  size.


• The range of reference portion sizes available may infl uence the
  estimates.


• The use of multiple photographs results in more accurate esti-
  mates.
  Respondents


• Respondents of all ages have been reported to have diffi culty
  with portion size estimates.


• Women have sometimes, but not invariably, been reported to be
  better able to estimate portion size than men, but this may
  simply refl ect the fact that they tend to handle food more often
  than men.