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OCR for page 19
Some Study
Design Characteristics
DIETARY METHODOLOGY
Selection among available methods for assessing food and/or nutrient intake
of households or individuals depends on available resources (funds, time,
and personnel), the objectives of the study, and the target population (Marr,
1971~. The effectiveness of any survey method in answering questions about
food or nutrient intake of a population is contingent upon a statistically
adequate sampling frame to assure that appropriate respondents are used and
that oversampling of targeted groups to meet specific needs is permitted.
In general, information on food intake is obtained by (1) inference, (2)
observation, and (3) verbal or written reports. Each has some inherent
strengths and limitations, so a combination of methods may be needed.
Inferred data are derived primarily from aggregate data provided by
commodity reports, commercial surveys of movement of food products in
and out of warehouses or markets, and national food balance reports. These
data provide information on trends in food consumption for a population
group. They do not provide information on average intakes or ranges of
intakes of individual consumers. Aggregate data can serve as a surrogate for
individual data if the above limitations are understood. However, aggregate
data are most useful in validating observations generalized to the total
population from individual data.
Direct personal observation is generally so costly of money, time, and
personnel that it is precluded in large-scale surveys and is used primarily to
validate reported data. The observations may be either obtrusive (such as
participant observation, which may in short-term studies result in modified
19
OCR for page 20
20 ASSESSING CHANGING FOOD CONSUMPTION PATTERNS
behavior) or unobtrusive (such as monitoring store purchases and correlating
them with specific individuals, which may constitute an invasion of pri-
vacy). Both modified behavior and invasion of privacy are serious obstacles
to the use of methods involving observation of food intake.
AS a result of the limitations of inferred and observed data, reporting is
generally regarded as the most feasible and cost-effective way of learning
about food intake of individuals and groups. Reported data are obtained by
oral interview or are recorded by the subject. Oral interviews involve retro-
spective accounts with the interviewer available in person or by telephone.
Food frequency information, a dietary history, and/or a 24-hour recall of
food intake may be recorded by or for the subject in a written, telephoned, or
tape-recorded format. Food intake records may be maintained in this manner
for from one to usually no more than 7 days.
Usefulness of a reporting method depends on validity and reliability of the
information reported, on the cost and speed of data collection and analysis,
and on the appropriateness for both long- and short-range goals of the
survey. Conclusions are only as good as the quality of the core information
on which they are based, so an ongoing effort to assess the validity of the
reported data is essential. Although there is considerable information on the
reliability of various techniques for obtaining written and oral reports, there
is very little information on validity of the data obtained. Limited studies
done on small subsamples of the population indicate a tendency to underre-
port at upper levels of intake and to overreport at lower levels. This
phenomenon, known as the "flat slope syndrome," casts doubt on the
interpretation of data as regards extremes of intake. The extent to which
actual and reported intakes vary appears to differ with the nutrient studied;
therefore, the "flat slope syndrome" may need to be evaluated across the
range of nutrients (Gersovitz et al., 19781.
There is an urgent need for methodological studies for assessment of the
validity of currently used dietary survey techniques in a range of cir-
cumstances and for identification and validation of alternative and innova-
tive methods for obtaining food intake information (Garn et al., 1978~.
While this report suggests four replications, questions remain regarding the
minimum number of days or other units of time for which reported observa-
tions must be made in order to assess usual dietary intake and produce an
accepted representation of actual intake. It has been reported in at least one
study that validity of record keeping by adults decreases after 4 consecutive
days (Gersovitz et al., 1978~. There does not seem to be a similar problem
when records are kept for several days intermittently (Beaton et al., 1979~.
Records of a single day's intake of individuals representing weekdays and
weekend days over a long enough interval to detect cyclic changes have
been found to give an acceptable estimate of usual food consumption
OCR for page 21
Some Study Design Characteristics
21
(Houser and Bebb, 19811. Probing to clarify records and enhance the accu-
racy of recall has resulted in substantial increases in the completeness of
records (Campbell and Dodds, 1967~. Analyses of food intake records using
James-Stein indicators show that nutrient intakes regress toward the mean
and that averages of 5 (and possibly fewer) days may give values that more
closely approximate usual intakes than do individual day intakes (Samonds
et al., 1978~. Other methodological questions for which there are only
limited answers and which require further research are how to ensure im-
mediate reporting, how to standardize estimates of serving sizes and de-
scriptions of food items, the extent to which the homemaker can report for
other family members, and the extent and nature of interviewer differences.
Written records are useful with literate, motivated subjects but not with
those who are illiterate or poorly motivated or those with impaired vision or
lack of neuromuscular coordination. Oral records are useful with these
groups, but they often necessitate either a costly personal interview or the
use of a recording device that may be technically baffling or too impersonal
to encourage continued participation. Interviews that provide an opportunity
for probing and permit clarification of food identification have been found to
increase reported intakes by as much as 25 percent and to prolong participa-
tion, but they may encourage subjects to strive to respond in a perceived
approved fashion.
A limited study of the validity of food records telephoned each day to a
nutritionist or a telephone answering service showed them to be as valid as a
24-hour recall or a 7-day food record (Raker, 1979~. Reports given to the
nutritionist who could probe for some specific information were judged to
be more valid than those from a recording device. The phone survey
technique has the advantages of assuring that records are kept each day and
of being equally applicable for literate and nonliterate subjects. However,
telephone recordings have limitations similar to those of written records and
limit the population surveyed to respondents who have telephones.
Currently available methods for determining food intake and the inherent
advantages and disadvantages of each are shown in Table 2. Dietary
methodologies must be refined to identify feasible methods of minimizing
their present limitations and disadvantages.
ESTIMATION OF USUAL INTAKES
The preceding discussion has defined many of the limitations of current
methods for measuring food consumption patterns of individuals and popu-
lation groups. Most methods in current use collect data relating to finite
periods of time, i.e., 1, 2, 3, or 7 days. For the proposed system it will be
necessary to collect data that describe the distribution of usual food intakes
OCR for page 22
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OCR for page 24
24 ASSESSING CHANGING FOOD CONSUMPTION PATTERNS
of the population group over an extended period of time. Statistically, this
measure may be described as an estimate of the inter individual (between
individual) variation. The relationship between observed variance and inter-
and intraindividual variances is described by the following equation:
o2 = ob2 + °w21n,
where o2 = observed variance, 0b2 = intraindividual (between individual)
variance, oW2 = intraindividual (within individual) variance, and n =
number of repeated observations for each individual. The last term (ow21n)
may be described as day-by-day variation in the intake of individuals. The
magnitude of this component of variance may be estimated if the study design
includes repeated measurements of 1-day intake for all, or an approximate
representative sample of the individuals. Preferably these replications of
1-day data should be independent of one another (e.g., 1-day observations
separated in time), but, with appropriate statistical treatment, related obser-
vations (e.g., the individual days of a 3-day record) may be used in estimating
the intraindividual variance. The design should include appropriate sampling
to measure the influence of seasons, holidays, and weekends on the patterns
of food intake.
With knowledge of total or observed variance and of intraindividual var-
iance, an estimate of interindividual variance (the measure of variation in
usual intake) can be derived from the above equation. Repeated observa-
tions on the same individuals is the preferred method for measuring usual
pattern of intake and its variation.
The reliability of the estimate of oW2 is a function of the total number of
repeated 1-day observations. From the statistical point of view, collecting
data from 100 people twice or 50 people three times (each pattern has 100
repeated days) leads to similar confidence limits in the estimate of intrain-
dividual variance as long as the subjects observed are representative of the
population. The decision on the desirable number of replications per subject
should involve consideration of the logistics of repetitive examinations
separated in time (i.e., as independent observations). The decision on the
total number of additional measurements (total replications) must be based
upon a consideration of needed confidence.
Since interindividual variance is calculated from intraindividual variation
and observed total variance, it follows that if the total sample is large and
hence the total variance has been reliably measured, the confidence of the
estimate of interindividual variation is inversely related to the ratio of
intra/interindividual variation. That is, as the magnitude of intraindividual
variation increases in relationship to interindividual variation, the reliability
of the estimate of intraindividual variation, and hence the number of repli-
OCR for page 25
Some Study Design Characteristics
25
cate 1-day intake estimates required for the population must increase if a
reliable estimate of interindividual variation is to be obtained. The statistical
problem of confidence limits is considered in survey design.
Beaton and co-workers (1979) have provided estimates of the partitioning
of variance for several nutrient sectors. It is apparent that this varies with the
nutrient and with the food item under study. Thus, design requirements are a
function of questions to be asked of the data.
It is extremely important that all sources of consumed food, including
food supplements, alcoholic and nonalcoholic beverages, water, and medi-
cations, be incorporated into the data. The several methods for collecting
individual food consumption data should be evaluated. In practice, no one
method is likely to be perfect; therefore, it is necessary to select and adapt
the best method to meet the needs of the particular study to be undertaken.
Combinations of 24-hour recall with two or more 3-day diet records, food
frequency questionnaires, telephone interviews, and/or extended diary rec-
ords could be considered.
It should be remembered, however, that for the proposed monitoring
system, repeated observations that permit measurement of the usual food
intake and the extremes or variation by and between individuals are essential
components. Internal quality control checks must be an integral part of this
or any system. Validity checks are extremely important and a concerted
effort should be made to develop and implement procedures to determine the
validity of various methods of assessing dietary intake, including combina-
tions of methods. Research is required to develop satisfactory systems for
determining the validity of measurements obtained.
ASSESSMENT OF NUTRITIONAL STATUS
The conventional method of determining nutritional status of individuals
and population groups relies primarily on biochemical assessment. The
proposed food consumption monitoring system can provide only a probabil-
ity estimate of nutritional status of a population group (by comparison of the
usual nutrient intake with appropriate nutrient requirement figures). It is not
possible from consumption data alone to identify the specific individuals in
the population who are in a particular nutritional state.
Both prevalence and severity of nutritional inadequacy and excess are of
concern in population assessment. In biochemical assessment, different
levels of nutrients (in cells, tissues, and fluids) are associated with different
probabilities of impaired function. Frequently, cutoff points are selected for
categorization of deficiency or excess. As the cutoff point for adequacy is
raised or lowered, the prevalence of deficiency appears to increase or de-
OCR for page 26
26 ASSESSING CHANGING FOOD CONSUMPTION PATTERNS
crease. However, it is the interpretation that has changed, not the prevalence
or severity of functional impairment.
It is possible to establish a series of cutoff points related to the deficiency
(or excess) of a nutrient, and it is possible to develop a series of prediction
curves expressing the expected prevalence of varying degrees of deficiency
(or excess) in the population. This can be done with biochemical data and
with combination dietary and nutrient requirement data. Using the proposed
system of estimating usual nutrient intake from usual food intake and relat-
ing this to usual nutrient requirement, information obtained from dietary
data ought to be similar in accuracy to information obtained from biochemi-
cal data. When the estimates of nutritional adequacy indicate the probability
that a particular population segment has a significant prevalence of inadequ-
acy (or excess) for one or more nutrients, it may be desirable to study that
population group in greater detail. Such studies must include biochemical
assessment if the intent is to identify specific individuals who are in poor
nutritional state.
RELATIONSHIP OF FOOD CONSUMPTION PATTERNS TO HEALTH
STATUS INDICATORS
The possible relationship between food consumption patterns and health
status provides the rationale for FDA interest in monitoring food consump-
tion. Yet the relationships are complex and not easily documented. This
section will attempt to provide some perspective on the types of indices and
methods used in the proposed system, which will give a picture of food-
related differences in health status in the population.
Three types of relationships between flood consumption and health status
are of concern. Firstly, health status is affected by food consumption
through the intervening variable, nutritional state. For these types of health
outcomes, dietary data have predictive value in population terms even
though they are not direct measures of nutritional status. Selection of
specific measures of nutritional status depends on the level of sensitivity
required, e.g., degree of iron saturation.
A second type of relationship is one in which consumption of specific
foods, substances, or combinations of these is linked to health status but not
through the intervening variable, nutritional status. Most toxicological
problems that are due to chance contamination of a specific production lot of
a food are of this type. For this kind of linkage to be monitored effectively,
it is essential that the food consumption data base be adaptable to very fine
disaggregation by specific commodity product type, brand, or other factors.
For some of these relationships, special ad hoc studies will have to be
undertaken as the information required is likely to be highly specific to
certain foods or population segments.
OCR for page 27
Some Study Design Characteristics
27
Food consumption patterns and/or nutritional status can also be con-
tributing or facilitating factors but not the sole etiologic factor in the
pathogenesis of diseases such as coronary heart disease, hypertension, or
other chronic degenerative diseases. This third kind of relationship is
characteristic of several of the leading causes of morbidity and mortality in
the United States. Generally, the contributory role of diet is not well quan-
tified. However, since diet may be the only factor that can be easily manip-
ulated, it is important to know if differences in food consumption are as-
sociated with differing incidences of these diseases. In making this determi-
nation, it is important to control the other variables (such as heredity, occu-
pation, etc.) known to affect the disease.
Irrespective of which of the three types of relationship is postulated, the
disease or condition of concern must meet certain basic criteria (Institute of
Medicine, 1973), which include the following:
1. have significant functional impact on those affected;
2. be relatively well defined and easy to diagnose in clinical and
nonclinical settings;
3. have a prevalence rate that is high enough to permit the collection of
adequate data from a limited population sample;
4. have a natural history that varies with utilization or consumption of
food and/or nutrients; and
5. have potential for documentation of influences of nondietary vari
ablest
Identification of a clear relationship between food consumption and health
status indicators will suggest means of dietary intervention for prevention,
and often treatment.
A systematic approach for relating a health status problem to food con-
sumption data is essential. A logical progression of steps for such linkage is
given below, recognizing that modification will be appropriate for specific
conditions. There are six steps, which may be concurrent:
1. conducting retrospective studies to establish increased relative risk of
disease (or decrement in a health status indicator) associated with differ-
ences in food consumption;
2. monitoring populations which have a particularly high or low preva-
lence or incidence of the disease indicator;
3. verifying the continued association in identified populations;
4. identifying populations with food consumption patterns indicating
they may be at risk and initiating prospective studies in these populations in
order to confirm the relationship and determine proportion of risk attribut-
able to food consumption;
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28 ASSESSING CHANGING FOOD CONSUMPTION PATTERNS
5. monitoring food consumption patterns of populations to detect those
populations at risk; and
6. developing and implementing intervention strategies.
For some conditions, in particular acute and chronic toxicities, steps 2 to
5 may not be needed before intervention is undertaken. In cases where the
association between health status and food consumption is firmly estab-
lished, steps 1 to 4 may not be necessary. However, the general model
provides a framework for an orderly approach to development of cost-
effective intervention strategies.
Retrospective studies to establish an association of health status with food
consumption are dependent on identifying probable associations through
laboratory, clinical, or epidemiological studies. Appropriate study design
must consider variables other than food consumption that may be related to
the etiology or expression of the health state. Dietary methodologies de-
scribed earlier will be used to determine current food consumption and to
test its relationship to acute toxicities, perinatal events, infant morbidity or
mortality, and the like. Different methodology is required to determine
long-term food consumption patterns and their effect upon chronic tox-
icities, chronic disease, growth, etc. A projected need to use data generated
by the proposed consumption monitoring system for this determination
would have significant design and sampling implications.
An important aspect of the proposed system for relating food consumption
to health status involves the use of existing health data on identified popula-
tion segments to determine unusual health patterns. The usual pattern of
food intake of these populations can then be determined in a special study or
by oversampling the identified segment in the ongoing survey.
Health and food intake data are preferably obtained from the same indi-
viduals. If this is not possible, data should be collected on individuals of the
same sample cell characteristics to provide as close a relationship as possible
between usual food intake information and health status. When indications
of a relationship between patterns of usual food intake and health status in a
population group are observed, confirmation may be necessary through the
use of special studies to obtain significant measures of nutritional and health
status. Biochemical and anthropometric data as well as extensive dietary and
medical histories of the individuals in the population under study may be
required.
Representative terms from entire chapter:
health status
