Pregnancy
Pregnancy Background
The most intensive period of human growth and development occurs during
the nine months of pregnancy (11). Assessing the food and nutrient intake
of pregnant women is complicated because conception triggers an array
of complex and sequential physiological changes that affect maternal nutrient
absorption and metabolism, energy and nutrient needs, appetite, and meal
patterns (12). Individual physiologic and behavioral responses to the
stress of reproduction vary widely (13), and both the type and amount
of food consumed may fluctuate depending on the period of pregnancy. Nausea,
which is estimated to occur in 50-80 percent of pregnancies (11), may
begin as early as 4 to 6 weeks gestation, usually peaks around 8 to 12
weeks, and then declines thereafter. Heartburn and constipation are other
common ailments that may trigger changes in usual food habits. Pregnant
women may develop food preferences and aversions due to changes in the
sense of taste and smell. Pica, an eating disorder characterized by the
compulsion to eat substances that are not food, affects more than half
of pregnant women in some locations in the southern part of the United
States (11). Maternal concerns about optimal fetal growth and development,
weight gain, and parenting may increase maternal awareness of nutrition
and focus personal and health care provider attention on food habits and
supplement consumption. Misrepresentation of intake, particularly with
regard to alcohol and supplements, may be an issue because of respondents'
fear of noncompliance.
Validation Studies in Pregnant Populations
Validation of dietary assessment methods in pregnant women is limited.
Table
2.1 presents a summary of 15 validation studies; additional information
on each study is included at the end of this chapter in Table
2.2. Two studies examining energy expenditure using the DLW (Doubly
Labeled Water) method to measure TEE (Total
Energy Expenditure) also evaluated reported energy intake. In 12 Cambridge
women, reported energy intake from 7-day weighed food records underestimated
TEE measured by the DLW method by 6 to 15 percent in the first trimester,
12 to 18 percent in the second trimester, and 24 percent in the third trimester
(14).
Although large differences between individuals in the metabolic cost of
pregnancy was found, application of the Goldberg cut-off limits based on
Energy Intake:Basal Metabolic Rate (EI:BMR) ratios identified under-reporters.
Energy intake measured by 4-day weighed food records kept before conception
by 22 Swedish women, underestimated TEE at 6 to 18 weeks gestation by 27
percent (15).
In a review of six different data sets containing 228 DLW TEE measurements
in pregnant and lactating women or non-pregnant, non-lactating controls
in Europe and Gambia (including the DLW studies in Table 1), Prentice and
colleagues (13)
recommend caution in the overinterpretation of dietary intake records.
Their data show food intake records can be "extremely misleading and
this is only revealed when there is some external validation such as the
doubly labeled water method" (13).
Three other validation studies in Table
2.1 and Table
2.4 included a biological marker to assess reported intake. Erythrocyte
cell membrane fatty acid content was compared with fat intake reported
on an FFQ (Food
Frequency Questionnaire) in a study of 185 New England women in the
first trimester (21)
and on 35 third-trimester Mexico City women (18).
Both studies found significant correlations between erythrocyte alpha
linoleic acid content and dietary intake of the fatty acid, and that women were
classified into the same intake ranges by both the FFQ and this biological
marker. Serum levels of caffeine and paraxanthine were examined as a biological
marker to validate caffeine intake reported on a 24HR (24-Hour
Recall), with correlations between the two measures comparable to
correlations between reported smoking and serum cotinine in pregnancy
(17).
No other studies validating energy or nutrient intake assessed by the 24HR
(24-Hour
Recall) method in pregnancy populations were found. The 10 FFQ validation
studies in Table
2.1 (and Table 2.4)
are difficult to compare because the populations differ, the FFQ instruments
differ, the studies cover various periods of pregnancy, and they differ
in their reference methods and in the number of days of dietary recording.
There are also statistical differences between the studies. Three FFQ studies
on European pregnant women compared FFQ intake with weighed or estimated
FRs (Food
Records) (19;20;23).
Each found the FFQ overestimated energy intake, but generally classified
the women into the same or adjacent nutrient intake category as the FR.
In the US, six studies have examined the validity of the Harvard
FFQ (HFFQ) modified in various ways for pregnancy in each study and
assessing intake usually for the past 1 to 3 months. Comparisons with multiple
administrations of 24HR interviews in 4 studies found the Harvard FFQ overestimated
energy intake (25-28).
However, compared with 4-day weighed FRs, the modified Harvard FFQ underestimated
energy intake by 10 percent during mid-pregnancy in a population of educated,
white Minnesota women (22).
A large study of low-income pregnant women comparing the HFFQ with three
24HR interviews concluded the HFFQ provides a reasonably accurate measure
for the majority of nutrients and can appropriately rank individuals relative
to one another even if absolute intakes may not be precise (27;28).
However, the exclusion of 14 to 18% of the women reporting caloric intakes
above 4,500 on the HFFQ suggests a significant proportion of the low-income
women were unable to complete the HFFQ adequately (28).
The NCI-Block HHHQ (NCI
Health Habits and History Questionnaire) was found more valid for white
women and black pregnant WIC participants than the HFFQ, based on correlations
between each FFQ and three 24HR interviews for energy and five nutrients.
Neither FFQ was valid in Hispanic women (25).
Although most of the FFQ instruments in the validation studies in Table
2.1 collected information on supplement intake, validation of reported
supplement intake was not discussed.
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