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. 2008 Jan;4(1):14-27.
doi: 10.1111/j.1740-8709.2007.00104.x.

Methodological challenges when monitoring the diet of pregnant women in a large study: experiences from the Norwegian Mother and Child Cohort Study (MoBa)

Helle Margrete Meltzer  1 , Anne Lise Brantsaeter, Trond A Ydersbond, Jan Alexander, Margaretha Haugen
Affiliations

Methodological challenges when monitoring the diet of pregnant women in a large study: experiences from the Norwegian Mother and Child Cohort Study (MoBa)

Helle Margrete Meltzer et al. Matern Child Nutr. 2008 Jan.

Abstract

The aim of this article is to describe the main methodological challenges in the monitoring of dietary intake in the Norwegian Mother and Child Cohort Study (MoBa), a pregnancy cohort aiming to include 100 000 participants. The overall challenge was to record dietary patterns in sufficient detail to support future testing of a broad range of hypotheses, while at the same time limiting the burden on the participants. The main questions to be answered were: which dietary method to choose, when in pregnancy to ask, which time period should the questions cover, which diet questions to include, how to perform a validation study, and how to handle uncertainties in the reporting. Our decisions were as follows: using a semi-quantitative food frequency questionnaire (FFQ) (in use from 1 March 2002), letting the participants answer in mid-pregnancy, and asking the mother what she has eaten since she became pregnant. The questions make it possible to estimate intake of food supplements, antioxidants and environmental contaminants in the future. Misreporting is handled by consistency checks. Reports with a calculated daily energy intake of <4.5 and >20 MJ day(-1) are excluded, about 1% in each end of the scale. A validation study confirmed that the included intakes are realistic. The outcome of our methodological choices indicates that our FFQ strikes a reasonable balance between conflicting methodological and scientific interests, and that our approach therefore may be of use to others planning to monitor diet in pregnancy cohorts.

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Figures

Figure 1
Figure 1
(a–d) Mercury intakes (μg day−1) calculated from the old and new questionnaires, with gamma distributions fitted by maximum likelihood estimation (n = 86). The fitted distributions are used for computing confidence intervals. (a) Daily Hg intake calculated from the new questionnaire, with gamma distribution fitted. (b) Plot of observed values vs. expected values from the gamma fitted in (a). (c) Daily Hg intake calculated from the old questionnaire, with gamma distribution fitted. (d) Plot of observed values vs. expected values from the gamma fitted in (c).
Figure 2
Figure 2
(a) The distribution of energy intakes (kJ/day) calculated from the food frequency questionnaire (FFQ), with gamma distribution fitted by maximum likelihood estimation. For estimation of gamma parameters, numbers were translated by 3000 kJ. This value gives the best fit for the data, and may be considered as a lower limit for the intakes in the ‘validation population’. The intakes in this group had a considerably lower dispersion than the whole MoBa population, the 99.8% confidence interval (CI) in this group roughly corresponding to the 98% CI of the whole population. (b) Plot of observed values vs. expected values from the gamma fitted in Fig. 1a.
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