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Methodologic studies during pregnancy: the reliability of the 24-hour dietary recall.

In this paper, we address the problem of the reliability of quantitative historical estimation of nutrient intake during pregnancy. The normal physiological changes of pregnancy include changes of both nutritional requirements (1) and of the biochemical indices that serve as standards of nutritional adequacy (2). For instance, it is probable that decreasing Hb concentration more usually reflects plasma volume expansion, rather than depleted red cell mass. It is therefore not too surprising that low Hb concentration, at least above 8 g/lOO ml, has been associated with higher birthweight (3). Fat stores increase, but fat is not laid down uniformly throughout the body (4). These normal physiological changes [comprehensively reviewed by Hytten and Leitch (5)] influence strategy for nutritional assessment. It is very difficult to quantify nutrient intake among free living individuals, a necessity in studies requiring large numbers of subjects. The rapid changes of pregnancy compound such problems. The most accurate dietary techniques are not applicable to epidemiological research for several reasons. Women who will endure the intensive observational study of a metabolic ward or similar closed environment are not likely to be representative of the population from which they are drawn. Also, activity and behavior under closed conditions are probably different than when not constrained (6), and the issues of greatest import during pregnancy (for instance, the relationship of maternal nutrition to fetal growth and survival) demand much larger numbers of subjects than can possibly be studied by the techniques of the metabolic ward (7, 8). Food diaries can be kept by large numbers of free-living individuals, but only atypical, highly motivated women will keep them accurately: the participant not only must record, but must also weigh or estimate all portion sizes. In addition, the procedure probably interferes with normal food selection (9). The diet history, developed by Burke (10), and the food frequency checklist (1 1), aim to characterize typical, long-term diet intake, but suffer from inaccuracy of memory. The 24-h dietary recall has therefore become the most frequently used technique to characterize diet. Such recalls are of greater accuracy than techniques dependent on longer memory and, like other historical methods, can be integrated into standard prenatal care, minimizing the impact on participant convenience and behavior. However, they may not represent typical diet, since one day’s diet may be an invalid or unreliable estimate of long-term eating patterns. We have therefore studied the reliability of repeated 24-h diet recalls, to judge their utility as a measure of dietary intake during pregnancy. The validity of a measure is the extent to which the measure accurately reflects some underlying truth. In practice, another, presumably more accurate, measure serves as a criterion ofvalidity, since the “truth” remains unknown. Reliability has been defined as the ability of a procedure to give “the same results when used repeatedly in the same situation” (12). Data cannot be valid unless they are reliable; the obverse is not necessarily true. While our analysis addresses reliability, a brief review of studies of validity may help