DESCRIPCIÓN DE LAS ACTIVIDADES

Nutritional epidemiology (see Appendix 1 – Glossary of terms) has been described as one of the most important disciplines in the study of human disease aetiology (Trichopoulos and Lagiou, 2001). However, the study of dietary intake is very complex involving “hundreds of foods and nutrients […] additives and contaminants” (Trichopoulos and Lagiou, 2001, p. 133).

Due to this complexity, dietary intake is methodologically difficult to capture and no gold-standard method for assessing total dietary intake exists to date (Trautwein and Hermann, 2005).

So how can a complex behaviour such as diet be represented and researched in a meaningful way? One solution is the use of global scores describing dietary pattern or dietary quality in order to combine intake data of different foods and nutrients (Willett, 1998).

1.5.1. Dietary patterns

In 1982, Schwerin and colleagues were the first to use dietary patterns to examine the influence of more than “single nutrients” (Slattery, 2008, p. 14) on health (Slattery, 2008). This was a result of a growing understanding that single or few nutrients are limited in explaining the influence of diet on health. Foods tend not to be eaten in isolation and the investigation of single nutrients would thus be limited or even misleading, if other present components or nutrients were not considered (Slattery, 2008; Hu et al., 1999).

Furthermore, the single-nutrient approach fails to consider interactions of different nutrients (e.g. iron absorption improved through vitamin C) and nutrients such as potassium and magnesium which are so highly correlated that it becomes difficult to examine their effect separately. A dietary pattern could actually become a confounder when nutrient intakes are associated with certain patterns, as is often the case (Hu et al., 1999).

Finally, as patterns are built on the basis of habitual food consumption, they might represent “real-world conditions” (Hu et al., 1999, p. 234) more accurately (Hu et al., 1999). Guidelines expressed as dietary patterns may be more easy to understand for people of low nutrition literacy and as such

- 21 - offer an effective health promotion strategy (Bowley and Blundell, 2016).

While the aim of dietary patterns, representing complex dietary behaviour in different intake categories or scores, is clear, there are surprisingly few attempts at defining what exactly a dietary pattern is. A lack of universally accepted definition has been noted as far back as 1989 (Oltersdorf et al., 1989).

Patterns have been described as:

“a specific constellation of variables among or within individuals that includes many single aspects in one index” (Oltersdorf et al., 1989, p. 241).

The more recent definition published by the authors of a US Department of Agriculture systematic review into the relationship between dietary patterns and health outcomes appears more in-depth and focuses on consumption frequency and habits (Essery Stoody et al., 2014). They defined dietary patterns as:

“the quantities, proportions, variety, or combination of different foods, drinks, and nutrients (when available) in diets, and the frequency with which they are habitually consumed” (Essery Stoody et al., 2014, p. 1).

1.5.2. Diet quality

Similarly, the concept of diet quality, sometimes also referred to as dietary quality, is another relatively new way of describing diet ‘as a whole’ rather than measuring single nutrients.

The concept of diet quality involves:

“the assessment of both quality and variety of the entire diet, enabling examination of associations between whole foods and health status, rather than just nutrients” (Wirt and Collins, 2009, p. 2473).

The concept also involves the use of a single score:

“These indices are more complex explanations of diet quality than are assessments that use only total energy intake or specific nutrient intake”

(Watts et al., 2007, p. 128).

- 22 - This means that diet quality can be assessed using composite indices that combine complex behaviour (diet and eating habits) into a single score (Thiele et al., 2004).

Different ways of assessing diet quality exist: scoring adherence to (national) dietary guidelines, rating the diversity of food choice in key food groups (see Appendix 1 – Glossary of terms), or scoring pre-defined food patterns known to protect or impair health (Wirt and Collins, 2009). Diet quality indices often use different foods, or group foods in different categories, and construct their scores differently making comparison across instruments difficult (Thiele et al., 2004).

1.5.3. Similarities of dietary patterns and diet quality The concepts of dietary patterns and diet quality have both been defined as data reduction methods with the goal of summarising diet in a broader way (Slattery, 2008). Principle component factor analysis is the most commonly used method to study dietary patterns (Northstone et al., 2007), but other multivariate methods have also been used to create scores. Dietary quality is most commonly measured with pre-defined scales or indices (Thiele et al., 2004).

Whichever method is used, the goal is to reduce the wealth of dietary intake data (usually gained from food frequency questionnaires, dietary recalls or weighed food records) into manageable and interpretable groups of foods (see Appendix 1 – Glossary of terms) that tend to be eaten in combination and to relate these to health outcomes.

1.5.4. Ambiguity of terms

The terms dietary pattern and diet quality are sometimes used interchangeably. For example, Poon et al. (2013) refer in their article and title thereof to ‘dietary pattern’ when they used a dietary quality index to measure it (Poon et al., 2013).

Moreover no ‘gold standard’ definition of either term currently exists despite an increasing amount of studies using them. While authors are clear on

- 23 - their wish to combine a complex behaviour into a composite measure (i.e.

‘why’ an assessment of dietary patterns or dietary quality is necessary), the delineation of what this involves exactly for example in terms of food groups (i.e. ‘how’ dietary patterns or quality should be assessed) is rather vague.

Perhaps it is this lack of clear definition that has lead authors to use the terms dietary pattern or dietary quality in ways that are not fulfilling the goal of representing complex behaviour in a simple measure. For example, Carmichael et al. (2003) developed a diet quality score which involved assessment of the intake of five selected micronutrients and percentage of kilocalories from fat and from sweets (Carmichael et al., 2003). Comparing this to the definition by Wirt et al. (2009) it can be argued that the diet quality score does not meet the criteria of assessing ‘quality and variety of the entire diet’ (see Section 1.5.2.).

Likewise, Fitzsimon et al. (2007) aimed to assess dietary patterns during pregnancy and the association with offspring asthma. However, dietary pattern assessment consisted of the average number of servings of three food groups (fruits, vegetables and oily fish) only (Fitzsimon et al., 2007).

This does not appear to meet the criteria of assessing more than ‘single foods or nutrients’ (see Section 1.5.1.) because major food groups such as dairy or grains were omitted.

1.5.5. Early studies of diet ‘as a whole’ in pregnancy The use of either concept for describing diet ‘as a whole’ (as opposed to

‘single’ foods or nutrients) is useful for the study of diet during pregnancy.

As outlined in section 1.5.1., the concept only emerged in the 1980’s, and it was a decade before it was applied to pregnant populations making this a relatively new field of research.

One of the first studies using dietary pattern or “nutrient profiles” (Wolff and Wolff, 1995, p. 122) to investigate diet of pregnant women was conducted by Wolff and Wolff in 1995, who investigated the effect of maternal dietary patterns on birth weight (Wolff and Wolff, 1995). The authors chose the concept of dietary pattern as opposed to the concept of single nutrients because this had proven a valid method in the field of birth weight research.

- 24 - In the past, pregnancy weight and gestational weight gain had been used as easily obtainable markers of maternal nutritional status or in the absence of dietary intake data. However, research repeatedly failed to show significant associations between these surrogates and maternal or child health outcomes. Wolff and Wolff therefore used dietary pattern as another easily obtainable marker of complex dietary behaviour which demonstrated a more pronounced and distinctive relationships with birth weight (Wolff and Wolff, 1995).

One of the first records of using dietary quality to assess pregnancy diet was a study by Gardner and Feldstein, who in 1990 assessed diet using the so called “Index for Nutritional Quality” to rate how closely women’s diets met intake recommendations of seven key nutrients (Suitor et al., 1990). The first to assess diet quality beyond single key nutrients and rating diet in its entirety in pregnancy were Bodnar et al., who in 2002 as part of the PIN cohort study (Pregnancy, Infection and Nutrition) developed the so called diet quality index for use in pregnancy (DQI-P) (Bodnar and Siega-Riz, 2002).

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