Keywords

FormalPara Key Points

  • Diet quality is a subjective term that is often used within the context of a deficiency or excess of nutrients or foods.

  • Attempts have been made to translate such subjectivity into objective measures, namely via scoring systems. Such scoring systems comprise nutrients and foods or food groups that are assumed to be either healthy or detrimental.

  • This chapter briefly describes a few scoring systems, namely the Healthy Eating Index (HEI), Diet Quality Index (DQI), Diet Quality Index-International (DQI-I), Healthy Diet Indicator (HDI), and the Mediterranean Diet Score (MDS).

  • These selective examples illustrate the nature of the scoring systems, their attributes, and applications.

  • Some of these scoring systems correlate with biomarkers, mortality, cognitive impairment, and other variables.

  • On the other hand some studies are negative, suggesting that the scoring systems may not be applicable to all situations.

Introduction

The human diet is composed of many different nutrients, which contribute to maintaining normal bodily functions. Components of the diet may be used to provide metabolic fuels (e.g., the breakdown of macronutrients to form adenosine triphosphate (ATP) which may be used to maintain the sodium potassium balance across cells), building blocks (e.g., amino acids for cytoskeletal proteins), or components of various cellular constituents (e.g., selenium as part of glutathione peroxidase).

In nutritional epidemiology, the focus has mainly been directed towards the role of such single dietary components [1]. This “reductionist” approach can reveal the relationship of individual nutrients or foods in disease development [1, 2]. However, this approach disregards the true complexity of the human diet, and therefore the true relationship between diet and disease [3]. This is because the food matrix is a composite mixture of individual components, many of which do not appear in the established lists of dietary reference values or recommended daily intakes. Additionally, within the body, there are many nutrient-to-nutrient interactions that further complicate the associations between single dietary components and disease [4]. For this matter, a “holistic” approach is usually undertaken to evaluate diet quality based on patterns of intake [3].

Dietary patterning can be defined as theoretically defined dietary patterns or empirically derived dietary patterns [1]. The latter is examined in “a posteriori” approach, where statistical methods such as factor and cluster analyses are used to generate patterns from collected food consumption data [1, 3, 5]. Theoretically defined dietary patterns (dietary indices/scores), on the other hand, are created “a priori.” They are made up of nutritional variables (foods and/or nutrients) and based on current nutrition knowledge [1, 3, 5]. These variables are quantified and summed to provide an overall measure of diet quality [1, 5]. The ensuing material will focus on predefined indices of overall diet quality. Unfortunately, the term diet quality has been defined and used in different ways. However, there have been various attempts to make the concept of diet quality more objective, quantitative, and measurable. Table 1.1 provides a basic breakdown of the attributes and key issues in the construction of predefined indices of diet quality [3].

Table 1.1 Characteristics of predefined indices of overall diet quality
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A variety of diet indices or tools have been developed to assess overall diet quality [6]. Typically, these indices are constructed on the basis of dietary recommendations, such as “servings” of food items in the US Department of Agriculture Food Guide Pyramid [4, 7].

Existing Indices of Diet Quality

There are several systems of scoring that have been validated by relating the index to health outcome. Table 1.2 provides an overview of existing diet quality indices and studies in which they have been used and/or evaluated [3]. Some of the most common of these indices are the Healthy Eating Index (HEI) [8], Diet Quality Index (DQI) [9], Healthy Diet Indicator (HDI) [10], and the Mediterranean Diet Score (MDS) [11]. The Diet Quality Index-International (DQI-I) [12], which is a derivative of the DQI, is a fairly recent predefined measure created for global monitoring and exploring diet quality across countries. Prior to the development of this index, cross-national comparison of diet quality had rarely been attempted [12].

Table 1.2 Overview of existing diet quality measures and studies in which they have been used and/or evaluated
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There are some indices, such as the Food-Based Quality Index (FBQI), that consist solely of foods or food groups. Other indices like the adapted DQIs consist of just nutrients. The majority of indices, however, comprise both food groups and nutrients [3]. Table 1.3 includes an overview of the attributes found in the scoring systems mentioned below [8–47].

Table 1.3 Overview of attributes included in theoretically defined indices of diet quality
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Healthy Eating Index

The HEI is a 10-component, 100-point measure of diet quality that assesses conformance to US dietary guidelines [48]. It is based on five different food groups (grains, vegetables, fruits, milk, and meat), four nutrients (total fat, saturated fatty acids (SFAs), cholesterol, and sodium), and a measure of the variety in food intake [8]. A HEI score of 80 or more indicates a good diet; scores between 50 and 80 suggest that a diet needs improvements, and scores less than 50 consider a diet to be poor [49]. Table 1.4 shows an overview of the HEI.

Table 1.4 Health Eating Index (HEI)
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Hann et al. [24] and Weinstein et al. [26] found that the HEI was associated with a wide range of nutritional biomarkers of micronutrients, i.e., alpha-carotene, beta-carotene, beta-cryptoxanthin, and vitamin C. They explain that consumption of these nutrients is a common indicator of fruits and vegetables, and therefore consumption of these food groups [24]. Both studies found no significant correlation between HEI score and cholesterol [24, 26]. A study by Dubois et al. [14] analyzed three different methods to measuring overall diet quality. They found that the HEI had a higher correlation with the Mean Adequacy Ratio (MAR = 0.287) of several nutrients, compared to the DQI and HDI.

Data on the relationship between the HEI score and mortality is lacking. However, there are four studies that have examined the relationship between HEI and disease risk [21, 22, 25, 30]. Harnack et al. [30] found no significant association between the HEI score with cancer incidence [30]. In the McCullough et al. [21, 22] studies, a weak inverse association between HEI score and chronic disease risk (cardiovascular disease (CVD) and cancer) was reported [21]. They did not report such an association with overall chronic disease risk in women, and only a weak inverse association with CVD risk [22].

The HEI is based on US dietary guidelines and, to a certain extent, measures how individuals follow these guidelines. However, further work is needed to firmly establish the HEI as a good predictor of health outcome. Nevertheless, the HEI shows correlations with plasma biomarkers such as alpha-carotene, beta-carotene, beta-cryptoxanthin, and vitamin C [24].

Healthy Diet Indicator

The HDI (Table 1.5) was developed in the Netherlands [3] and is based on the World Health Organisation’s dietary recommendations for the prevention of chronic disease [50]. The HDI is made up of nine micro- and macronutrient components [3, 10]. It uses a 9-point measure of four nutrients (SFAs, polyunsaturated fatty acids (PUFAs), mono- and disaccharides, and cholesterol) and five food groups (complex carbohydrates, dietary fiber, fruits and vegetables, and pulses, nuts, and seeds). Typically, the higher the overall HDI score, the better the diet quality.

Table 1.5 Healthy Diet Indicator (HDI)
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Huijbregts et al. [10, 27] reported the HDI to be inversely associated with all-cause mortality in men [10] but not women [27]. Huijbregts et al. [28] also suggested a healthy HDI score to correlate with better cognitive function in elderly men. Dubois et al. [14] reported that the HDI score only slightly correlated with MAR (0.079). Moreover, Haveman-Nies et al. [29] found no association between HDI score and albumin, hemoglobin (Hb), or waist circumference.

Diet Quality Index and Diet Quality Index-International and Health Outcome

The DQI is made up of eight components (Table 1.6) and is based on the US recommendations from Diet and Health [51]. The DQI uses a 16-point measure, where a high score is indicative of a poor diet quality (unlike the previously mentioned dietary indices).

Table 1.6 Diet Quality Index (DQIa)
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Dubois et al. [14] showed the DQI to only marginally correlate with nutrient adequacy. One study by Seymour et al. [13] found that a high DQI score was positively associated with all-cause mortality. Moreover, persons with a high DQI score had lower CVD-mortality, but no association was found between cancer mortality and the DQI score [13].

The DQI-I is an adapted version of the DQI [12]. It is divided into four major components, including, variety, adequacy, moderation, and empty calorie foods (Table 1.7). The total DQI-I score ranges from 0 to 100, where a score of “0” reflects an extremely poor diet, and a score of “100” (highest possible score) indicates a high quality diet [12]. The differences between the original DQI and the DQI-I indices are significant. The DQI-I not only is more extensive than the DQI but also has attributed different weights to the individual components [3]. Thus, the DQI-I incorporates both nutrient and food perspectives of the assessed diet, providing a more grounded tool to describe the diversity of consumption observed from country to country [12]. Kim et al. [12] have suggested that the DQI-I can indentify dietary problem areas. Like the DQI, a high score on the DQI-I, suggests a good diet quality.

Table 1.7 Diet Quality Index-International (DQI-I)
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Mediterranean Diet Score

The MDS is an eight-component, 8-point measure of diet quality (Table 1.8). It is based mainly on food groups, and is supplemented with a ratio of the fatty acid composition of the diet (monounsaturated fatty acids (MUFAs) and SFAs) [1]. A high MDS score indicates a good diet quality.

Table 1.8 Mediterranean Diet Score (MDS)
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A study by Lasheras et al. [33] evaluated the relationship between MDS and mortality among Spanish elderly. They found that the MDS was only significantly associated with a reduced risk of death in persons under 80 years [33]. Another study evaluating French adults was also reported to exhibit lower mortality following a Mediterranean diet in intervention studies [52, 53]. Furthermore, Osler and Schroll [31] found an association between plasma carotene and MDS. However, no association was reported between plasma cholesterol, high density lipoprotein (HDL), or vitamin E with the score [31].

Dietary patterns are influenced greatly by cultural differences. Thus, it is important to take into consideration these differences when choosing diet quality indices to measure diet quality of a certain population. Though the MDS may seem pertinent in predicting mortality, especially in European Mediterranean populations, it may be better to adapt or develop a score that tailors to local diets in Western populations, like the UK [3].

Subjective and Non-scoring Systems

It is important to emphasize that many published studies employ the term diet quality or other aspects of quality without using indices or numerical scoring systems. Such studies may be subjective but based on a firm scientific foundation. For example, diets low in iodine may be deemed as poor or low quality as the consequences of iodine deficiency can be devastating. Iodine deficiency, for example, is well known to be associated with increased rates of stillbirths, spontaneous abortions, cretinism, hypothyroidism, impaired cognitive function, etc. This does not mean to say that the terms poor or low quality have been misused but rather they have been used within a different context. However, it is increasingly likely that the use of diet quality scoring systems will gain wider usage, and new ones developed as the scientific dialogue between diet and disease progresses.

Conclusion

Diet quality is a subjective term that is often used within the context of a deficiency or excess of nutrients or foods. Attempts have been made to translate such subjectivity into objective measures, namely via scoring systems. Some of these scoring systems correlate with biomarkers, mortality, cognitive impairment, and other variables. On the other hand some studies are negative suggesting that the scoring systems may not be applicable to all situations or populations.