Diet diversity is highlighted in dietary guidelines, yet diverse diets can vary substantially in quality. Understanding its role within overall diet quality is essential. This systematic review investigated diet quality scores incorporating a distinct diet diversity component (DQDS) and their associations with type 2 diabetes (T2D), cancer, cardiovascular diseases (CVD), obesity and all-cause mortality among healthy adults in high-income Western countries. A systematic search of the NCBI Database up to October 2025 identified prospective studies quantifying dietary intake and assessing a priori defined DQDS in relation to the outcomes of interest. Study quality was evaluated using the Scottish Intercollegiate Guidelines Network checklist. Due to substantial heterogeneity, findings were synthesised narratively. Sixteen reports were included, applying eight DQDS, including the original Healthy Eating Index, Diet Quality Index variants, national Dietary Guidelines Indices and the Healthy Food Diversity-Index. Considerable variability existed in the diet quality dimensions measured (adequacy, balance, moderation, diversity) and in how diet diversity was operationalised (simple food-group counts or evenness-based metrics). Associations between DQDS and all-cause mortality (n = 5), cancer (n = 4), CVD (n = 3), obesity (n = 4) and T2D (n = 3) were inconsistent, with studies reporting null or inverse relationships. Current evidence remains insufficient to determine whether diet quality scores that include diet diversity are linked to chronic disease outcomes. Heterogeneity in the conceptualisation of both diet quality and diet diversity limits comparability across studies, highlighting the need for conceptually aligned, multidimensional DQDS in future research to clarify these relationships.
Nickel et al. (Tue,) studied this question.
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