Dietary Inflammatory Index, Sleep Duration, and Sleep Quality: A Systematic Review

The inflammatory potential of the diet, as measured by the Dietary Inflammatory Index (DII®), has been repeatedly shown to be associated with various inflammatory markers and mental and physical health outcomes. Of specific importance, several cross-sectional studies revealed mixed results regarding the correlations between the DII and sleep outcomes. Hence, in the current paper, a systematic review that examines the associations between the DII, sleep duration, and sleep quality was performed. The PubMed database was systematically searched for studies published up to November 2023 following PRISMA guidelines. Only cross-sectional studies that assessed the DII, sleep duration, and sleep quality across healthy and unhealthy cohorts were included. Eleven and seven studies were included in the systematic review for sleep quality and duration, respectively. The results of the present systematic review show that pro-inflammatory diets may be associated with poor sleep outcomes (duration and quality); however, as the current literature is inconsistent and limited, further cross-sectional studies in larger cohorts are necessary to (i) explore this relationship to address this heterogeneity and (ii) explore populations that are more sensitive to diet-induced inflammation.


Introduction
Poor diet, a leading risk factor for noncommunicable diseases, is linked to poor sleep outcomes [1].For instance, previous studies have shown the benefits of fats; group B vitamins, magnesium, and l-tryptophan; foods containing tryptophan, melatonin, and phytonutrients(e.g., cherries, kiwifruit, milk) [2][3][4]; and dietary patterns such as the Mediterraneanstyle diet [5] and/or a high-quality diet [6] on sleep duration and sleep quality.
The aforementioned nutrients, food groups, and dietary patterns are also known to be associated with reduced systemic chronic inflammation (SCI) biomarkers, including platelet and leukocyte counts, neutrophil-to-lymphocyte ratios (NLRs), and C-reactive protein (CRP) levels [7][8][9][10].For instance, large-cohort studies in healthy adults from Italy, the UK, and the US, assessing diet using food frequency questionnaires, found that adherence to a Mediterranean dietary pattern or higher healthy dietary scores was associated with lower markers of systematic chronic inflammation [7][8][9].Additionally, a meta-analysis of five cross-sectional studies conducted in older adults (≥64 years)) revealed significant inverse associations between adherence to a Mediterranean dietary pattern and circulating CRP [10].Moreover, evidence from meta-analyses of randomized controlled trials across various age groups have further supported the cross-sectional evidence above by showing reductions in CRP, interleukin-6 (IL-6), and interleukin-1 β (IL-1β) [11,12].
As evidenced above, the findings remain highly inconclusive, and to the best of our knowledge, this area of research has not been systematically reviewed.Despite the heterogeneity of the studies included in this review, our objective was to carry out a systematic review and assess the strength of the scientific evidence supporting the associations between sleep outcomes (namely sleep duration and sleep quality) and the DII.

Study Selection
The inclusion criteria were (1) the DII, a literature-based dietary score that quantifies the inflammatory potential of diet [17], being measured by 24-hour dietary recall, dietary survey data, or a food frequency questionnaire; (2) sleep duration or sleep quality, assessed by both subjective and objective measures; (3) cross-sectional studies; and (4) healthy or unhealthy (sleep and/or metabolic disorders) participants of any age or gender.
The exclusion criteria included (1) randomized controlled and quasi-experimental trials, case reports, letters to editors, conference papers, theses, personal opinions or commentaries, and (2) animal, in vitro, and ex vivo studies.

Search Strategy and Data Sources
An electronic literature search was carried out on PubMed and Scopus in order to identify appropriate studies.The literature search was conducted until the beginning of November 2023.The search strings used in the search included (DII OR "dietary inflammatory index" OR "inflammatory diet" OR "anti-inflammatory diet") AND (sleep*).Manuscripts were selected according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) diagram [31,32].Papers were selected independently by one reviewer (PH) based on the inclusion and exclusion criteria specified above.Information regarding (1) Publication details (authors, year, journal), (2) Participant characteristics (number of participants recruited, number of participants included in the study, gender, age range, and health status), (3) Study design, (4) Measures (DII, sleep duration, and sleep quality), and (5) Notes (factors that might affect results and/or data quality) were extracted from all publications.

Study Characteristics
We identified 45 publications and screened them for eligibility based on the inclusion and exclusion criteria.Thirty-two studies were excluded.Eleven and seven studies that met all the inclusion criteria were included in the current review for sleep quality and sleep duration, respectively (Figure 1).

DII and Sleep Duration
Seven studies assessed the association between sleep duration and dietary inflammation using the DII.Three studies assessed sleep duration by using objective methods (i.e., using actigraphy) [24,26,29].Four studies assessed sleep duration subjectively using either the Pittsburgh Sleep Quality Index (PSQI) [19,20,33] or self-reported hours of sleep per night [21].Summaries of all the studies are presented in Table 1.

DII and Sleep Duration
Seven studies assessed the association between sleep duration and dietary inflammation using the DII.Three studies assessed sleep duration by using objective methods (i.e., using actigraphy) [24,26,29].Four studies assessed sleep duration subjectively using either the Pittsburgh Sleep Quality Index (PSQI) [19,20,33] or self-reported hours of sleep per night [21].Summaries of all the studies are presented in Table 1.
One study assessed the correlation between sleep duration and dietary inflammation in pregnant participants with overweight or obesity and showed no association between sleep duration and dietary inflammatory scores [29].

DII and Sleep Quality
Twelve studies assessed the link between sleep quality (as measured by the Pittsburgh Sleep Quality Index, where higher scores indicate poor sleep quality [34]) and the DII.Summaries of all the studies are presented in Table 1.
Seven of these studies were conducted in healthy adults across the lifespan [19,20,23,24,26,30,33].Studies that have been conducted mainly in young adults found null associations between sleep quality and the DII [19,26,30,33].However, in middle-aged adults, Wirth, Fekedulegn, et al. (2022) [24] showed that more pro-inflammatory diets were correlated with higher improved subjective sleep quality.By using representative samples across age groups (but by mainly recruiting young-to-middle-aged adults), Godos, Ferri, Caraci, Cosentino, Castellano, Shivappa, et al. (2019) [20] showed that individuals in the highest quartile of the DII (i.e., individuals who follow more pro-inflammatory dietary patterns) were less likely to have adequate sleep quality and, similarly, Wang et al. (2022) [23] showed that more pro-inflammatory diets were linked with poor sleep quality (in individuals with poor sleep quality).The remainder of the studies were conducted in young-to-middle-aged unhealthy adults [18,22,25,27].While studies conducted in overweight and/or obese individuals found significant associations between higher scores on the DII and poor sleep quality [18,22] (apart from Tabrizi and Farhangi, 2021 [28]), other studies showed no significant association between the DII and sleep quality in individuals with sleep apnea [27] and fibromyalgia [25].

Risk of Bias
The quality of the studies included in this review was evaluated by one reviewer (PH) using the Agency for Healthcare Research and Quality (AHRQ) checklist [35]; Table 2.The 11-item AHRQ checklist has "yes", "no", or "unclear" classifications.The studies are classified as "high quality" (8-11 items with a "yes" response); moderate quality (4-7 items with a "yes" response); and "low quality" (0-3 items with a "yes" response).The applied quality appraisal revealed eleven studies to be of high quality, one study to be of moderate quality, and two studies to be of low quality.

Discussion
In terms of sleep duration, small cross-sectional studies in healthy adults (n < 500), reported no significant correlation between DII scores and sleep duration [20,24,26,33].Similar findings were reported in pregnant participants with overweight or obesity [29].One study reported an association between higher DII scores and shorter sleep duration amongst university employees in Iran [19]; however, it is important to highlight that the majority of participants (i.e., 85% and above) reported a short sleep duration of less than 6 h, which may not represent the broader population [36].Hence, this limitation raises concern for the generalizability of their findings.A large-scale survey (n > 30,000) conducted by Kase et al. (2021) [21], on the other hand, revealed that higher dietary inflammation scores, indicative of a pro-inflammatory diet, were associated with both shorter (less than 6 h) and longer (more than 9 h) sleep durations.Notably, this study utilized data from the National Health and Nutrition Examination Survey, where sleep duration was assessed through self-reported hours of sleep at night on weekdays or workdays.In contrast, the study by Behbahani et al. (2022) [19] utilized the Pittsburgh Sleep Quality Index (PSQI) to evaluate self-reported sleep duration, although specific details about variable information are not clarified in the study.Given that different sleep assessments (e.g., actigraphy, diary, and retrospective questionnaires) are known to yield different estimates of sleep duration [37], findings should be interpreted with caution.
Taken together, the observed discrepancies across studies may be attributed to variations in study size, sample representation, and different methods used to assess sleep duration, with some studies relying on subjective self-report measures.Notably, studies involving both healthy and unhealthy participants that measured sleep objectively using actigraphy reported no significant link between sleep duration and dietary inflammatory scores [24,26,29].This highlights the importance of standardized methodologies in future research.
In terms of sleep quality, small-scale (n < 430) cross-sectional studies in healthy adults did not show an association between sleep quality and DII [19,30,33] (apart from Wirth, Fekedulegn, et al., 2022 [24]).However, it is important to note that, in the Wirth, Fekedulegn, et al., 2022 study [24], participants (i.e., police officers) were exposed to several stressors that can affect sleep, including working night and evening shifts, shift changes, and higher depression and anxiety levels (which is in line with previous work showing health disparities in this population [38]).Hence, their inconsistent results could be attributable to these confounding factors.On the other hand, in large-cohort studies (n > 2000) with healthy adults, an association between reduced sleep quality and adherence to pro-inflammatory dietary patterns was found [20,23].Therefore, null results reported above could be explained by the lack of power and sample representatives.Nevertheless, a correlation between reduced sleep quality and following pro-inflammatory dietary patterns may reflect the mediatory role of inflammation in the associations between diet quality and sleep quality.In fact, a previous study shows the mediating role of various inflammatory markers (such as platelet and neutrophil counts, CRP levels, and NLR) on the diet and sleep quality relationship in generally healthy older adults [8].Hence, reducing dietary and/or circulating pro-inflammatory biomarkers via dietary interventions may offer a promising primary and/or alternative approach to improving sleep quality.
Additionally, studies conducted in obese and/or overweight individuals showed an association between reduced sleep quality and adherence to pro-inflammatory dietary patterns [18,22] (apart from Tabrizi and Farhangi, 2021 [28]; in which the null finding could be driven by the different statistical analysis technique, namely, the structural equational modeling, used).However, a similar pattern of results was not observed in studies conducted in populations with pain-related disorders or sleep-related breathing disorders [25,27].Given that (i) obesity and/or being overweight are states of low-grade systemic chronic inflammation [39] and (ii) systemic chronic inflammation (i.e., elevated pro-inflammatory biomarkers such as CRP) is associated with poor sleep quality [8], the results could reflect that the DII could be linked to sleep quality only in individuals with compromised inflammatory status.However, more research that elucidates the relationship between the DII and sleep quality in other metabolic conditions (such as non-communicable diseases) that are characterized by persistently high concentrations of circulating pro-inflammatory biomarkers is warranted.
The biological mechanisms regarding the association between the DII score and sleep duration and sleep quality could involve (i) cytokine responses, (ii) the neuroendocrine and autonomic pathways that link sleep with the immune system, and (iii) the role of inflammatory peptides in the homeostatic regulation of sleep (for a review, please see [40]).However, it is also important to highlight that various sociodemographic factors (such as low socioeconomic status and reduced access to healthy food and healthcare) could also contribute to the DII and sleep association, as they have all been shown to be associated with poor dietary choices (hence leading to anti-inflammatory dietary patterns) and poor sleep outcomes [41,42].
There are various limitations of the current review.Firstly, the studies involved in this review had no standard way of utilizing the DII and/or PSQI, such that these measures were used in a categorical and/or continuous manner.Therefore, due to the heterogeneity of the extracted data, no quantitative meta-analysis could be performed.Secondly, FFQ, dietary recall, and PSQI measures are subject to response/recall bias, as both sleep outcomes and dietary intakes are known to be under-reported [43,44].Thirdly, there was heterogeneity in terms of DII score calculations, such that the number of food parameters included for estimating the DII differed substantially across studies.Fourthly, seven studies were conducted in female-only samples, limiting the generalizability of the findings.Fifthly, it is important to note that the studies included in this review did not systematically account for factors that may contribute to poor sleep (such as caffeine intake, lack of exercise, poor diet, stress, etc.); hence, it is highly possible that the results could be driven by the impact of these factors on sleep duration and quality.Finally, due to the cross-sectional nature of the studies, causality cannot be assumed.

Conclusions
The current review offers a thorough assessment of the literature on the association between sleep duration and quality and the DII and shows that pro-inflammatory diets may be associated with poor sleep outcomes (duration and quality).However, as the current literature is heterogenous, future studies are required to (i) replicate previous findings in large cohorts across age groups, utilizing all 45 food parameters that are required to estimate the DII, preferably by using objective sleep outcomes, and (ii) examine the potential benefits of adhering to anti-inflammatory diets on sleep outcomes and beyond.

Table 1 .
Summary of the studies involved in the review.

Table 2 .
[35] of bias assessment by using the Agency for Healthcare Research and Quality checklist[35].