Review ArticleConsumption of a healthy dietary pattern results in significant reductions in C-reactive protein levels in adults: a meta-analysis
Introduction
The study of the diet-disease relationship has seen a major shift in focus from single nutrients to that of healthy dietary patterns [1]. The shift recognizes that individual foods or nutrients are not eaten in isolation, and there are potential synergistic effects of multiple components within the diet [2]. This shift is reflected in dietary guidance, with recent reviews by the 2015 US Dietary Guidelines Advisory Committee identifying healthy cuisine-based dietary patterns, such as the Mediterranean diet, to be associated with reduced risk of chronic disease [3]. Likewise, disease risk is being examined in a more integrated fashion. For example, the metabolic syndrome (MetS) represents a cluster of risk factors including abdominal obesity, raised blood pressure, insulin resistance, and dyslipidemia [4]. The known relationship between MetS, and cardiovascular disease and type 2 diabetes [5] highlights the importance of understanding the effects of dietary patterns on disease progression. Of particular relevance is the state of chronic, low-grade inflammation indicative of ongoing pathology, which underpins the MetS (Fig. 1).
A suite of biomarkers has emerged in the literature to indicate changes in this underlying low grade inflammation which characterizes the components of the MetS (Fig. 1). For example, tumor necrosis factor–α (TNF-α), secreted by adipocytes, is a pro-inflammatory cytokine which propagates the inflammatory response and is associated with low insulin sensitivity [6]. Pro-inflammatory cytokines stimulate the hepatic production of C-reactive protein (CRP), another marker of inflammation [7] and an independent predictor of cardiovascular events [8], [9], [10], [11]. The inclusion of CRP in prediction models based on traditional risk markers improves the prediction of cardiovascular events [12]. Another adipokine, resistin, increases the expression of pro-inflammatory cytokines including TNF-α thus stimulating the inflammatory response [13] and serves as a biomarker of atherosclerotic risk [14]. Retinol binding protein 4 (RBP4), an adipokine associated with increased body mass index and visceral adipose tissue, may serve as an early detection biomarker for type 2 diabetes mellitus [15]. In contrast, adiponectin is a hormone with anti-inflammatory [16] and insulin sensitizing effects [17], [18], making it a potential biomarker of health, with the high-molecular-weight (HMW) form the most physiologically active [19]. The ratio of adiponectin to leptin has also been proposed as biomarker of the metabolic syndrome [20], and has also been suggested as a method of assessing insulin resistance [21].
These biomarkers provide insight into the progression and amelioration of MetS, so they are increasingly used as indicators of the effect of dietary interventions on health. For example, the Mediterranean diet (a healthy, cuisine-based dietary pattern) has been associated with a reduced incidence of MetS [22], [23], and meta-analyses show effects on markers of inflammation [24], [25]. These need to be updated with the latest results from the landmark PREDIMED study [26], [27]. Moreover, given the emergence of dietary guidance surrounding dietary patterns, the effects of healthy dietary patterns in general needs to be explored, as well as considerations of novel biomarkers of inflammation such as HMW adiponectin. The aim of this meta-analysis was to determine the effect of interventions targeting healthy dietary patterns on a range of biomarkers associated with underlying adiposity, insulin resistance, and inflammation in adults. It was hypothesized that consumption of a healthy dietary pattern characterized by higher consumption of vegetables, fruit, wholegrains, and lower consumption of red meat would result in favorable changes in these biomarkers.
Section snippets
Approach for the meta-analysis
This systematic literature review and meta-analysis followed the requirements of the PRISMA statement [28] (Supplemental Table 1). The review was registered in PROSPERO, the international prospective register of systematic reviews (http://www.crd.york.ac.uk/PROSPERO; registration number: CRD42015019236).
Description of the included studies
A total of 737 articles were assessed for eligibility in the review. Following application of the exclusion criteria, 18 articles describing 17 studies were included in the review. Fig. 2 displays the process of study selection [28].
Included studies assessed the effects of dietary patterns such as the Mediterranean diet (n = 13), the Nordic diet (n = 2), the Tibetan diet (n = 1), and the Dietary Approaches to Stop Hypertension (DASH) diet (n = 1) on outcomes, with studies grouped according to
Discussion
The dietary patterns identified in this review were characterized as plant food based (mostly fruit, vegetables, and wholegrains and with little red meat). CRP was the only biomarker to emerge with substantial evidence for a relationship with healthy dietary patterns, namely that healthy dietary patterns reduced CRP levels compared to control diets. Sub-group analysis showed the effect was demonstrated with the Mediterranean diet, and in studies with intervention duration of three months or
Disclosure statement
The authors declare they have no conflict of interest
Acknowledgment
This study was supported by the Smart Foods Centre, School of Medicine, University of Wollongong.
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