**1. Introduction**

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality worldwide. It is well known that high concentrations of circulating high-density lipoprotein-cholesterol (HDL-C) are inversely correlated with the risk of CVD [1]. While raising HDL-C is a theoretically attractive target, there is no evidence from randomized trials that increasing plasma HDL-C concentrations reduces CVD risk [2].

Several studies have shown that high-density lipoproteins (HDLs) are highly heterogeneous in size, shape, density, lipid and protein composition [3]. HDL particles undergo continuous remodeling through interactions with other circulating lipoproteins and tissues [4]. HDL-associated proteins have been considered until now to predominate in determining the particle structure and biological functions: cholesterol removal, anti-inflammatory, antioxidant and endothelial cell protection [5]. For example, paraoxonase 1 (PON1) is a HDL-associated protein mediating HDL functionality that has been negatively correlated with unfavorable outcome in stroke patients [6]. However, the latest cutting edge lipidomic technology has revealed important roles of lipid components in HDL function. Sphingomielin-1-phosphate (S1P) is the most studied bioactive lipid bound to HDL and it has been negatively correlated with the severity of coronary artery disease [7]. Recent evidence linked increased odds of acute coronary syndrome to low cholesterol efflux capacity (CEC), pro-oxidant and pro-inflammatory HDL particles and low HDL

**Citation:** Grao-Cruces, E.; Varela, L.M.; Martin, M.E.; Bermudez, B.; Montserrat-de la Paz, S. High-Density Lipoproteins and Mediterranean Diet: A Systematic Review. *Nutrients* **2021**, *13*, 955. https://doi.org/10.3390/ nu13030955

Academic Editor: Mario Barbagallo

Received: 23 February 2021 Accepted: 12 March 2021 Published: 16 March 2021

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levels of S1P [8]. In addition, HDL-C was found to be highly related to cardiovascular risk when it was carried by small HDLs, suggesting that cardiovascular risk is highly influenced by HDL size [9]. Taken all together, it is becoming clear that circulating HDL-C plasma concentrations is not an appropriate marker of CVD risk and therefore do not represent a reliable therapeutic target. Targeting HDL functionality rather than HDL-C concentrations may represent a more promising therapeutic target.

Different strategies are followed in order to modify plasma HDL-C and cardiovascular health. Healthy diets have beneficial effects on lipid profile, replacing saturated fatty acids (SFA) with either monounsaturated (MUFA) or polyunsaturated fatty acids (PUFA) have been shown to reduce plasma HDL-C [10]. This objective is achieved with Mediterranean diet (MD), rich in olive oil (OO) fruits and vegetables, nuts, legumes and whole cereals, fish and red wine. Data from observational and randomized controlled trials supports that MD protects against CVD, MD has showed capacity to improve lipid profile related to HDL-C [11]. The purpose of this systematic review was to investigate the effect of the MD on changes in HDL structure and functionality in humans.

#### **2. Materials and Methods**

This systematic review was conducted following the Preferred Reporting Items for Systematic reviews and Meta-analysis (PRISMA) [12]. This review was registered on PROSPERO (Registration No. CRD42020218784). Available from: https://www.crd.york. ac.uk/prospero/display\_record.php?ID=CRD42020218784 (accessed on 23 February 2021).

#### *2.1. Searching Strategy*

A comprehensive search was conducted in four databases (PubMed, Scopus, Web of Science and Cochrane Library), searching all years of record up until October 2020. The language restriction was English and Spanish. Search terms were categorized into 3 key concepts: study population, HDL modulation, and nutritional intervention; specific terms used were: (adult OR "middle-aged" OR young OR m?n OR wom?n OR obese OR healthy) AND ("Mediterranean diet" OR "olive oil") AND ("HDL remodel\*" or "HDL function\*" OR "HDL dysfunction\*" OR "HDL change" OR "HDL component"), respectively. Whereas, reference lists of retrieved articles were manually searched for relevant publications (Table 1).



HDL, high-density lipoprotein; MD, Mediterranean diet.

#### *2.2. Selection Criteria*

Published studies included in this review were require to adhere to the following criteria: (1) original research; (2) adult human studies; (3) articles including HDL changes in composition, HDL component modifications or HDL functionality changes; (4) dietary intervention with MD or MD-related foods; (5) obese, dyslipidemic and healthy humans.

#### *2.3. Data Extraction and Reliability*

The PRISMA recommendations were followed. Firstly, titles were screened and abstracts were analyzed in order to identify relevant articles. Then, articles chosen were downloaded and reviewed in detail by different researchers.

#### **3. Results**

#### *3.1. Search and Selection of Studies*

The searching and selection strategies are detailed in Figure 1. A total of 55 records were identified through database searching and 38 through other sources, search of reference list of retrieved articles. Duplicates were removed and a set of 34 records were left to select the ones to be screened. From these 34, 7 records were excluded due to article type and 27 records were susceptible of full-text assessment. Consequently, 13 records were excluded with criteria, and 14 records were assessed for eligibility. Finally, 1 record was excluded because of the methodology used and 13 records were analyzed.

**Figure 1.** Flow diagram of record selection.

### *3.2. Comparison between Studies*

Nine different study populations have been found in the records chosen, whereas, ten nutritional interventions were studied (Table 2). Two records studied the same 296 individuals with three nutritional interventions [13,14]. Otherwise, after the interventions, Hernaez et al., 2019, made an evaluation of foods consumed by 196 individuals from the initial study population, in order to create different groups [13]. Individuals in each study population were different between studies, healthy individuals were included in a total of 5 study populations [15–19], and the rest of the study populations included high cardiovascular risk individuals, which suffered from dyslipidemia, obesity or hypercholesterolemia [13,14,20–25].


#### **Table 2.** Study characteristics.

MD, Mediterranean diet; VOO, virgin olive oil; HPCOO, high-polyphenol content olive oil; LPCOO, low-polyphenol content olive oil; EPA, eicosapentaenoic acid; FVOO, functional virgin olive oil; FVOOT, functional olive oil with thyme; UK, United Kingdom.

> Intervention time varied between studies. The longest intervention time was 1 year [13,14] and the shortest 4 days [17].

#### *3.3. HDL Functionality Modulation*

A set of 7 records measured cholesterol efflux capacity (CEC), CEC measurements were conducted on THP-1 monocyte-derived macrophages [13–15,18,20], on J-774A.1 macrophages [22–25] and on primary cultures [19]. PON, lecithin cholesterol acyl transferase (LCAT), and cholesteryl ester transfer protein (CETP) activities were determined in a set of 5 [13,14,16,20,22–25], 3 [15,16,22–25] and 4 [13–16] populations, respectively, by enzymatic activity measurements.

Specific MD-related foods were able to modulate HDL functions (Table 3). An increase of CEC was showed after virgin OO (VOO), whole grains and nuts [13] eicosapentaenoic acid (EPA) [20] and extra VOO [18] consumption compared to baseline. Enzyme activity was variable after different interventions: CETP activity was decreased by diets enriched with legumes, fish and VOO [13]. PON1 activity was increased by nut, legume and fish enriched diets [13], EPA consumption [20], lycopene rich and supplemented diets [16] and functional VOO with thyme (FVOOT) intervention [22–25]. LCAT activity increased with lycopene supplemented diets [16]. Although there was no change after VOO, functional VOO (FVOO and FVOOT) interventions in LCAT activity, it was significantly higher after FVOOT in comparison with VOO intervention [22–25].

