Research Article
Serum phospholipid fraction of polyunsaturated fatty acids is the preferred indicator for nutrition and health status in hemodialysis patients

https://doi.org/10.1016/j.jnutbio.2016.07.021Get rights and content

Abstract

Long chain (LC) polyunsaturated fatty acids (PUFA) are major components of cell membrane phospholipids (PL) and serve as precursors for numerous bioactive lipid derivatives. Fatty acids (FA) are routinely analyzed in biological samples to assess composition of tissues, cells, and lipid fractions. In human studies, serum or plasma is often used because of their easy procurement. However, the lipid pool in serum and plasma is a mixture of triacylglycerol (TG), PL, cholesterol and its esters, and other components. Herein, we report findings from a serum FA analysis after fractionation of polar and neutral lipids by solid phase extraction in a large cohort of 400 hemodialysis patients. LC PUFA were found concentrated in the polar fraction compared to the total or the neutral lipid fraction. When correlated with clinical markers of disease, a greater number of significant correlations were found for PUFA in polar compared to total or the neutral fraction. We also observed that polar lipids are a reliable reflection of LC PUFA status compared to the total or neutral fractions because the latter are diluted by non-essential FA. The relative amounts of LC PUFA in the total and neutral fractions reflect the contribution of TG in blood that varies with diet, age, and physiologic state. Our data indicate that LC PUFA in the polar fraction are superior indicators of bioactive FA-status than in the total or the neutral fraction and should be used to establish important links between PUFA status, their bioactive substrates in hemodialysis patients.

Introduction

The study of fatty acid (FA) composition of tissue phospholipids (PL) is important because long chain (LC) polyunsaturated fatty acids (PUFA) associated with these lipids serve critical biological roles as the precursors for numerous bioactive lipid derivatives and in cell-to-cell communications [1], [2]. The PUFA-derived mediators include prostaglandins, leukotrienes, thromboxanes, endocannabinoids (EC), and other families of oxylipins. In human studies, serum or plasma is commonly used to evaluate fatty acid status because of its availability and the FA composition of circulating PL is the most reliable estimate of PUFA and essential fatty acid status PL [3].

Analyses of LC PUFA are routinely performed to assess dietary status and tissue composition related to their biological importance. The analysis begins with the extraction of lipids from the sample and in some cases isolation of lipid classes, for example, the polar lipids such as PL from those that are non-polar lipid compounds such as triacylglycerols (TG). Thus, the blood lipid pool is a mixture of TG, PL, cholesterol and its esters, and other lipid constituents including FA. The FA composition of serum PL has become an acceptable indicator for assessing nutrition status and to predict dietary fat intakes and de novo lipid synthesis [4], but its importance in dialysis patients is not well established.

It is recognized that LC n–3 PUFA, by virtue of their biochemistry and numerous derivatives, participate in a number of key biological processes. An example are the oxylipin derivatives that participate in cell membrane physiology, cell membrane fluidity, cell signaling pathways, inflammation, and gene expression [1], [3], [5], [6], [7]. The importance of LC n-3 PUFA to cardiovascular health is also of great scientific interest. With its anti-inflammatory, anti-arrhythmic, lipid-lowering, and anti-hypertensive effects, LC n-3 PUFA have been described in many reports to possess important cardioprotective benefits, especially for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) [8], [9], [10]. This is especially relevant for the chronic hemodialysis population, which has exceptionally high rates of cardiovascular disease [11].

Higher amounts of PUFA are recognized to be associated with PL relative to the amounts found in neutral lipids like TG [12]. Because LC PUFA rarely accumulate in the neutral lipid fraction, only serum total lipids or polar LC PUFA will be useful as clinical biomarkers in hemodialysis subjects, a population with a large burden of illness [13]. Similar findings have been noted in non-dialysis patients [13]. Because LC PUFA in the polar lipid fraction is a better and more stable reflection of total LC PUFA than what is found in other glycerol lipids such as TG, we firmly support that when studying the biological actions of PUFA the focus of attention should be on the polar lipid fraction. LC PUFA in the polar lipid fraction are subject to many biochemical pathways that liberate (such as by phospholipase A2) PUFA for biosynthesis of potent biological mediators via the cyclooxygenase (COX), lipoxygenase (LOX), EC pathways and others [1], [2].

Therefore, the overarching hypothesis of this study is that the FA composition of serum polar lipid fraction best represents overall PUFA status in chronic hemodialysis patients and is therefore a better indicator of dietary status and well-being. The aims of this study were to confirm in hemodialysis patients that (1) greater amounts of LC PUFA are found in the polar lipid fraction, (2) LC PUFA in the polar fraction provide greater clinical relevance for essential FA status and dietary status of the subject or patient than other fractions, and (3) LC PUFA in the polar lipid fraction reflect more stable levels of LC PUFA than what is found in other glycerol lipids such as TG. The nutrition and clinical significance of our analyses is to achieve the best representative appraisal of PUFA to assess diet status and disease risk.

Section snippets

Study population and collection of serum samples

The population of 10,044 individuals who participated in the Accelerated Mortality on Renal Replacement (ArMORR) project, a nationally representative prospective cohort study of patients who initiated chronic hemodialysis at one of 1056 dialysis centers in the US, have been previously described [14]. This study was approved by the Institutional Review Board of the Massachusetts General Hospital, Boston, MA, ClinicalTrials.gov Identifier: NCT00505180.

Lipid class fractionation

Human serum samples were processed to

Results

The 400 ArMORR subjects used for this analysis were part of a separate study that included 100 deceased who died of sudden cardiac death during the first year on hemodialysis and 300 patients who survived (Supplemental Table 1). LC PUFA levels are higher in the polar fraction than in the total lipids (Table 1A) and neutral fraction (Table 1B). Levels of the two most common LC PUFA, AA and DHA, were consistently higher in the polar fraction compared to total lipids or neutral fraction, as were

Discussion

In this investigation we found that in hemodialysis patients the polar fraction of serum lipids, which is rich in phospholipids, is preferentially enriched with LC PUFA [13] and is more strongly associated with some markers than found with the neutral fraction of PUFA. These observations coincide with the general population; even though the absolute LC n-3 PUFA content in hemodialysis patients is generally lower compared to the common population [13]. Recently, other investigators reported that

Conflict of interest

There is no conflict of interest to report.

Authorship

Allon Friedman and Bruce Watkins designed the experiment; Bruce Watkins drafted the manuscript; Hector Tamez, Julia Wenger, Ravi Thadhani provided the samples used in the research. Analyses were performed in the Advance Technology Laboratory, Nutrition and Molecular BioSciences, University of Connecticut.

Acknowledgment

This work was supported by NIH grant DK084403 and the National Kidney Foundation.

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    An abstract (#1072.16) of this work was presented at the 2013 Experimental Biology Meeting at Boston MA.

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