Immunometabolism of long-chain omega-3 fatty acids in humans

Open Access
Author:
Flock, Michael Ryan
Graduate Program:
Nutritional Sciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 18, 2014
Committee Members:
  • Penny Margaret Kris Etherton, Dissertation Advisor
  • Kumble Sandeep Prabhu, Committee Member
  • Connie Jo Rogers, Committee Member
  • Michael Henry Green, Committee Member
  • Trent L Gaugler, Special Member
Keywords:
  • nutrition
  • omega-3 fatty acids
  • red blood cells
  • inflammation
  • cardiovascular disease
  • immunometabolism
  • immunology
Abstract:
Over the last couple decades, there has been a surge in research describing the role of fatty acids in immune function due in large part to advances in molecular and biochemistry techniques. Long-chain omega-3 polyunsaturated fatty acids (n-3 PUFAs), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have emerged as potent modulators of metabolic and immune processes, which has to led to their increased use as a potential therapeutic option for the prevention and treatment of inflammatory conditions. Mechanisms underlying the immunometabolic effects of EPA and DHA are multifaceted and require further investigation. Questions regarding the appropriate dose, form, and delivery of n-3 PUFA need to be examined in order to provide effective n-3 PUFA recommendations for individuals seeking approaches to ameliorate inflammation and/or support immune health. The objective of the first study was to model the incorporation of supplemental EPA+DHA intakes within dietary ranges into red blood cell (RBC) membranes of healthy adults, and identify factors that modify this response. This was a randomized, placebo-controlled, double-blind, parallel study conducted in 125 healthy adults comparing 0, 300, 600, 900, and 1,800 mg/d of EPA+DHA given as fish oil supplements for approximately 5 months. The RBC membrane content of EPA+DHA (Omega-3 Index [O3I]) increased in a dose-dependent manner (p<0.0001), with the dose of EPA+DHA alone accounting for 68% (quadratic, p<0.0001) of the variability in response. Dose adjusted per unit body weight accounted for 70% (linear, p<0.0001) of the change. Additional factors that improved prediction of treatment response were baseline O3I, age, sex, and physical activity. Collectively, these explained 78% of the response variability (p<0.0001). The objective of the second study was to determine the effects of supplemental EPA+DHA on serum inflammatory marker concentrations (i.e., TNF-α, IL-6, and CRP) in healthy adults, and secondly, to evaluate the associations between RBC membrane fatty acid content and inflammatory markers. There were no significant differences in IL-6, TNF-α or CRP concentrations between the 5 groups following supplementation; however, a marginally significant treatment effect on TNF-α was observed (p<0.08). Higher quartiles of DHA content in RBC membranes were associated with lower TNF-α at baseline (p = 0.001), whereas the lowest quartile of docosapenatenoic acid (DPA) content had significantly higher CRP (p<0.001). Increased arachidonic acid (AA) content was associated with higher TNF-α and IL-6 (p<0.05); however, increased linoleic acid (LA) content was associated with lower IL-6 (p<0.05). There were no significant associations between changes in RBC content of n-3 or n-6 PUFA and changes in inflammatory marker concentrations. Our findings indicate that EPA+DHA intake, across a range of nutritionally-achievable doses, has no dose-response effect on circulating TNF-α, IL-6, or CRP in healthy adults after 5 months of supplementation. However, observed baseline associations warrant further investigation. The objective of the third study was to evaluate changes in gene expression following a low-dose in vivo lipopolysaccharide (LPS) challenge in healthy adult men, and secondly, to explore the relationship between subject characteristics, RBC fatty acids, and gene expression. Eight healthy adult males with diverse O3I profiles were intravenously administered sterile protein-free LPS (0.6 ng/kg body weight). Peripheral blood mononuclear cells (PBMC) collected at 0, 2, 4, and 24 hours post-LPS injection were assessed for changes in inflammatory gene expression. Increased pro-inflammatory gene expression occurred 2 hours after LPS administration, whereas anti-inflammatory gene expression increased at 4 hours. RBC content of docosapentaenoic acid (DPA), but not EPA or DHA, was associated with increased IL-1β expression at 4 hours as well as larger reductions in IL-1β expression between the 4 and 24 hour time points (p<0.01). In conclusion, consuming EPA+DHA in the form of fish oil supplementation dose-dependently increases the O3I and several factors in addition to dose (i.e., body weight, baseline O3I, age, physical activity, and sex) further explained the variability in O3I response. These results can be used to estimate an individual’s required supplemental intake for achieving target cell membrane levels of n-3 PUFA. Higher intakes of EPA+DHA (i.e., ≥1,800 mg/d) and increased cell membrane content of n-3 PUFA may provide anti-inflammatory benefits as evident by lower circulating inflammatory markers; however, additional clinical studies are needed to assess the effect of higher cell membrane content of n-3 PUFA, including DPA, on inflammatory responses.