Differential Effects of n-3 and n-6 Fatty Acids on Carotid Plaque and Its Progression: Analyses from The Multi-Ethnic Study of Atherosclerosis

By Kristen N Smith, PhD, RD, LD; Mary R Dicklin, PhD; Kevin C Maki, PhD, CLS, FNLA

 Background:

The process of atherosclerotic development is multifactorial involving inflammation, endothelial activation, oxidative stress and lipid accumulation in the arterial wall resulting in plaque accretion and vessel occlusion.1 Data have shown that atherogenesis can occur as early as adolescence2 and support the need for the identification and control of modifiable risk factors.  Long chain omega-3 (n-3) and omega-6 (n-6) polyunsaturated fatty acids (PUFAs) have been hypothesized to have antiatherogenic properties, yet the available evidence is inconsistent and no large multiethnic studies have examined n-3 and n-6 levels in plasma in relation to subclinical atherosclerosis outcomes.

Overall, n-3 PUFAs (including plant-derived alpha-linolenic acid [ALA] and fish oil eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]) have been considered cardioprotective and their plasma concentrations inversely associated with inflammation and endothelial activation.3-8 Cohort studies have shown n-3 PUFAs to be associated with lower risk of cardiovascular outcomes.9,10 However, null results in recent meta-analyses and randomized controlled trials11-13 have raised questions regarding the benefits associated with n-3 FAs.

Compared to n-3 PUFAs, n-6 PUFAs have not been studied as thoroughly, but have been suggested by some to be proinflammatory and proatherogenic.14 This impression of n-6 PUFAs likely stemmed from the observation that the essential n-6 PUFA, linoleic acid (LA) can be converted to arachidonic acid (AA) – which is a substrate for eicosanoid lipid mediators that promote vascular disease.14,15 More recently this perception has been changing16 and results from observational studies suggest that LA and AA may both have cardiovascular benefits.6,17-19 Conversely, n-6 PUFAs such as gamma-linolenic acid and dihomo-gamma-linolenic acid (mainly created through de novo synthesis from LA) have been associated with inflammation and endothelial activation,6 although researchers are unsure whether higher levels have any impact on atherosclerosis and no large prospective cohort studies have investigated plasma n-6 PUFAs and subclinical measures of atherosclerosis.

This study of participants without apparent cardiovascular disease at baseline examined “whether objectively measured plasma levels of n-3 or n-6 PUFAs were associated with the presence of carotid plaque or occurrence of plaque progression during a median 9.5-year study period, and whether race/ethnicity modified any observed associations.”1

 Methods:

The MESA cohort was gathered from six regions in the United States: Forsyth County, NC; Northern Manhattan and the Bronx, NY; Baltimore City and Baltimore County, MD; St. Paul, MN; Chicago and the village of Maywood, IL; and Los Angeles County, CA. This cohort consisted of men and women of diverse backgrounds between the ages of 45 and 84 years at baseline, who were free from clinical cardiovascular disease. The present investigation included 3327 MESA participants and assessed whether plasma n-3 or n-6 levels were associated with the presence of carotid plaque or occurrence of plaque progression during a median 9.5-year study period.

Fasting samples of blood and plasma were taken throughout the study period and B-mode carotid ultrasonography was conducted at Exam 1 (2000-2002) and Exam 5 (2010-2012) to evaluate carotid artery intima-media thickness and plaque.

Results:

Baseline assessments (stratified by presence of plaque) helped to identify a profile of people less likely to have carotid plaque:

  • Lower mean age (p<0.001)
  • More likely to be women (p=0.003)
  • More likely to have never smoked (p<0.001)
  • Lower mean systolic blood pressure (p<0.001)
  • Less likely to have diabetes mellitus (p<0.001)
  • Lower levels of total cholesterol (p<0.001)
  • Greater levels of plasma LA (p<0.001)
  • Greater levels of plasma DHA (p=0.03)

Profiles were also created of subjects who did not experience plaque progression over the course of the study. This group of individuals had the following characteristics:

  • Lower mean age (p<0.001)
  • Lower body mass index (p=0.016)
  • Lower systolic blood pressure (p<0.001)
  • Lower levels of total cholesterol (p=0.04)
  • Fewer current and former smokers (p<0.001)
  • Fewer individuals taking medication(s) for hypertension (p<0.001)
  • Fewer individuals taking medication(s) for lipids (p<0.001)
  • Greater levels of high-density lipoprotein cholesterol (p=0.002)
  • Greater levels of plasma LA (p=0.003)
  • Greater levels of plasma DHA (p=0.005)

Assessment of quartiles of plasma n-3 and n-6 PUFA concentrations found that subjects in the second quartile of n-3 ALA had an 11% greater risk of having plaque than the quartile 1 referent (p=0.02) but there were no noted relationships for the third and fourth quartiles vs. the referent for the presence or absence of plaque. Participants within the fourth DHA quartile had a 9% lower risk of having carotid plaque compared to the referent (p<0.05), and a 12% lower risk of carotid plaque progression than the referent (p=0.002). There were no significant differences between n-6 PUFA quartiles (2nd through 4th vs. referent) for carotid plaque progression.

At baseline, participants in the top quartile of the n-3/n-6 ratio were at a significantly lower risk of for having carotid plaque (p=0.03) and those in the highest quartiles of EPA+DHA and total n-3 PUFAs were at reduced risk for plaque progression (p=0.01 and p=0.02, respectively). There were no significant relationships between total n-6 PUFAs and either the presence or progression of carotid plaque.

Interaction analysis did not detect a modifying effect of race/ethnicity on associations between PUFAs and baseline carotid plaque or plaque progression.

 Comment:

Overall, these results support an association between plasma levels of n-3 PUFAs, particularly DHA, with lower prevalence and risk for progression of carotid plaque.  No consistent positive or inverse associations were found for n-6 PUFAs and carotid plaque presence or progression. The results of this cohort investigation provide evidence for the ongoing recommendation to incorporate n-3 PUFAs into the diet for preventing or delaying atherogenesis. The lack of significant associations for n-6 PUFAs with plaque presence or progression suggest neither pro- nor anti-atherogenic properties for these fatty acids.

 

References:

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