DFA: EPA/DHA and Cognitive Health (2011)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To evaluate the association between erythrocyte membrane concentrations of n-3 polyunsaturated fatty acids, blood concentrations of total mercury and the incidence of dementia and Alzheimer's disease.

Inclusion Criteria:
  • ≥65 years of age
  • Non-demented persons as determined by physician evaluation
  • Living in the community or in institutions from 36 urban and surrounding rural areas in all Canadian provinces
  • Completion of clinical evaluation 
  • Blood samples provided
  • Written informed consent. 


Exclusion Criteria:
  • ≤65 years of age
  • Not a resident of Canadian provinces
  • No clinical evaluation completed or blood samples provided
  • No signed written informed consent.
Description of Study Protocol:


Recruited from communities and institutions from 36 urban and rural areas in all Canadian provinces


The prospective cohort study entitled Canadian Study of Health and Aging (CSHA) provided a representative sample of persons aged ≥65 years. A sub-sample of 663 non-demented CSHA subjects with a complete clinical examination, blood samples and follow-up information was eligible for analysis of laboratory measurements. The association between erythrocyte membrane concentrations n-3 PUFAs, blood concentrations of total mercury and the incidence of dementia and AD. 



Statistical Analysis

  • Total n-3 PUFAs, DHA, EPA and mercury were treated as continuous variables and in quartiles
  • To explore the combined effect of mercury and n-3 PUFA concentration, four groups were defined:
    • Mercury and n-3 PUFA concentrations above the median
    • Mercury and n-3 PUFA concentrations below the median
    • Mercury concentration above the n-3 PUFA concentration below the median
    • Mercury concentration below and n-3 PUFA concentration above the median
  • P-values obtained using chi-square tests for categorical variables and T tests or nonparametric Wilcoxon's rank sum test for continuous variables
  • Cox proportional hazards regression models used to examine the association between n-3 PUFAs and incident dementia or AD.
Data Collection Summary:

Timing of Measurements

  • Information on risk factors collected at baseline in 1991 using self-administered questionnaire
  • Clinical exam including Modified Mini Mental State Examination conducted at baseline and follow-up visits in 1996-1997 and 2001-2002
  • Non-fasting blood samples taken at each phase of the CSHA; 1991, 1996-1997 and 2001-2002.

Dependent Variables

Incidence of dementia or AD

Independent Variables

  • n-3 PUFAs concentration
  • DHA concentration
  • EPA concentration
  • Blood mercury concentration.

Control Variables

  • Age
  • Gender
  • Years of education
  • Body mass index
  • Apolipoprotein E allele (APOE) carrier status
  • Alcohol consumption
  • Smoking history
  • Medical history. 
Description of Actual Data Sample:
  • Initial N: 701 subjects
  • Attrition (final N): 663 subjects
    • Cognitively normal: 58.8% women
    • Incident dementia: 66.4% women)
  • Age:
    • Cognitively normal subjects: 80±6 years of age
    • Incident dementia subjects: 82.5±6.6 years of age
  • Ethnicity: Not discussed
  • Other relevant demographics:
    • Years of education
      • Cognitively normal: 10±4.1 years
      • Incident dementia: 9±4 years
      • P<0.01
      • Subjects from study sample had slightly fewer years of education than those not included (P<0.01)
  • Anthropometrics:
    • BMI (kg/m2)
      • Cognitively normal: 25.9±4.6kg/m2
      • Incident dementia: 25.2±5.2kg/m2
      • P=0.09 
  • Location: Canada.


Summary of Results:

 Key Findings

  • 149 incident cases of dementia, including 105 cases of AD
  • Distributions of n-3 PUFAs and DHA among the study sample were normal
  • Concentrations of total n-3 PUFAs, EPA and DHA did not differ significantly between groups
  • Incident cases of dementia had a significantly lower blood concentrations of mercury
  • Age at the moment of providing blood was significantly higher among dementia cases, mean number of years of education was smaller and the proportion of carriers of APOE was higher
  • Compared with the lowest quartile, hazard ratios for the association with incident dementia or AD were not different for the higher quartiles of total n-3 PUFAs, EPA or DHA in models adjusted for sex and education and when further adjusted for total blood mercury concentrations
  • No association observed between n-3 PUFAs and dementia or AD in final models adjusted for all confounders except mercury
  • Hazard ratios for dementia and AD were significantly decreased by 43-51% among those with mercury concentrations in the higher two quartiles in models adjusted for sex and education and additional adjustment for n-3 PUFA concentrations and other confounders
  • Completely adjusted risks for those with above median concentrations of both n-3 PUFAs and mercury were significantly reduced by 43% for dementia and by 46% for AD compared with those with both mercury and n-3 PUFA concentrations below the median
  • Risks were not significantly reduced for those with high mercury and low n-3 PUFA concentrations or for those with low mercury and high n-3 PUFA concentrations.
  Model 1
95 % CI
Model 2
95 % CI
Dementia n-3 PUFAs and mercury low 1.0




Dementia n-3 PUFAs high, mercury low 0.96

0.63, 1.48


0.65, 1.58

Dementia n-3 PUFAs low, mercury high


0.4, 1.08


0.41, 1.15

Dementia n-3 PUFAs and mercury high


0.39, 0.91



0.36, 0.89


AD n-3 PUFAs and mercury low 1.0




AD n-3 PUFAs high, mercury low 1.16

0.71, 1.89


0.69, 1.89

AD n-3 PUFAS low, mercury high 0.66

0.36, 1.21


0.35, 1.21

AD n-3 PUFAs and mercury high 0.56

0.33, 0.95



0.31, 0.92


* Significant reduction as compared to subjects with both mercury and n-3 PUFA concentrations below the median 

Other Findings

Proportion of APOE carriers was higher among dementia cases.


Author Conclusion:

Analysis from the CSHA found no evidence of a reduced risk of dementia or AD among subjects with higher blood concentrations of total n-3 PUFAs, DHA or EPA after adjusting for mercury blood concentrations or after adjustment for other confounders. The study also showed that even subjects in the highest quartile of mercury concentrations, dementia or AD risk was not increased.

Funding Source:
Government: Canadian Institutes of Health Research
University/Hospital: Fonds de la Recherche en Sante du Quebec, the Reseau Quebecois de recherche sur le Viellissement and the Chaire de Geraitrie de l'Universite Laval
Reviewer Comments:
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? Yes
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? Yes
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? Yes
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) No
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? No
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? Yes
  6.6. Were extra or unplanned treatments described? Yes
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? Yes
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes