Ageing, cognition and omega-3 fatty acids : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Psychology at Massey University, Wellington, New Zealand
The evidence for omega-3 polyunsaturated fatty acids (n-3 PUFA) in fish oil supplements being a safe therapeutic agent is steadily growing. However, there is still a lack of evidence around the cognitive effects of n-3 PUFAs in older adults with Mild Cognitive Impairment (MCI), the moderating effect of the Apolipoprotein E (APOE) ɛ4 allele on cognition and well-being, and the popularity of fish oil supplements in New Zealand (NZ).
The primary aim of this thesis was to conduct a clinical trial to investigate the cognitive effects of a high dose docosahexaenoic acid (DHA) fish oil supplement in older adults with MCI, and to examine how the presence of the APOE ɛ4 allele affects the efficacy of fish oil. The trial involved a 12-month randomised, double-blind, placebo-controlled design with testing sessions at baseline, 6-months, 9-months and at the end of the trial. Seventy-two adults with MCI between the ages of 60 and 90 were recruited from Wellington, NZ. Participants were either given a DHA fish oil supplement containing 1491 mg of DHA and 351 mg of eicosapentaenoic acid (EPA) per day or a placebo supplement containing sunflower oil. Outcome measures included the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), the National Adult Reading Test (NART), the Coin Rotation Task (CRT), the California Older Adult Stroop Test (COAST), the Trail Making Test A and B (TMT), Digit Span Backwards Test (DSBT), the Geriatric Depression Scale (GDS), the Geriatric Anxiety Inventory (GAI), the 36-Item Short Form Health Survey Questionnaire (SF-36), height, weight and blood pressure as well as red blood cell (RBC) fatty acid profiles. ANCOVAs, t-tests and chi-square tests were used to test for differences between the DHA and placebo groups.
The secondary aim of this thesis was to conduct an online cross-sectional survey to investigate how popular fish oil supplements are within a sample population of New Zealanders, to determine why fish oil supplements are used, what dose fish oil users take, and where fish oil users store their supplements. The aim of the survey was to test the hypothesis that fish oil users are more likely to display healthier dietary and lifestyle habits.
The final analysis (n = 60) of the trial found no evidence of a treatment effect using the cognitive measures, although it did find a treatment effect on systolic blood pressure (p = .03, ƞ2 = .08), and a treatment interaction for APOE ɛ4 carriers on depression (p = .04, ƞ2= .07) and anxiety (p = .02, ƞ2 = .09) scores in favour of the DHA group. The survey found that in a sample of 334 New Zealanders, 66.8% reported using supplements and 21.9% reported using fish oil supplements. The survey found that respondents who regularly eat oily fish are most likely to take fish oil supplements (p < .01), and that only 26% of fish oil users reported taking a dose that would meet dietary recommendations. Over half of fish oil users reported taking fish oils to ‘improve brain functioning’ and only 6.8% of fish oil users reported storing fish oil supplements in the refrigerator.
The evidence from epidemiological studies suggests that fish consumption is protective against cognitive decline and dementia, and yet the evidence from clinical trials investigating the cognitive effects of n-3 PUFAs in older adults has been inconsistent and at times conflicting. It appears that the protective effect of fish may be more than the n-3 PUFA content, and that fish consumption may be part of an overall healthier diet and lifestyle, along with regular physical activity, strong social connections and an emphasis on educational attainment and engagement in cognitive activities.