Our results do not support the hypothesis that high fish consumption or omega-3 PUFA intake is associated with reduced psychological distress. The associations, if anything, could be non-linear. An effect modification by health behaviour is also suggested. The beneficial health effects of high fish consumption or omega-3 PUFA intake may manifest only in individuals with detrimental health behaviour such as smoking or a sedentary lifestyle. However, results concerning the observed non-linear associations and an effect modification were inconsistent across the sub-groups. Thus, our main finding is that high fish consumption does not protect from psychological distress. Regardless of the measures of fish consumption and omega-3 PUFA intake, the main finding was consistent in all the three cross-sectional data sets.
The use of three separate data sets and several methods to assess fish consumption and omega-3 PUFA intake increased the validity of the results. In addition, the comprehensive data sets made it possible to evaluate the effects of several confounding and modifying factors. However, it is possible that we were unable to see potential associations due to a cross-sectional setting. The possible associations could be established in longitudinal settings. The nationally representative Health 2000 Survey is a clear strength of our study. The response rates were high in the Health 2000 Survey and the Finntwin16 Study. The response rate was low in the Fishermen Study questionnaire, reducing generalizability of the results. However, these data include additional measures of fish consumption. The professional fishermen and their families represented a population with high habitual fish consumption wherein the health effects of fish should most likely to be manifested. Conversely, fish consumption among the young adults of the Finntwin16 study was low, with less than one-third reporting eating fish a few times a week. Regarding fish consumption, we thus had three different kinds of data. The nationally representative Health 2000 Survey enabled us to investigate the whole range of fish consumption in the general population, e.g. including the extreme ends. Therefore somewhat higher mean fish consumption in Finland37
compared with some other countries in Europe38
did not cause a problem. A limitation of the present study is that both the Health 2000 and the Fishermen sub-studies were based on convenience samples. Results based on these data cannot be generalized since bias introduced by subject participation is possible. In addition, the number of participants in the Fishermen sub-study was low.
In the present study, no hypothesized associations were observed between fish consumption or omega-3 PUFA intake and psychological distress. One reason might be the fact that the study outcome was recent psychological distress instead of more severe mental disorders. The GHQ was developed as a screening tool to detect individuals who are likely to have or to be at risk of developing psychiatric disorders.39
More specifically, it assesses changes in an individual’s ability to carry out daily functions and the emergence of a new psychological disorder rather than life-long personality characteristics. Previous research has mainly focused on more severe psychiatric disorders such as depressive symptoms, depression or even schizophrenia.40
In agreement with our findings, a recent study suggested that omega-3 PUFAs may not have a role in the aetiology of minor depression.11
However, according to two clinical trials, supplemental EPA and DHA have reduced stress measured by the GHQ in smokers19
and measured by the Perceived Stress Scale and the Daily Hassles and Uplifts Scale in a patient population.41
It is noteworthy that in the first trial, the number of participants was very small (n
= 7) and in the second trial, the observed benefits were additional to psychiatric care.
We used the 128-item FFQ from two data sets to measure fish consumption and dietary fatty acid intake and were able to take into account total energy intake in the analyses. This was not done in the majority of the previous studies that showed associations between high fish consumption or dietary intake of omega-3 PUFAs and lower rates of depression or fewer depressive symptoms. In only one of these studies on elderly men were fish consumption and omega-3 PUFA intake adjusted for total energy intake.4
On the contrary, total energy intake was used as a covariate in three out of six questionnaire-based studies reporting no linear associations between high fish consumption and lower rates depression or fewer depressive symptoms.10,12–16
Hence, in these studies, total energy intake was a confounder. In our study, associations were not observed regardless of energy-adjustment.
An FFQ is designed to rank individuals, not to measure absolute food consumption or nutrient intake, although the unit is g/day. In addition, fish consumption is usually over-reported in FFQ data.26,27
Therefore FFQ-based intakes cannot be compared with recommendations. However, an advantage of an FFQ is that it evaluates the usual long-term diet, which is conceptually more important in epidemiological studies than absolute intake. In addition, the FFQ used in the present study has been calibrated against a 14-day food record and it was found to meet the requirements of epidemiological studies.27
In the Fishermen Study, high serum DHA was associated with greater distress on the contrary to the hypothesis. However, this can be a coincidence due to the small number of participants or the very high levels of serum DHA in the Fisherman sub-study population. A rise in distress scores was seen after serum DHA concentration reached 6% both in men and women. Extremely high DHA concentrations may not be beneficial for mental well-being. Mental disorders such as depression have been associated with over-activity of the inflammatory response of the immune system increasing pro-inflammatory cytokines production.42
Omega-3 PUFAs have been suggested to inhibit some of these cytokines, especially of tumour necrosis alpha (TNF-α) and interleukin-1 beta (IL1-β).43
However, in one recent study, DHA was reported to have pro-inflammatory effects by an induction of Th-1-like immune response.44
For this reason, the study’s authors suggested that the treatment of depressed patients with fish oils should be avoided, since DHA could aggravate the immune regulatory system response in depression. Similarly, high fish consumption and omega-3 PUFA dietary intake were associated with more depressed mood in a Spanish study, which used a validated, semi-quantitative FFQ and energy adjustment.16
The authors suggest that one possible explanation might be the contamination of fish by mercury or other toxic compounds such as dioxins and polychlorinated biphenyls. More research is needed to assess whether a high intake of environmental contaminants could increase the risk of depressive symptoms among participants with high habitual fish consumption.
Instead of linear associations, we found some non-linear associations between fish consumption or omega-3 PUFA serum concentrations and psychological distress scores. These findings are in line with some previous studies suggesting that individuals with moderate consumption of fish or moderate omega-3 PUFA intake have a lower risk of mental disorder when compared with individuals with high consumption/intake.13,16,45
In addition, stratified analyses revealed that some behavioural factors, such as physical activity, modified the studied associations. It is possible that fish consumption and omega-3 PUFA intake are associated with reduced psychological distress when lipid peroxidation has reduced serum levels of omega-3 PUFAs as a result of smoking or illness. However, the results concerning non-linear associations and effect modification were not consistent in any of the studied sub-groups or data sets. Therefore, the results of the present study suggest that there is no association between fish consumption or omega-3 PUFA intake and psychological distress. A failure to detect the hypothesized linear associations may simply reflect a more complex association between lifestyle and mood.
According to our results, there was no indication that high fish consumption or omega-3 PUFA intake would be associated with reduced distress either at the population level or in a high fish consumption group. Hence, the current evidence does not justify for recommendations to eat high amounts of fish to prevent symptoms of psychological distress.