As a general rule, fish is a very valuable addition to the diet and something that many of us probably don’t get enough of.
But, a more unusual connection is the potential for fish to actually help in preventing depression.
This relationship was highlighted in a recent study by Li, Liu and Zhang, which was published in the British Medical Journal.
The outcomes of the study suggest yet another reason why people should be including fish in their diet.
The Study Itself
This particular study was a meta-analysis. With this type of paper, the authors do not directly conduct any research.
Instead, the approach involves searching the scientific literature and looking at other studies that have been conducted.
Often authors will look at the approaches that the other studies took, as well as their outcomes and the differences between them.
In this case, the authors found a total of 26 studies that looked at fish consumption and depression. Across all of the studies, the total number of participants considered was 150,278.
The key result of the study was that the risk ratio when comparing highest level of fish consumption to lowest level was an average of 0.83 (the confidence interval for this outcome was 0.74 to 0.93).
With a risk ratio, you are comparing to a value of 1.0, where anything under that value indicates reduced risk.
So, a value of 0.83 means that people consuming more fish had a lower risk of depression than those who consumed less or no fish.
Specifically, their risk was 17% lower.
The authors also noted that similar results were found when looking at just the cohort studies or just the cross-sectional studies (those are two different forms of observational research).
As part of their meta-analysis, the authors of this study looked into the potential of publication bias.
Scientific papers go through a rigorous process before they are published. This process includes peer-reviewing and can require authors to edit their papers many times.
There are also many more papers submitted than there are papers actually published.
So, some people do get their papers published, while others might not.
In some areas, there can also be a publication bias.
For example, a paper that offers support for an already popular theory may be more likely to get published than one which promotes a new or controversial theory.
That issue can sometimes make it seem like there is much more scientific support for a given theory than there actually is.
By testing this area, the authors of this study were able to get an idea whether publication bias influenced the observed results or not.
The authors did not find any evidence of a publication bias.
That reinforces the hypothesis that fish consumption can significantly decrease the risk of depression.
Two key aspects of a meta-analysis are finding and choosing which studies to feature.
In this case, the authors used the keywords: ‘depression’ or ‘depressive disorder’ or ‘depressive symptoms’ and ‘fish’.
These keywords were used to search a number of key databases and the authors considered all studies published up till March of 2015 (the time the study would have been actually conducted).
The authors also examined the reference lists of any studies they used to find any relevant studies that missed the initial search.
To figure out which studies to use, the authors had a number of criteria.
I’ve simplified them a bit, but the criteria were as follows:
- Studies had an observational design
- Study participants were from the general population
- Studies focused on fish consumption
- Study outcome was depression
- The study outcomes were given in the form of changes in risk
In some ways, this choice of studies is limiting, because it means the authors aren’t considering experimental studies at all, or the impact that fish consumption might have on people who already have depression.
Nevertheless, the criteria are important, because if the studies chosen were too different from one another, there wouldn’t be a way of accurately comparing them.
Strengths and Weaknesses
The meta-analysis design allows the authors to consider the outcomes of a number of different studies, which is a major strength.
This type of study is particularly useful for illustrating what the current scientific consensus is for a given field.
In this case, the authors clearly showed that there is significant evidence for fish consumption decreasing the risk of depression.
Another strength is the number of studies that were included. This can help to reduce sampling error and improve the reliability of the outcomes.
However, there were still some limitations of the study.
One was that the authors were only considering observational studies.
As I’ve mentioned many times, observational studies can be powerful, but they don’t’ test cause and effect.
So, the relationship the authors found could be the result of something else.
For example, people at risk of depression might tend to eat less healthy food in general, which could mean that they eat less fish.
If that was the case, then the observed results could still be present, but the cause would be human behavior, not the impact of fish.
A second limitation was that the authors looked at differences between the highest and lowest levels of fish consumption in the studies.
This means that there is little indication about how much fish caused the observed effects, or even what type of fish. Likewise, it isn’t possible to know how frequently participants consumed fish without looking at every study individually.
A final limitation was that there were differences between the studies.
This includes differences in the amount of fish consumed, and also differences in depression diagnosis and measures.
The fact that studies define and measure depression differently does make the overall analysis less viable.
Mechanism for Fish Affecting Depression
The authors didn’t talk about the topic much, but they did highlight one potential mechanism for how fish could affect depression.
This focused on the presence of omega-3 fatty acids in fish, and their potential role in the brain.
Indeed, some research studies have associated omega-3 fatty acid consumption with decreased depression risk and depressive symptoms (4,5,6,7) although not all studies have found this outcome (8,9,10).
Take Home Message
No research study is perfect, and despite its limitations, this particular study was well-designed and provides valuable information about fish consumption and depression.
The nature of the study means that there isn’t enough information to give people specific advice on how much fish (or what fish) to eat to decrease their depression risk.
Nevertheless, the study does support the idea that fish consumption can decrease the likelihood of people developing depression.
So, this is one more study that suggests that people should consume fish on a regular basis.
What do you think? Does a connection between fish and depression make sense, or is it just an artifact of the observational approach?