With clinical trials and meta-analyses failing to support significant health benefits from fish oil and omega-3 supplements in reducing all-cause mortality and preventing various chronic diseases of aging, fish is back in the news. The news reported on a meta-analysis of data from 38 published observational and intervention studies from 15 countries, involving nearly 800,000 people. The study was a head spinning stream of contradictions and junkscience favorites that may be fun to read.
Eating mackerel, sardines or other oily fish two or more times a week can curb your risk of stroke, new research suggests…
Those who ate two or four servings of oily fish a week had a 6% lower risk of cerebrovascular disease (stroke or mini-stroke), compared to those who ate less than one serving of fish a week. Researchers found the more fish people ate, the lower their risk of stroke. Men and women who ate five or more servings a week had a 12% lower risk.
The study authors said they’re not totally sure why adding oily fish to the diet helps keep strokes at bay, but think it could be because of interactions with the vitamins and essential amino acids. They also said it could be because fish is replacing other foods, such as red meat, which are bad for vascular health. It could also be that fish-eaters tend to have healthier diets generally.
These were null findings. The pooled relative risks were weak, untenable and no greater than might have been found by random chance. That should have ended speculations about a valid causal link right there. But it didn’t.
The study published in the British Medical Journal had the same problems plaguing many meta-analyses, such as lumping together studies of differing methodologies; no consistency among the 38 study databases in how the data was gathered, measured, reported or interpreted; or in their scientific rigour and duration of follow-up. None of the clinical trials culled from had been designed to investigate strokes as a primary outcome, either.
The authors reported that their review demonstrated “moderate, inverse associations of fish and long chain omega 3 fatty acid consumption with risk of cerebrovascular events.” In contrast to their conclusions, however, the authors reported that “dietary, circulating biomarkers and supplements of long chain omega 3 fatty acids were not significantly associated with risk of cerebrovascular disease.” So, there was no support for omega 3 fatty acids being the explanation.
Their untenable associations were then turned into causation and then reverse causation, with advice to eat more fish to lower stroke risks.
The authors went on to speculate as to how eating fatty fish might help reduce strokes through some unidentifiable, complex interplay of vitamins, amino acids and trace elements in fish. There is no evidence to support such specious claims, however. Never the less, the implications of their findings, they wrote, support beliefs in the preventive effects of eating all-natural whole foods, as well as fish and fish oils:
Our findings are in line with current dietary guidelines (that is, to encourage fish consumption for all; and intake of fish oils, preferably from oily fish, to people with pre-existing or at high risk of coronary heart disease) and favor propositions that future nutritional guidelines should be principally “food based.”
The more plausable explanation for professed correlations between eating seafood and better long term health outcomes is that fish consumption is a well known marker for people who are generally healthier to begin with and of higher socioeconomic status, both of which have been linked with better outcomes.