“Both of those pieces of evidence are not hooking up at the moment,” said Professor Clara Chow, president of the Cardiac Society of Australia and New Zealand.
“The evidence from clinical trials has been confusing.”
How much fish should you eat?
The Heart Foundation recommends eating two to three serves of fish a week, with evidence suggesting diets high in fish cut the risks of heart disease, stroke, eye degeneration and dementia. That should provide between 250 and 500 milligrams of omega-3 fish oil per day.
Fish that contain more than 500 milligrams of fish oil per 150 gram serve:
- Atlantic salmon
- Australian salmon
- Fish that contain 300 to 400 milligrams of fish oil per 150 gram serve:
To try to resolve the issue, a team of scientists from the Madras Diabetes Research Foundation compiled data from four very large studies, allowing them to track a huge pool of patients around the world for nearly a decade.
The study participants each filled in food surveys asking them how much fish they ate. The team then correlated their health outcomes.
The data, published on Tuesday in JAMA Interal Medicine, suggested a very slight protective effect.
But the signal wasn’t strong enough to be conclusive.
There are several theories why.
The underlying studies were not designed primarily to look at the link between fish and heart disease. Also, people are notoriously bad at remembering what they ate last week.
And there is some evidence fish works best to prevent heart attack, rather than overall cardiovascular disease, which the scientists were measuring.
“I don’t think it’s a very good paper,” said Professor Nicholls.
Or perhaps, as several experts suggested, not all types of fish are equal.
Grilled fish is likely more healthy than, say, fish and chips or a fish sandwich.
“Most of the fish eaten in south-east Asia is fried – and that probably does not give you cardiovascular benefits,” said Professor Thiruma Arumugam from the Centre for Cardiovascular Biology and Disease Research at La Trobe University.
And some fish, like salmon and mackerel, contain much higher levels of omega-3s than others. In a small subset of patients studied, oily fish seemed to be strongly proactive while other fish did not.
A 2019 study found patients with heart disease or diabetes given four grams of the fish oil EPA daily – a very high dose – had significantly lower rates of heart attack or death.
When his patients tell him they are taking fish oil, Professor Nicholls asks to see the bottle.
“There is fish oils and there is fish oils. Most you can buy over the counter really contain very low doses of omega-3s,” he said. “If you’re taking them to prevent a heart attack, most of those preparations won’t do that.”
What’s the bottom line?
Professor Nicholls said the Heart Foundation’s guidelines are still the right ones.
“A healthy diet includes the consumption of two to three servings of fish a week.”
Science and health explained and analysed with a rigorous focus on the evidence. Examine is a weekly newsletter by science reporter Liam Mannix. Sign up to receive it every Tuesday.
Liam is The Age and The Sydney Morning Herald’s science reporter