Google+ Facebook Twitter Twitter

Omega-3 fats: major reviews, minor effects

Lee Hooper takes us behind the headlines to look at the findings of two systematic evidence reviews of the benefits of omega-3 fats.

In the world of dietary healthcare, omega-3 fats have a solid reputation for their protective and enriching qualities. Derived from fatty fish, nuts, seeds and a few other sources, they are said to offer defence against a range of conditions, including heart disease, cancer and stroke. As a result, but also because the western diet has tended to move away from the natural sources of these fats, omega-3 supplements have become big business – the worldwide market was worth $33bn in 2016 and projected to hit $57bn by 2025.    

However, the accepted wisdom of the benefits of omega-3 fats has been thrown into doubt with the publication of the two new systematic evidence reviews. The first, published in the British Journal of Cancer, has found that omega-3 and omega-6 fats and total dietary polyunsaturated fat have little or no effect on cancer diagnosis and death rates, and may even raise the risk of prostate cancer slightly. The second study, in the Cochrane Database of Systemic Reviews, revealed that the key omega-3 fats (EPA and DHA from fish, and ALA from plants) also have little effect on heart disease diagnoses and deaths – any positive impact is small.    

“Recent large and long-running trials have shown us there is a beneficial effect, but such a small one”

Best evidence

Both reviews were conducted by a team of researchers drawn largely from Norwich Medical School at the University of East Anglia and were headed up by Lee Hooper, a Reader in Research Synthesis and Nutrition.

The impetus for the reviews came from the World Health Organization (WHO): “The question was posed for us by WHO as they want to update their guidelines on polyunsaturated fat, including omega-3s and omega-6s,” says Hooper. “So they naturally wanted to know what the best evidence is on the health effects of all those across a wide range of outcomes.”

The two papers just published are part of a series of reviews looking at the effects of these fats on all sorts of conditions, with diabetes and depression among them. “Our research question came about because they are keen to know whether they should be encouraging people to eat more omega-3s and polyunsaturated fat in general,” says Hooper. “In trying to address their question we embarked on a rather longer adventure than expected.”

The research teams started work on the reviews in 2016 with a trawl of the various databases. “We pulled in trials from as far back as the 1960s. I think those are probably the oldest,” says Hooper. “The late 1950s and early 1960s were when people started to look at fats and health in some detail and they ran some really good trials at that time. Plenty of other trials have been carried out every decade since.” The most recent trial included in the reviews was published in July 2019. In all, they looked at 86 randomised control trials (162,796 participants in all) for the heart disease review, and 47 (108,194 participants) for the cancer review.

In delivering omega-3 and other fats to the participants, most of the trials used supplements. “It’s just much easier to do that than to encourage people to eat more oily fish,” says Hooper. “Of course, we were interested in oily fish trials too, and we pulled them in where we could, but there just aren’t many of them. So the evidence relates more to omega-3 supplements because that’s what was largely given during most of the trials.”

Not delivering

Could the results reflect some key difference between the supplements and the natural sources? “Not that we are aware of,” says Hooper. “There is nothing specific about the supplements. They come in many different forms. Some early trials gave straight oil – people had to drink it, which sounds pretty grim. But there is not a consistent coating, for example, that you could say is a potential issue. In some trials the oil was concentrated and is some it was enriched, but the effects are remarkably consistent in that we don’t see many effects. And even when we do, they are very small. So I don’t think it’s the supplements that are the issue. It’s that omega-3 itself is not delivering what it is believed to deliver.”

This hasn’t come as a huge shock to Hooper, who conducted an earlier systematic review as part of her PhD. “I was surprised then that we saw little effect of omega-3 on cardiovascular outcomes. I was advising people to eat lots of omega-3 because I was convinced they were doing good. Recent large and long-running trials have added a lot of information, but the results still look consistently unhelpful. They have shown us there is a beneficial effect, but such a small one.”

Small effect

What might WHO make of this? “They will look at the evidence in its entirety, including our evidence, possibly balancing it with the evidence from cohort studies. They are also looking at data for pregnant women and children. All of that will come together when they bring out their advice.”

In the meantime where does that leave dietary advice? “The advice to eat at least two portions of fish a week, one of which is oily, is still good. Oily fish has lots of healthy stuff in it besides omega-3. It’s a good source of protein, selenium, iodine and calcium and so on. But I think taking a supplement is another matter. They do bring triglycerides down, but what they don’t seem to do is provide much protection from cancer or heart disease. It’s a small effect for something that can cost people a lot of money. They might be better off spending it on something that might be better for them – such as pair of running shoes!”   

Lee Hooper     

1981, BSc in Biochemistry, University of East Anglia     

1989, Diploma in Dietetics, Leeds Polytechnic     

1989-1999, Heart Health Dietitian, Shropshire,     

1998, Diploma in Systematic Reviews Methodology, UCL & Royal Free Hospital     

1999, Diploma in Advanced Dietetic Practice, British Dietetic Association     

1999-2002, Research Associate, University of Manchester     

2002-2005, Lecturer, University of Manchester     

2005, PhD, Systematic reviews in diet and cardiovascular disease, University of Manchester     

2005-2008, Lecturer, University of East Anglia     

2008-date, Senior Lecturer, University of East Anglia     

2014-date, Reader in Research Synthesis and Nutrition, University of East Anglia.

Image credit | iStock


Related Articles