Editorial: Omega-3: Best taken with a big pinch of salt
IF I told you that one cheap pill could boost your brain power, protect you from heart disease and cancer, and even alleviate depression, all with no known side effects, would you want it? Who wouldn’t?
You’ve probably heard of the pill’s main ingredient: omega-3, a substance found in fish oil and other natural products (see diagram). If the flood of headlines and adverts from food and supplement manufacturers are to be believed, you need only boost your intake of omega-3 and all these benefits will be yours.
Advertisement
Omega-3 supplements first appeared in the early 1980s. Given they are still going strong 30 years later, you would be forgiven for thinking that claims of their beneficence have all been substantiated. Yet several new studies, as well as recent reviews of existing evidence, call this received wisdom into question. So before you splash out on supplements and food fortified with extra omega-3 it might be worth taking a closer look at the evidence. Do any of the claims stand up under scrutiny?
Omega-3 is the name of a family of fatty acids made of chains of carbon atoms of varying length. They cannot be synthesised in the human body, and so must be obtained from our diet. Three members of the family are particularly important to human health. Short-chain alpha-linolenic acid (ALA) is a key molecule found abundantly in green leafy vegetables, walnuts and flax (linseed), rape (canola) and soybean oil, broccoli and algae. It is the vital precursor molecule that gets converted by all mammals into two important long-chain relatives in the omega-3 family, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
DHA is vital for human health and is present in membranes of many different types of cells, in particular in the brain and retina. EPA has anti-inflammatory effects. Humans convert between 5 and 10 per cent of ALA to EPA and less than 4 per cent of ALA to DHA. DHA and EPA can also be obtained directly by eating animal products, particularly seafood. Algae make large amounts of EPA and DHA and these fatty acids accumulate up the marine food chain, with the highest levels found in predator fish like mackerel and tuna.
Eye problems
Research into the role of essential fatty acids began in the 1920s. Early studies found that primates deprived of ALA showed depleted levels of DHA, which caused abnormalities in retinal function and visual acuity. Other symptoms included extreme thirst and exceptionally dry hair and skin.
The area was reinvigorated in the late 1970s when Nobel laureate John Vane showed that prostacyclin – a molecule involved in preventing blood from clotting – could be derived from EPA. A flurry of interest followed, including a key paper by Daan Kromhout, published in The New England Journal of Medicine in 1985, showing that even a modest intake of fish was associated with low rates of heart disease (). Several subsequent large studies have supported the finding that omega-3 can reduce the chance of a repeat heart attack.
Omega-3’s role in protecting people against heart disease seemed clear-cut – at least it did until Lee Hooper, a nutrition researcher at the University of East Anglia, UK, reviewed the results of many of these studies. In 2006 she published her controversial conclusion: “Long-chain and shorter chain omega-3 fats do not have a clear effect on total mortality combined cardiovascular events, or cancer.”
“It might work in some groups of people, but not necessarily for all,” she says, and points out that one study, carried out by the UK’s Medical Research Council Epidemiology Unit in Cardiff, found men who had angina were more likely to die if they were taking fish oil supplements than when on a placebo or eating oily fish. “It wasn’t a perfect study but these results worried us,” she says.
However, in 2008 a joint report by the World 91ɫƬ Organization and the UN Food and Agriculture Organization (FAO) on fats and fatty acids concluded that omega-3 may contribute to the prevention of coronary heart disease. Tom Sanders, a nutrition researcher at King’s College London, who contributed to the WHO report, says that since Hooper’s review was published two large studies have shown that purified EPA and DHA can ward off cardiovascular disease. He says an analysis of studies focusing on those who have already had one heart attack generally indicate that if you take oily fish or fish-oil supplements for around two years, the risk of having a second heart attack is reduced by around 18 per cent – even taking into account other confounding variables, such as obesity and smoking.
Sanders, who acted as a consultant for the FAO/WHO report, says the latest analysis of randomised controlled trials shows that taking fish or fish-oil supplements also reduces the risk of having any form of cardiovascular disease by 11 per cent: “We concluded that the evidence is somewhere between probable and convincing that omega-3s can protect you from cardiovascular disease.” Hooper though still stands by her 2006 conclusion. “We just don’t have good enough evidence,” she says.
Sanders admits that we still don’t know how omega-3s may confer this protective effect. Because the trial used fish oil, it is still unclear whether any protective role was down to EPA, DHA, ALA or some combination of these.
There is certainly no shortage of claims when it comes to other benefits of omega-3s, yet they are even harder to back up with solid evidence, let alone explain. For example, Joseph Hibbeln, a psychiatrist from the National Institutes of 91ɫƬ in Bethesda, Maryland, has found a correlation between the amount of fish a country eats and the rate of depression there. “There is a large difference in the risk of depression predicted by how much fish people consume,” he says. “I was ecstatic when I got these results.”
Germans, he found, eat less than 10 kilograms of fish per person per year, and 5 per cent are depressed. By contrast, the Japanese eat around 65 kilograms, and less than 1 per cent of them are depressed. This has led some researchers to suggest that omega-3 supplements could be used to cure depression. Hibbeln has even linked rates of fish consumption with murder rates in a paper published in the journal Lipids in 2004 and believes eating oily fish can reduce aggression, citing a paper published in The British Journal of Psychiatry, which made the claim that adding omega-3 from fish oil plus minerals to prisoners’ diets reduced levels of violence.
Sanders and Hooper say these correlations have been vastly over-hyped. “These claims take the results much further than the science actually warrants,” says Sanders. The old mantra, that correlation doesn’t imply causation, should be heeded in other cases too (see “Omega-3 myths”).
“The claims take the results much further than the science warrants”
One of the most common claims for omega-3s is that they can boost brain power. In the UK, Madeleine Portwood from Durham County Council and Alex Richardson from the physiology, anatomy and genetics department at the University of Oxford carried out a trial on nearly 300 children, half of whom took a placebo while the other half took omega-3 fish oil supplements. Portwood and Richardson assessed the children’s reading, writing and ability to concentrate and found that there was a significant improvement in those taking omega-3 – but this was seen in only 40 per cent of the children on supplements.
A further study run by Portwood and Durham County Council concluded that pupils who took omega-3 supplements in the year before their GCSE exams got better results than those who did not. Sanders thinks this second study is virtually worthless because there was no placebo control group. “No conclusions can be made about the cognitive benefits of fish oil,” he says. As things stand, the claim that taking fish oil can improve mental performance is little better than speculation. None of these results has been replicated and systematic reviews of the evidence fail to find consistent proof of the claims made. A paper published last month reported the results of a two-year double-blind placebo-controlled study, , that tested the effects of omega-3 supplements on cognitive ability in a group of 867 people aged between 70 and 80. People on omega-3 supplements showed no improvement over the placebo group.
So, apart from possibly protecting you from cardiovascular disease, do omega-3s have any other positive effects? Hooper says omega-3s probably help reduce joint pain caused by rheumatoid arthritis. She points to a review paper published in 2007 in the journal Pain, covering 17 randomised, controlled trials of omega-3s on joint pain. “There is evidence that omega-3s’ anti-inflammatory properties remove morning stiffness and reduce the amount of anti-inflammatory drugs needed,” she says. “But it doesn’t stop the progression of arthritis and it does not protect the joint or prevent further deterioration. It only dampens down the pain.”
Crucial role
The crucial role of omega-3 during fetal development is well established. Numerous studies have shown that omega-3s, particularly DHA and EPA, are key in the development of the brain and eyes in the fetus and early childhood. As a result, the WHO/FAO report recommends pregnant women slightly increase their EPA and DHA intake from a standard 0.25 grams per day for the average adult, to 0.3 grams per day with at least 0.2 grams being DHA.
Should pregnant women therefore take supplements? Women are better than men at converting ALA to DHA and the fetus is also capable of higher rates of conversion than adults are. Sanders says this means DHA supplements aren’t necessary as long as the expectant mother is eating a balanced diet that includes some omega-3 from fish or vegetarian sources.
In adults, omega-3s do slowly get oxidised, and need to be replaced. The WHO recommendation of 0.25 grams per day should more than adequately cover this. Is it worth boosting your intake beyond that? At present there is slim evidence this would have any other effect than moderate the risk of repeat heart attack, and that can be better achieved by other means. Sanders has studied rates of cardiovascular disease in vegetarians – who generally have very low levels of DHA or EPA in their diet – and says they have far lower rates simply because of their healthier lifestyle and the fact that they are less likely to be smokers.
Even if evidence does eventually emerge to suggest that boosting your baseline level of omega-3s can provide clear-cut health and lifestyle benefits, it’s highly unlikely that there would be enough fish to provide omega-3s for the planet’s burgeoning population. And Sanders thinks it’s probably fine to rely on our natural capacity for converting omega-3s from plants into the long-chain omega-3s that fish oil is so rich in. Just 1 gram of ALA – for example, from 10 grams of rapeseed oil, which is less than a tablespoonful – should meet the WHO’s recommended levels to prevent deficiency. The WHO also says there is “convincing” evidence that taking between 0.25 and 2 grams of DHA per day could help reduce risk of a fatal heart attack, but, as Sanders says, there are other equally effective ways of protecting yourself from that risk.
The bottom line, as with many nutrition-related questions, is to simply maintain a balanced diet.
Editorial: Omega-3: Best taken with a big pinch of salt
Omega-3 myths
ADHD
Claim: omega-3 levels are low in children with attention-deficit hyperactivity disorder and supplements can improve core symptoms
Current thinking: systematic reviews found there were too many inconsistencies between studies to draw any reliable conclusions
Aggression
Claim: countries with a high level of omega-3 in their diet have the lowest murder rates
Current thinking: correlation is not disputed, but critics of the study say this is insufficient evidence to claim causation
Alzheimer’s disease
Claim: DHA may delay onset of Alzheimer’s, based on evidence from rat studies
Current thinking: two recent large studies found omega-3 supplements had no effect on cognitive function in normal ageing or on the incidence and treatment of dementia
Brain boosting
Claim: fish oil supplements significantly improve reading, spelling and behaviour
Current thinking: a systematic review of all studies found insufficient evidence to identify any effect. The largest study to date reported no effect on cognitive function in later life
Cancer
Claim: DHA can reduce tumour size in rats
Current thinking: a systematic review by the World Cancer Research Fund found little evidence of any effect of omega-3s on cancer. The only reliable study suggested eating fish could reduce the risk of thyroid cancer, but the reviewers believe this was almost certainly due to the iodine content of fish, not omega-3
Depression & other mental health problems
Claim: countries with high levels of omega-3 in their diet have low levels of depression and suicide
Current thinking: when results from randomised controlled trials were systematically reviewed, no evidence of effects on mental health, including schizophrenia and bipolar disorder, were found
