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Fish oil oxidises and turns rancid more easily than other oils, as it is chemically unstable. Photo / 123RF

Pregnant women taking fish oil capsules should buy them in small lots and keep them in the fridge, a group of medical researchers say. By Nicky Pellegrino.

Omega 3 fatty acids are essential in the diet and particularly important for pregnant women, as there is growing evidence that consuming adequate amounts may reduce the risk of the child developing metabolic problems, such as diabetes, later in life. Fatty fish is the best source and many pregnant women choose to take fish oil supplements. However, caution is advised, say a group of scientists from the University of Auckland's Liggins Institute.

Fish oil oxidises and turns rancid more easily than other oils, as it is chemically unstable. In 2015, institute researchers tested 32 products and found that more than half exceeded the recommended levels of oxidation. The question that remained was how much that matters for the pregnant women who may be taking them.

An animal-based study was conducted. Pregnant rats were given fish oil in a range of doses and oxidisation. All the animals were healthy during pregnancy and the pups looked well when they were born, says paediatric endocrinologist Ben Albert. But in the group that received human-relevant doses of highly oxidised oil, 6 per cent of the offspring died within the first two days of life.

The theory is that the rancid oil may have affected the quality or volume of the mother's milk, although that isn't known for sure at this point.

So, how can consumers tell whether a fish oil supplement they are buying is fresh? "To be honest, I think you can't tell," says Albert. "Oxidised oil doesn't necessarily smell any different and often flavourings have been added. In an earlier study, we looked at whether the price of the oil, best-before date and country of origin made a difference, and it really didn't."

The fish oil in capsules has a long supply chain. Much of it comes from Peruvian anchovies and is processed in China before possibly being transported to Australia or New Zealand to be packaged. It isn't known where exactly along that path any oxidation might occur, but since heat and light speed the process, Albert says it makes sense to be careful how products are stored at home.

"Buy a small container that you're going to finish in a short amount of time, rather than a huge one," he advises. "And keeping fish oil capsules in the fridge is a really good idea."

The natural health product industry has been quick to respond to this latest Liggins Institute research. It has said that the oils given to the pregnant rats were artificially oxidised to higher limits than would ever be found in products on New Zealand shelves and called the study "flawed and of little value to anyone".

But Albert stands by the work and believes the industry is missing a "good news" story.

The study showed that even at the maximum recommended limits of oxidation, the oils were fine, he explains. "We saw no sign of unwellness in any of the animals. That suggests those recommended levels are appropriate and the oils are safe."

Liggins Institute research has shown that even when pregnant rats are fed an unhealthy high-fat diet, consuming the Omega 3 in fish oil can have a protective effect on the metabolic health of their offspring later in life. And it is soon to release the results of a study using fresh, safe fish oil in pregnant women.

"It's important that women try to have more Omega 3 in pregnancy and I certainly wouldn't want anyone to think that if they've taken fish oil, they've harmed their baby, because there's no evidence that is the case," says Albert. "But if you take a fish oil product, you want it to be within the safe limits of oxidation and we should have a system that enables people to confidently buy one that is."

The Ministry of Health advises pregnant women to eat fish to supply essential fatty acids. It warns that increased mercury levels in long-lived and larger fish can be unsafe for the baby, but says there is little concern with canned fish such as tuna, salmon and sardines.

The advantage of eating fresh or cannned fish, says Albert, is that if the fish is going off, you'll be able to tell and won't eat it.