Fresh is best

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

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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 oxidization. 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% 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."