Caution urged over taking fish oil supplements in pregnancy

26 July 2016

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Fish oil supplements


Highly oxidised (“off”) fish oil fed to pregnant rats caused almost 30 percent of their newborns to die, a New Zealand study has found.

The researchers wanted to investigate the health effects of “off” fish oil after an earlier study they did found most fish oil supplements sold in New Zealand were off to some degree.

Female rats in the latest study were given either unoxidised fish oil, a highly oxidised fish oil, or water (the control group) daily throughout pregnancy. Almost 30 percent of baby rats born to mothers who had highly oxidised oil died within the first two days. Rat pups in this group were eight times more likely to die than those in the control group.

Giving pregnant rats the unoxidised fish oil did not increase mortality rates in their babies, indicating that the lethal effect on newborns came from the chemicals that omega-3 fatty acids break down into during oxidation.

Omega-3 fatty acids are known to be chemically fragile or “unstable”, and can easily break down when exposed to natural conditions such as light, heat and oxygen.

The earlier, highly read study, published in Scientific Reports last year, tested 36 brands of fish oil supplements capsules. It used the international industry standard tests of oxidation. Eighty-three percent were oxidised beyond international recommended levels. How “off” they were had nothing to do with best-before date, price, or the country they came from.

Four studies from North America, South Africa and Europe have also uncovered high levels of oxidation in fish oil supplements.

“Once we discovered so many supplements were oxidised, we decided to focus on the health effects of oxidised fish oil during and after pregnancy,” says research fellow Dr. Ben Albert, from the Liggins Institute at the University of Auckland.

“Pregnancy is a critical period when considering the safety of medications, and the same should apply to dietary supplements. Chemicals that may be harmless to mothers could potentially disrupt developmental processes in the womb.”

“We were surprised by the death rate,” says study lead Professor Wayne Cutfield, also from the Liggins Institute. “We’d expected some negative health effects on the rat offspring, but we didn’t expect them to die.”

 “We don’t know exactly why the newborn rats died,” Dr. Albert says. “Because we didn’t expect them to die, we didn’t design the study to look for reasons.”

At weaning, the mothers given oxidised fish oil also had greater insulin resistance, which in humans can lead to type 2 diabetes.

Dr. Albert emphasises that the results of this study cannot be directly applied to humans.

“Obviously, rats are not humans. Also, it’s important to note that the fish oil dose we gave to the rats was higher than doses humans take, but it was the dose commonly used in fish oil studies in rats.

“Also, the level of oxidisation was at least double what we detected in most products in our earlier study of fish oil supplements. Commonly, a more potent formulation is given to determine if there is any effect. Then, you design follow-up experiments to test different doses at different degrees, to see exactly when the effects start to show up,” he said.

“In future studies, we hope to examine what happens in pregnant rats when you vary how oxidised the fish oil is, and to understand exactly how the oxidised fish oil harms the baby rats.”

Up to one in five New Zealand women take fish oil supplements during pregnancy, according to the latest estimate.

“While some women take fish oil during pregnancy to try to improve the development of their child’s brain, there’s no convincing evidence that this helps,” says Dr. Albert.

“Oxidised fish oil is unlikely to carry serious health risks in humans,” says Professor Cutfield.

“But at the moment, we just don’t know the health risks to the unborn baby. And our first study showed it’s not possible to know if fish oil is oxidised when we buy it. This suggests it may be wiser for women not to take fish oil supplements in pregnancy.

“Clearly, further research is needed to assess any risk to humans,” he says. “In the meantime, pregnant women might consider eating fresh fish for omega-3 oils.”


Key points:

  • Main finding: 30 percent of rat newborns died within two days of birth after their mothers were fed oxidised (“off”) fish oil during pregnancy
  • Giving unoxidised fish oil to pregnant rats did not increase mortality rates in their offspring
  • Follows earlier study by the same team that found most fish oil supplements on the market in New Zealand were oxidised to varying degrees in the sample tested; the degree of oxidisation had nothing to do with price, use-by date or country of origin
  • Five independent studies, including ours, conducted across four continents that all show frequent oxidation above recommended limits
  • Omega-3 fatty oils, for which fish oil supplements are taken, easily break down (oxidise) when exposed to natural conditions such as light, heat and oxygen
  • Further research is needed to assess any potential risk of “off” fish oil supplements to pregnant women and their babies