Pregnancy food allergy claims unfounded

“Mothers-to-be can reduce the chances of their babies developing food allergies by eating a diet rich in oily fish and nuts”, The Daily Telegraph has today claimed. The newspaper says that researchers found that when mothers-to-be ate a diet high in a particular group of polyunsaturated fatty acids this “allowed more broken down food substances and bacteria to pass into the blood stream”. In turn, they say this would trigger the baby's immune system to produce antibodies.

However, this report is actually based on an animal study looking at the effect of feeding pregnant pigs a diet rich in omega-3 fatty acids. It found that at 28 days after their birth, the piglets whose mothers were fed a diet rich in the omega-3 fatty acids in pregnancy had more ‘permeable’ intestines, allowing more substances to pass into the blood. However, scientists did not look at the effect of this on allergy or any other health outcomes in the pigs. In the article, the authors themselves acknowledge that they do not know whether these changes would be beneficial or harmful.

Overall, it is also not clear to what extent these findings represent what would happen in humans and do not provide sufficient evidence on which to base any dietary advice about omega-3 fatty acids for pregnant women.

Oily fish are one source of omega-3 fatty acids, but it is important to remember that pregnant women are advised to consume no more than two portions of oily fish a week due to the relatively high levels of mercury it can contain.

Where did the story come from?

The study was carried out by researchers from France’s SENAH centre for Livestock Systems and Animal and Human Nutrition and other research organisations in France. The study was funded by INRA (the French National Institute for Agricultural Research) and was published in the peer-reviewed Journal of Physiology.

The Daily Telegraph’s article places too much emphasis on the potential implications this research has for humans. The earlier parts of the article do not make it clear that this research was in pigs, and use words such as “mother-to-be” and “baby” that could make it seem that the research was in humans or directly relevant to humans. The article only alerts us to the fact that this research was in pigs in the penultimate paragraph.

What kind of research was this?

This was animal research that looked at how feeding omega-3 fatty acids to pregnant pigs affected their offspring’s gut permeability, which is the gut’s ability to allow substances to pass into the blood stream. Omega-3 fatty acids are found in oily fish such as salmon and tuna, and in some plant oils such as linseed oil.

The barrier that stops larger molecules from exiting the gut and entering the blood stream is called the intestinal epithelial barrier (IEB). This barrier is reported to play a key role in the development of the immune system in newborns, as it regulates whether certain molecules can enter the blood stream.

The researchers say that if the barrier allows greater amounts of molecules to get through (is more permeable) this runs the risk of exposing the body to more toxins and increases the risk of inflammation. However, the greater exposure to molecules in the blood that comes with having greater permeability may also allow the immune system to start to build up a tolerance to molecules absorbed from foodstuffs.

The researchers say that the effect that maternal diet in pregnancy has on the permeability of this barrier is not well understood. They also say that omega-3 polyunsaturated fatty acids (PUFAs) have a beneficial effect on gut inflammation disorders, and that a small trial in humans showed that omega-3 fatty acid supplementation during later pregnancy and lactation may reduce risk of allergies in the first year of life in children with a family history of allergic disease. On the basis of these results the researchers wanted to focus on how newborn pigs’ IEB permeability was affected by their mother’s consumption of omega-3 polyunsaturated fatty acids during pregnancy and lactation.

It would not be possible to carry out this type of research in humans and the differences between species mean that the results may not be fully representative of what happens in humans.

What did the research involve?

The researchers fed 12 pregnant pigs either a lard-based diet (the control group) or a diet based on linseed oil, which is high in omega-3 polyunsaturated fatty acids. The diets provided the same amount of calories and fat. The pigs received this diet during pregnancy and lactation. The researchers then tested intestinal permeability in the piglets at birth, and at 3, 7, 14, 21 and 28 days after birth.

The researchers also carried out various experiments to test whether the gut nervous system might be involved in any changes to intestinal permeability seen. This included looking at the effects of maternal diet on gut nerves and response to chemicals targeting the nervous system in the piglets, and the effects of omega-3 fatty acids on rat gut nerve cells in the laboratory.

What were the basic results?

The researchers found that intestinal permeability in all the piglets increased up to day 14 after birth, and then decreased. However, at 28 days the piglets whose mothers had been fed the omega-3 rich diet had higher intestinal permeability.

The researchers found that chemicals targeting the nervous system produced different effects on the intestinal permeability of those piglets whose mothers were fed the omega-3 diet and those whose mothers were fed the control diet. One chemical increased intestinal permeability in the control piglets but not the omega-3 piglets, while another chemical decreased intestinal permeability in omega-3 piglets but not control piglets.

The omega-3 piglets also had different proportions of different types of gut nerves from the control piglets. One derivative of omega-3 fatty acid was also found to have a similar effect on the proportions of different types of rat gut nerves when added to them in the laboratory.

How did the researchers interpret the results?

The researchers concluded that feeding pregnant and lactating pigs an omega-3-rich diet increases intestinal permeability in their offspring. They say that this is likely to be due to changes in the nerves supplying the gut. They also say that “the beneficial versus harmful consequences of this increased intestinal permeability remain to be elucidated”.

Conclusion

This study suggests that feeding pigs an omega-3-rich diet during pregnancy can influence intestinal permeability in their offspring after birth. However, despite press coverage suggesting that the study was of importance to pregnant women, it should be noted that it is unclear to what extent the changes seen in these pigs would be representative of what would happen in humans. Also, the study did not explore what, if any, knock-on health effects these changes would have on pigs.

Within the research paper the authors themselves acknowledge that they cannot tell whether the changes would be beneficial or harmful to health. This is in contrast to the claim attributed to one of the study’s authors in The Daily Telegraph, which says that they feel their research adds to the evidence that consuming fish or walnut oil during pregnancy will “accelerate the development of a healthy immune system to ward off food allergies". This claim is not supported by this current research, which is of limited scope.

In light of these major limitations, this research is not sufficient basis on which to suggest any dietary advice about omega-3 fatty acids for pregnant women. One source of omega-3 fatty acids is oily fish. Currently in the UK, pregnant women are advised to avoid eating more than two portions of oily fish a week as these types of fish can contain high levels of mercury.