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Mum's high-fibre diet may protect child against asthma

Asthma clue Eating a high-fibre diet during pregnancy changes a mother's gut bacteria, which in turn may help protect her unborn child from developing asthma, new research suggests.

Bacterial breakdown of fibre in the gut produces chemicals that dampen the activity of inflammatory genes in the foetus, report the researchers in today's issue of Nature Communications.

"This is the best evidence to date that a high-fibre diet can protect against the development of asthma," says lead author, Dr Alison Thorburn, an immunologist and microbiologist from Monash University.

Asthma and other inflammatory diseases are less prevalent in cultures that eat a high-fibre diet, she says.

For example, previous research has found that children from Burkina Faso in Africa, who have low rates of asthma, eat a high-fibre diet and have move diverse gut microbes than children from Italy.

Importantly, adds Thorburn, these gut microbes enable the African children to digest fibre and produce high levels of the short-chain fatty acid acetate, which is a known anti-inflammatory molecule.

But, she says, asthma also tends to run in families, and mostly affects children -- one out of every four children have asthma in Australia.

This suggests it is inherited, but there is no asthma gene as such.

Thorburn wondered whether a low-fibre diet in Western mothers was still somehow predisposing their unborn children to asthma, and whether a mother could protect her children from asthma by switching to a high-fibre diet.

High-fibre diets

To investigate this question, Thorburn and colleagues carried out both animal and human studies.

First they exposed pregnant mice to high, normal or low-fibre diets in their third trimester, and then challenged their pups with an allergen known to induce the equivalent of human asthma in mice.

Those pups whose mothers had been on a low-fibre diet developed the asthma-like condition, whereas those whose mums had been on a high-fibre diet did not.

"It was completely wiped out," says Thorburn. "Something that was coming through from the mother's diet was protecting those offspring."

The researchers confirmed that the pregnant mice fed the high-fibre diet had a distinctive group of microbes that produce the anti-inflammatory chemical acetate.

They then showed that acetate entered the uterus and affected the expression of genes in the offspring involved in inflammation, so that they were less likely to get asthma after they were born.

Epigenetics

"These findings potentially explain one aspect of the 'inheritance' of asthma, via diet and epigenetic effects," write the researchers.

Epigenetics involves chemical modification of genes, changing the way they are expressed.

For example, mothers who smoke have children who are more likely to get asthma and this is known to occur through an epigenetic mechanism -- chemicals in smoke activate genes involved in inflammation.

In addition to their animal study, Thorburn and colleagues carried out a small human study that analysed the blood of 40 pregnant women.

They confirmed that those women who ate more fibre had more acetate in their blood and found those who had high levels of acetate were more likely to go on and have children protected from asthma symptoms.

Thorburn and colleagues say their findings could explain the low incidence of asthma seen in children growing up on a farm.

"[This] may relate to dietary differences between rural and urban settings, or may relate to microbes encountered in the farm environment that are geared for high [short-chain fatty acid] production (that is, faeces from livestock that mostly digest fibre)," write the researchers.

Caesarean risk?

Some researchers believe that children born by caesarean section are more prone to developing asthma because they are not exposed to the mother's gut microbes via the process of natural birth.

However, Thorburn and colleagues tested this assumption by delivering pups from mothers fed high-fibre diets by caesarean and then raising them with foster mothers.

These pups were never exposed to their original mothers' gut microbes through the birth process or through breast feeding, yet were still protected from asthma.

Thorburn says caesareans may not be linked to asthma after all, rather the asthma seen in some children born by caesarean could have been brought on by the antibiotics, given to women during emergency caesareans, which would affect the gut bacteria in their infants.

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