Taking vitamin D supplements during pregnancy and giving them to babies may help reduce New Zealand's high rate of allergies in children, important new research indicates.

The trial results also suggest, for the first time, that the supplements could even help reduce child asthma. About a quarter of New Zealand children have asthma, a condition that can be triggered by allergies, including to house dust mites.

Auckland University researchers have linked vitamin D supplementation to lower rates of sensitisation to house dust mites.

In a trial of 260 pregnant women, the participants were randomly assigned to receive low or high dose vitamin D drops or a placebo. Once each baby was born, he or she continued, until 6 months old, in the same arm of the trial that the mother had been in. At 18 months old, the babies had a blood test to measure antibodies to allergens, including house dust mite allergen, known to be involved in respiratory diseases.

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The proportion who were sensitised to house dust mite allergen was significantly lower in the vitamin D groups than in the placebo group, according to results of the trial, published in the international journal Allergy.

"Based upon a careful review of the records of the children's visits to their family doctor, we also saw that this vitamin D supplementation reduced the proportion of children making primary care visits which their family doctor thought were due to asthma," said one of the researchers, Associate Professor Cameron Grant, a Starship hospital paediatrician.

"Diagnosing asthma at such a young age is a difficult thing to do so we have to take this family doctor visit data with a grain of salt and certainly could not say for sure that the study shows the vitamin D prevents asthma," says Dr Grant.

"But it's the first study to show that correcting poor vitamin D status during pregnancy and infancy might prevent childhood asthma."

More than half of New Zealand newborns have low levels of vitamin D. Dr Grant said vitamin D supplementation was likely to be helpful only in those who were deficient.

New Zealand health authorities have a difficult task in balancing public advice to avoid excess sun exposure to reduce skin-cancer risk, while still getting a healthy amount of sunlight to produce enough vitamin D in our skin. Out diet is low in vitamin D and vitamin D supplements are not widely used.

Dr Grant recommended that all pregnant women and young infants take a vitamin D supplement during winter and spring.

But there was limited local availability of vitamin D supplements although they can be imported through internet shopping; the drops used in the trial were imported specially.

"We do need to have available products that are appropriate for pregnant women and young children and that are easy to give. There's a lot of work to be done to address this issue."

Vitamin D supplements

• Auckland trial of vitamin D supplements versus placebo in pregnant women and their babies

• Vitamin D was linked to babies having lower rate of being sensitised to house dust mite allergen

• House dust mite allergy can trigger asthma

• Researcher recommends vitamin D supplements for all pregnant women and young infants during winter and spring