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What you need to know about your vitamin D levels in winter

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Vitamin D is essential for bone health, and exposure to winter sun is a good way to maintain it.

You may have read recent reports about a worldwide “epidemic” of vitamin D deficiency and wondered if that applies in New Zealand.

Vitamin D is present in some foods, but the bulk of it is synthesised in our skin when we are exposed to sunlight. About 5% of adults in this country are deficient, but a further 27% have below the recommended level, according to figures from the Ministry of Health. Inevitably, our levels vary between summer and winter, unless we spend a lot of time outdoors.

When Pamela von Hurst, of Massey University’s school of sport, exercise and nutrition, measured the winter vitamin D levels of primary schoolchildren, she was heartened by the results.

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“But those kids were spending the whole of lunchtime, even in winter, running around outside in shorts and T-shirts,” says von Hurst, who suspects it would be a different story for adolescents, who aren’t forced outdoors in their lunch breaks and tend to be dressed in hoodies and long sleeves.

Von Hurst says some young schoolchildren had lower levels because they had darker skin, which reduces the dermal production of vitamin D, or because they were less active and had excess body fat.

Vitamin D is essential for the health of our bones and teeth, but it has also been linked to everything from better brain performance and improved cardiovascular health to a lower risk of cancer and a stronger immune system.

Some of the more conclusive evidence comes from research into the immune system, particularly respiratory diseases.

“The picture is becoming clearer and more positive there,” says von Hurst. “If you’ve got good vitamin D status, you’re less likely to develop a respiratory illness and more likely to recover better if you do get one.”

Vitamin D’s role in cancer prevention and cardiovascular health is less clear, because these conditions develop over a far longer period, meaning clinical trials struggle to deliver conclusive results.

Pamela von Hurst. Photo/Supplied

“But how much evidence do you need before you make sure you get enough vitamin D?” says von Hurst. “Sunshine is free, supplements are cheap, so why not?”

The evolutionary story is that, as cave dwellers, humans would spend summers hunting and gathering food and building up body fat, then, in winter, retreat to caves and slowly burn the fat, releasing vitamin D. These days, life isn’t like that. Many of us spend the bulk of our time indoors and, when we do go outside, we coat ourselves in sunblock to protect against skin cancer.

There is no evidence this is causing a deficiency, but it may be affecting our summer high of vitamin D, meaning we don’t have enough stored to get us through winter. If colds and flus strike in August and September, for instance, there is a possibility that low vitamin D is part of the reason.

For pregnant women who are planning to breastfeed, it is particularly important to have an optimum level of vitamin D.

Exposing large amounts of skin to the sun – arms and legs rather than just the face – for short periods is the best way to increase your vitamin D level. Before 10am and after 4pm in summer and the middle of the day in winter are the best times to accomplish this. There isn’t an exact prescription for the length of exposure required as lots of factors come into play, such as skin pigmentation, age and location.

Food sources include oily fish, dairy products and fatty cuts of meat. However, if you are deficient, diet alone won’t improve your status.

Von Hurst says she doesn’t usually take supplements, but she doses herself with vitamin D in winter. Those who are at higher risk of deficiency – people with darker skin, those living in the south of the country, or those who cover up for religious reasons – may want to consider following her example. Some supplements are extracted from sheep wool, but there are also algae-based products suitable for vegans.

If you are concerned about vitamin D levels over winter, you can request a blood test at Labtests without a GP referral. It costs $52.

This article was first published in the June 22, 2019 issue of the New Zealand Listener.