In winter, we need to go out of our way to catch enough rays to keep up vitamin D production.
In winter, we need to seek enough sun to maintain vitamin D production.
How long does vitamin D last in the body? Do we need daily sun exposure to keep levels topped up, or is once a week in the sun sufficient?
Vitamin D has long been known for its important role in bone metabolism. However, mounting evidence links vitamin D levels to the severity and frequency of respiratory infections in children and adverse pregnancy outcomes. Recent research also delves into its possible links with colon cancer and autoimmune disorders, such as multiple sclerosis and diabetes mellitus. But the problem is how to balance vitamin D production in the body with protecting our skin from sun damage?
Vitamin D is found in small amounts in foods such as oily fish – for example, salmon, tuna, sardines, eel and warehou – milk, dairy products, eggs and liver. Some margarines, spreads, dairy substitutes and liquid-meal products also contain added vitamin D.
However, food sources don’t meet our vitamin D needs, which leaves us dependent on our skin producing vitamin D when exposed to sunlight. Unfortunately, that same exposure can damage our skin.
Pamela von Hurst, the co-director of Massey University’s vitamin D research centre and an adviser to the Ministry of Health, says the vitamin lasts just over a month. “Some research shows it has a half-life of up to 90 days, but other studies show a half-life of 28 days.” A possible reason for the wide disparity may be variations in individual health and dietary intake, she says.
“If your calcium intake is really low, you’re going to be using a lot more vitamin D because it’s going to be required for all the roles that it has around regulating calcium levels. If you’re ill, you’ll use a lot more vitamin D,” says von Hurst.
Our body’s ability to stockpile vitamin D may partly explain why South Islanders tend to have similar vitamin D levels to people in other regions, even if they are exposed to lower ultraviolet radiation levels in winter. “South Islanders build up really good stores in summer. They get a lot more sun – as a rule it’s a lot warmer, sunnier and drier in summer than it is in the north.”
So, how much sun exposure should we aim for to optimise our vitamin D levels without endangering our skin? “Frequent, brief exposure would be the recommendation, and never put yourself at risk of burning,” says von Hurst.
In winter, expose your arms, face and neck during the middle of the day. About 30 minutes is ideal and unlikely to cause sunburn. “Go for a nice brisk walk that warms you up
and you’re more inclined to bare a bit of skin,” says von Hurst.
“In the South Island, the sun is strong enough that you’ll still make vitamin D. But given the weather’s often not sunny and it’s often too cold to expose enough skin, perhaps some supplementation in addition to sun exposure is advisable through winter.”
The maximum dosage of vitamin D supplement available over the counter is 1000IU, says von Hurst. “Anyone who is at risk of a deficiency should be taking 2000IU daily for the sake of their bones.”
Between September and April, sun protection is recommended – shade, a hat and clothing that screens the face and neck, along with sunscreen and sunglasses – especially between 10am and 4pm. A daily walk is still a good idea, but instead of midday, aim to be outdoors in the early morning or late afternoon.
Von Hurst has the perfect solution for her sun exposure this winter. She’s going to Spain for an international vitamin D workshop in Barcelona to present findings from a recent clinical trial investigating links between autism and levels of vitamin D and omega-3. Although she can’t yet reveal the research findings, she says the trial produced some, “quite interesting results”.
In winter, expose your arms, face and neck for about 30 minutes during the middle of the day.
Pamela von Hurst: vitamin D lasts about a month.