Minutes of exposure for vitamin D

How many are needed? The answer is at your fingertips ....

Ever wondered how long you need to stay in the sun to maintain healthy levels of vitamin D?

A just-published paper by some of my colleagues in Australia and the UK - but strangely none from New Zealand - has the answer (or at least the best answer available given our current knowledge of the subject) for New Zealand and Australia. They calculated the sun-exposure time needed to maintain levels of vitamin-D, as opposed to calculating the amount of vitamin-D is produced from a given amount of UV radiation. Their approach therefore presumes that the amount of vitamin D circulating in the blood is adequate at the end of summer. A corollary of this approach is that you should try to build up your levels of vitamin-D before the onset of winter when UV levels become low, because it takes several weeks to decay away. Of course, the trick is to build up those levels without incurring skin damage from excessive UV levels. As I’ve showed before, a delicate balance is needed.

Their results are based on measurements of UV radiation, rather than unrealistically assuming clear-sky conditions. Here I want to focus on the NZ situation, which is more problematic due to its higher latitude and therefore lower winter UV amounts. The measurements they used for New Zealand were those set up and maintained by our NIWA Lauder group. Sites included Leigh (North Auckland), Paraparaumu (near Wellington), Christchurch, Lauder and Invercargill. Because clouds typically cut back about 30 percent of the UV radiation at these sites, the exposure times needed under clear-sky conditions would be about 30 percent shorter than they calculate.

The diagram below gives their calculated exposure times per day needed to maintain current level of vitamin D for a fair-skinned person who is out in the sun 4 day per week wearing shorts and T-shirt (they estimate that the exposure times required are two or three times longer for darker skins). Results are shown as a function of time-of-day and month for two sites: one in north of New Zealand, and one near the southern tip of the country.

Near noon in summer, there are no vitamin-D worries anywhere in New Zealand. Your levels of vitamin D can always be maintained with less than 5 minutes of exposure, which is comfortably less than the exposure time for visible skin damage (erythema). It’s not so easy for darker skinned people, particularly if a large proportion of their skin isn’t exposed to sunlight.

It’s an entirely different story in winter. While adequate levels of vitamin-D can be maintained with exposure times less than 30 minutes per day throughout the year in the north, that’s not so in the south. At Invercargill in June and July, exposure times of more than 1 hour per day would be required - even in the unlikely event that you’re wearing only that t-short and shorts. For more realistic attire in those cold winter months there’s no way that you’d be able to maintain vitamin-D without some loss. Unrealistic exposures would also be required in the months of May and August. Some supplementation or food fortification of vitamin-D is desirable in winter (please don’t try to do it with sunbeds. They’re dangerous).

The situation northern NZ is similar to sites in southern Australia (e.g., Adelaide, Melbourne, and Sydney). At more northern Australian locations - like the winterless north (e.g., Brisbane) - you never need to worry about getting enough UV for vitamin D synthesis. On the contrary it’s hard to avoid getting too much UV for skin health.

The good news from the paper is that their calculated exposure times to maintain vitamin-D are in pretty good agreement with those estimated by the UVNZ and GlobalUV smartphone apps. That means you don’t need to carry graphs like the one above around with you all the time. The exposure times required are always at your fingertips (as long as your fingers are connected to a phone 😊).