Many New Zealanders understand how important vitamin D is for human health. However, they also believe that as a sunny country, kiwis are likely to have plenty of sunshine to get adequate amounts of vitamin through sun exposure. Unfortunately this isn’t the case.
Vitamin D is made when UV (more precisely, UVB rays) react with a compound (7-dehydrocholesterol) in the skin. The best rays for UV synthesis have wavelengths between 270–300 nm. These wavelengths are present when the UV index is greater than 3.
As outlined in the article linked to below, UVB does not hit the ground if it is below 45 degrees from the horizon:
The angle of the Sun above the horizon (at sea level) also affects the production of vitamin D because the atmosphere is thicker at lower angles and absorbs more UV. At angles greater than 45° above the horizon (at sea level), vitamin D production will be occurring, although some recent research suggests that vitamin D production may occur at angles as low as 30°.
https://www.sciencelearn.org.nz/resources/1313-vitamin-d-and-uv
Obviously, we need to optimise our sun exposure to ensure that we get the highest levels of vitamin D, so we need to be aware of when are the best times to go out in the sun without risking skin cancer.
The amount of time a person needs to produce enough vitamin D for good health depends on:
- the person’s distance from the equator
- the season of the year
- the amount of melanin in their skin — darker skin requires longer than fair skin.
The following chart suggests that you should find other sources of vitamin D in the winter, particular if you live in the South Island.
https://niwa.co.nz/sites/niwa.co.nz/files/uvi_web_plots.pdf
Time of day is important too:
To get an optimal vitamin D supplement from the sun at a minimal risk of getting cutaneous malignant melanoma (CMM), the best time of sun exposure is noon. Thus, common health recommendations given by authorities in many countries, that sun exposure should be avoided for three to five hours around noon and postponed to the afternoon, may be wrong and may even promote CMM
….
In view of this, short, nonerythemogenic exposures around noon should be recommended rather than longer nonerythemogenic exposures in the afternoon. This would give a maximal yield of vitamin D at a minimal CMM risk.
https://pubmed.ncbi.nlm.nih.gov/18348449/
Some suggest that moderate sun bed use in winter may be effective in maintaining healthy vitamin D levels:
The winter levels of vitamin D are 10 to 100% lower than the summer levels in most populations, but can be brought up to summer levels by moderate sun bed exposure. Doses of 200 IU of vitamin D per day are not sufficiently large to maintain a summer vitamin D status in winter. At high latitudes (>40 degrees) the sun provides no vitamin D in winter.
https://pubmed.ncbi.nlm.nih.gov/19667143/
Note, you should never stay out in the sun, or under sun bed for long enough to burn. Those unable to access the sun due to health or cultural reasons should ask their doctor for a vitamin D prescription. Unfortunately in NZ pharmacies are not allowed to sell vitamin at levels that are effective for human health.