Vitamin D plays an important role in our bodies:

  • It improves general health.
  • It promotes the formation and strengthening of bones (a deficiency will cause bone softening diseases, which then causes rickets in children and osteomalacia in adults).
  • It positively affects the immune system.
  • It helps protect against some cancers, such as colon cancer.
  • It is of benefit in auto-immune diseases such as multiple sclerosis.

Where do we get vitamin D from?

Very few foods are naturally rich in vitamin D. The best sources are fatty fish like salmon, sardines, tuna and eel; fish oils like cod liver oil; mushrooms and eggs. Some countries, like the USA, add vitamin D to foods like milk, soy milk and cereal grains.

Research shows a clear link between sunlight and the synthesis of vitamin D in our bodies, so most people need sun exposure to produce enough vitamin D for best health.

A deficiency of vitamin D in the body can happen:

  • if there is too little vitamin D in foods and drink
  • if too little time is spent outdoors in the sun
  • due to various health disorders that limit absorption or conversion of vitamin D in the body.

Why is UV important?

Not many people consume sufficient vitamin D for their body’s needs, so most people need some sunlight to boost their vitamin D to the levels required for best health. For example, if we do not produce enough vitamin D by consuming it or being in the sun, our bodies can’t extract calcium from the food we eat, so our bodies then take it from our bones. This is why a lack of vitamin D causes brittle bones and diseases like osteoporosis.

Also, medical research since 1970 has found an increasing amount of evidence that supports the hypothesis that vitamin D has significant, protective effects against the development of some cancers (particularly bowel cancer). One study in 2006, using data on over 4 million cancer patients from 13 different countries, showed a marked difference in cancer risk for a number of different cancers between countries classified as sunny and countries classified as less sunny.

Nature of Science

Scientists often find themselves in the middle of controversy. There are different opinions about many issues, for example, whether UV is good or bad. Scientists judge issues by the amount and quality of data that is used to support claims.

What role does the Sun play in vitamin D production?

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. 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°. At higher altitudes, there is less (thinner) atmosphere to absorb UV from the Sun.

So how much time does it take to produce enough vitamin D?

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.

Most people will have adequate vitamin D levels just from incidental exposure – even outside the peak UV times of 11am and 4pm. In fact, it is thought that as little as five minutes per day of incidental sun exposure (such as walking outside to the mail box or to catch the bus) is sufficient for someone who burns easily, and up to 20 minutes is sufficient for a person with darker skin.

Too much vs. too little?

So, how do you balance the clear evidence that UV causes skin cancer with the evidence that vitamin D is also beneficial to our bodies (and food sources alone may be insufficient)? When considering this dilemma, we do need to remember that, here in New Zealand:

  • our peak UV levels are 40 percent higher during summer than at corresponding latitudes in the northern hemisphere (NIWA research)
  • a significant proportion of the population has skin types that burn easily (due to our genetic heritage)
  • our rates of melanoma incidence and of deaths from melanoma are among the highest in the world.
    Published 29 July 2008