Health effects of sunshine

Tuesday January 8 2008

“The health benefits of sunshine outweigh the risk of skin cancer for pasty-faced Britons,” reported The Daily Telegraph today. A new study has found that although certain internal cancers are more common in countries closer to the Equator, people in these countries were less likely to die from these diseases, and that this benefit may outweigh the risk of skin cancer in populations who have less sun exposure.

This study addresses an important question about the balance of benefits and risks from exposure to sunlight. It did this by comparing different countries’ levels of sun exposure to their rates of cancer.

Although this study might generate some theories that deserve further investigation, it falls short of proving that sunlight exposure is responsible for the incidence or prognosis of cancer in different countries. There are simply too many other differences between these populations that could account for these differences.

Only studies that looked at individuals not populations would provide convincing evidence. As it is, people should continue to follow recommendations that suggest that they avoid extensive sun exposure, and in particular avoid getting sunburned.

The study looked at vitamin D produced through exposure to sunlight and did not examine oral supplementation.

As mentioned by the UK Scientific Advisory Committee on Nutrition in its 2007 report, there is “controversy over the recommended safe upper limits [for vitamin D oral supplements] in Europe and the USA”. Although the report states that “prolonged sunlight exposure does not lead to excess production” of vitamin D, high doses of oral vitamin D supplements have been “shown to have toxic effects”. These effects are primarily related to an increased risk of hypocalcaemia.

The report also warns, “Patients with sarcoidosis are abnormally sensitive to vitamin D [and] although the condition is uncommon, it would be a potential hazard if affected individuals were to take supplementary vitamin D… and would be the same for those with primary hyperparathyroidism.”

Although recent assessments of vitamin D have “suggested that vitamin D is not toxic at intakes well above current upper safe limits”, the UK Expert Group on Vitamins and Minerals has said that for guidance purposes only, a level of 25μg/d supplementary vitamin D would not be expected to cause adverse effects in the general population when consumed regularly over a long period.

Where did the story come from?

Dr Johan Moan and colleagues at the Institute for Cancer Research in Oslo, the University of Oslo, and the Brookhaven National Laboratory in New York carried out this research. The study was funded by the Sigval Bergesen D.Y. og hustru Nankis Foundation, The Research Foundation of the Norwegian Radiumhospital, and Helse-Sør Norway.

The study was published in the peer-reviewed medical journal: Proceedings of the National Academy of Sciences of the USA.

What kind of scientific study was this?

This study is an ecological study that looked at the relationship between exposure to the specific wavelength of sunlight that the body needs to make vitamin D (referred to as vitamin D-generating radiation from the sun) and the incidence of cancer in countries with different latitudes (how far north or south of the equator they are). The researchers also discussed findings from other studies that addressed similar questions.

The researchers obtained information from an international database on the incidence (number of new cases per year) of cancer between 1987 and 1997 in six countries where the majority of the population are white – UK, Denmark, Norway, Sweden, Australia, and New Zealand. They also obtained data from the World Health Organization on the number of deaths from cancer each year in these countries for the period 1989 to 1999. The incidence of malignant melanoma in Norway between 1960 and 2003 was also obtained from a national cancer database.

The researchers plotted graphs to compare the cancer incidence in these countries with their latitude. They calculated the ratio of cancer death rates to incidence rates two years previously for each country. They did this as a “crude measure of prognosis”, that is to give them a rough estimate of what proportion of people diagnosed with cancer die in the following two years. This is only a rough estimate because the people dying from cancer may not be the same people who were diagnosed with cancer two years earlier. The researchers plotted this ratio against latitude to look for relationships between these factors.

Each country’s exposure to “vitamin D-generating radiation from the sun” was calculated by looking at how much of this radiation the sun produced and how much would be transferred to the body.

These estimates took into account the wavelength of light needed to make vitamin D in the body, direct sunlight exposure and exposure to diffuse reflections of the sun’s rays. They also took into account the shape and orientation of the human body relative to the sun (this was estimated to be a vertical cylinder shape, with the top and bottom of the cylinder not being exposed), and the known depth of the ozone layer and the average daily cloud cover over each country.

They then compared the exposure to vitamin D-generating radiation against the incidence of cancer in each country.

What were the results of the study?

The researchers found that living further away from the Equator (having a greater latitude) reduced their exposure to “vitamin D-generating radiation from the sun” compared to living closer to the equator. People living in the UK were estimated to be exposed to 3.4 times less vitamin D-generating radiation than people living at the equator.

In the six countries assessed, there was a decrease in the incidence of malignant melanoma the further that a country was from the equator. There was also a trend towards an increased incidence of some internal cancers (colon, breast, prostate, and lung cancer) the closer a country was to the equator. However, when they looked at the rate of deaths from cancer in these six countries, they found that the closer a country was to the equator, the lower the ratio of cancer deaths to cancer incidence. They interpreted this as showing that although people might be more likely to develop cancer in the countries closer to the Equator, they were less likely to die from cancer. When they looked at a larger group of countries however, they found that they could not reliably identify differences in incidence of cancer according to latitude.

What interpretations did the researchers draw from these results?

The researchers concluded that because of the benefits associated with having sufficient vitamin D, increased exposure to sunlight may improve prognosis of cancer, and may “possibly” give more benefit than risks. They suggest that this message should be given to those “at risk for vitamin D deficiency”.

What does the NHS Knowledge Service make of this study?

This type of study does generate interesting theories about the relationship between an exposure and an outcome (in this case sunlight exposure and cancer). However, as it does not follow individuals over time to see how exposure affected outcome, and instead looks at these factors at the population level, it cannot prove that the exposure causes the outcome.

There are many differences between these countries other than their latitude, which could contribute to the differences in cancer incidence and deaths from cancer. Latitude may also correlate with other factors, such as a country’s socio-economic status, which might be responsible for the trends seen, rather than exposure to sunlight.

The authors acknowledge that although sunlight exposure varies according to latitude, studies have not shown a clear difference in the average vitamin D levels in the body in a population according to the latitude of where they live.

It is important to have an adequate intake of vitamin D, however, it is also important to avoid overdoing exposure to the sun. People should avoid extensive sun exposure, and in particular avoid getting sunburnt.

Analysis by Bazian
Edited by NHS Choices