After my recent article on whether or not sunscreen prevents skin ageing, several people responded that I should have added something about the evidence that sun exposure makes you live longer somewhere in the article. The idea that sun exposure is good for your longevity comes primarily from a Swedish study that was published in The Journal of Internal Medicine in 2014, so I thought it would be interesting to dissect that study in order to see what it actually shows.
30,000 women living in the south of Sweden were recruited between 1990 and 1992 and then followed for almost twenty years to see whether they lived or died. At the start of the study they were asked four different questions, which were used to determine how much they exposed their skin to the sun. First, they were asked how often they sunbathe in summer. Second, they were asked how much they sunbathe in winter, for example if they go on holiday to the mountains. Third, they were asked how much they use tanning beds. And fourth, they were asked if they go abroad to swim and sunbathe. For each of these questions, there were four options, pretty much ranging from “never ever” to “all the frickin’ time”. Based on the answers to these questions, the participants were given a score from 0 to 16 – with a 0 meaning that they never sunbathed, and a 16 meaning that they did it a lot.
Ok, so this was an observational study. Nothing active was done to the participants, and there was no randomisation to the different sunbathing groups. The participants were just asked a bunch of questions about how they lead their lives and then followed over time to see what happened to them. A problem with this type of study is confounding – i.e. the fact that people who vary from the average in one way, for example spending more time out in the sun, will also tend to vary from the average in lots of other ways. Maybe they exercise more, or earn more money, or eat more healthily, or something else.
It’s pretty obvious, just from looking at the four questions, that they are designed in such a way that people with high incomes and good health will be more likely to score high, and people with low incomes and poor health will be more likely to score low. You need to have a high income and pretty good health to go on holidays abroad, and the answers to three of the four questions are written in such a way that there is likely to be a strong correlation with how many holidays people take abroad.
The researchers have tried to compensate for confounding by asking the participants about how much they earn, how much they smoke, how much alcohol they drink, and so on, and then adjusting the results based on this information. Confounding variables are like pokémon – you gotta catch ’em all. The problem is that that is pretty much impossible, because no matter how many confounders you think of, and adjust for, there are always going to be a bunch more that you didn’t think of.
When we look at what answers the participants have given to these questions, we see that the people who spent a lot of time in the sun are, as we might expect, very different from the people who spent little time in the sun. There were in fact big differences between the groups in pretty much every variable the researchers cared to look at. Just to take a few major examples, the women who spent more time in the sun were more highly educated, earned more money, weighed less, and exercised more.
Let’s take a look at the results and then try to determine what they actually mean.
At the end of twenty years, 27% of the women who spent the least amount of time in the sun had died, as compared with only 6% of the women who spent the most amount of time in the sun.
That is, on its face, a pretty enormous difference. Cased closed, right? The women with the least amount of sun exposure were almost 400% more likely to die over the course of twenty years.
Well, not so fast. Like I said, there were big differences between the groups, that could explain the difference in outcome, and the analysis needs to be adjusted for these differences. After the researchers adjusted the analysis for differences in age, smoking habits, marital status, education level, income, and whether or not they’d been treated with certain drugs (anti-diabetics, anti-coagulants, or cardiovascular disease drugs), the difference dropped down massively, so that the least sun exposed were “only” 67% more likely to die than the most sun exposed. When the researchers also adjusted for differences in exercise and body weight, the difference between to groups dropped down further, so that the least sun exposed were 52% more likely to die.
Ok, 52% is not 400%, but it’s still a pretty big difference, right?
Yes, it is, if it’s real. But here we come to the big problem with observational studies. Although the researchers have corrected for a number of confounding factors, there are many remaining that they haven’t corrected for. For example, they’ve only corrected for a few underlying disease states when they’ve adjusted for use of anti-diabetic, anti-coagulant, and cardiovascular disease drugs. There are lots of co-morbidities that they haven’t adjusted for.
Nor have they adjusted for differences in physical function – people who are wheelchair bound or have some other physical handicap that limits their ability to move around will probably sunbathe less than people who don’t, and will also tend to have a shorter life expectancy due to their handicap. Nor have the researchers adjusted for differences in diet at all.
Perhaps most importantly, they haven’t adjusted for differences in vitamin D-levels. Vitamin D has been found to play an important role in health in lots of different ways. For example, a meta-analysis of randomised trials published in the British Medical Journal in 2017 found that people with vitamin D deficiency who were treated with a daily vitamin D supplement halved their rate of infection.
Unfortunately, the researchers in this study didn’t measure vitamin D levels at the start of the study. If they had, and they’d adjusted the analysis for vitamin D levels, then I can imagine that any difference between the groups would have disappeared completely – in which case you wouldn’t need to go out in to the sun in order to get the benefits of sunbathing, it would be enough to just take a vitamin D supplement.
A good general rule with observational data is to be very skeptical of it unless you see at least a doubling or halving of risk even after you’ve adjusted for all confounders you can think of. Here we see less than a doubling, so we should be skeptical and assume the difference is due to residual confounding until more convincing evidence can allow us to conclude otherwise. That is especially true when there are several big, important confounders remaining that the analysis hasn’t been adjusted for.
Ok, so what can we conclude?
It’s possible that sunbathing has a beneficial effect on longevity. But the claim that it does is based on observational data that is riddled with confounding, and the size of the effect is not large enough for any strong conclusions to be drawn. The possible benefit needs to be weighed against the known harms, in the form of more rapid skin ageing and increased risk of skin cancer. If there is a benefit, then it is most likely thanks to the fact that sun exposure increases vitamin D production, in which case you can get the same benefit, without the harms, by taking a supplement.
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