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.
https://sebastianrushworth.com/2022/07/13/can-sun-exposure-make-you-live-longer/