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.
Love reading how you take apart the various studies….it’s refreshing and funny.
Were they only asked those questions once at the beginning of the study? Behavioral patterns can change a lot over the course of 20 years. Also, did they wear sunscreen? I agree that measuring Vit D levels would be more useful.
That is a good point, they were only asked about these things once, as usual with cohort studies, and it is quite an assumption to think that people’s behaviour is completely stable over 20 years.
There is another benefit from sunlight as well as Vit D and that is nitric oxide production with it’s positive effects on the cardiovascular system.
Neil
Endless biased useless research! Compare Swedish with Swedish, with the ones who are wealthy enough to take longe holidays with those who are not able to have more leisure border the most unscientific research!
So you say that body synthesising vitamin D from sun is equivalent to a supplement for D. I highly doubt that is this the case, but if you have data on this i am happy to listen.
I take 4000 IU of D3 daily but from the start of this regimen several years ago I’ve wondered if I’m not benefiting from intermediate metabolites in the biosynthesis chain. I thought we were taking the compound at or near the end of the biosynthesis but D3 is one of the first substances synthesized.
“In the presence of sunlight, a form of vitamin D3 called cholecalciferol is synthesized from a derivative of the steroid cholesterol in the skin. The liver converts cholecalciferol to calcidiol, which is then converted to calcitriol (the active chemical form of the vitamin) in the kidneys.”
I’ve always viewed D3 supplements as better than nothing but not as good as regular moderate exposure to sunlight. I still do to some extent, but feel better about taking D3 instead of being out in the sunlight. I suspect moderate sunlight has many health benefits, as does moderate daily exercise.
Thank you for your work.
I was wondering about any relationship between sun exposure and mental health impacting physical health or longevity?
Many people in the Northern climates (me included) report feeling better mentally in the summer vs winter and often would like a sunny break in the winter. Is there something driving our need for sunshine?
SAD Seasonal Affective Disorder
there are cases that the vitamin D do not get through the body,so exposure to the sun is the only choice.
The return to flight post assessment for Nasa is the amount of smoke inhaled in the body during one’s life time,in an airport this can mix specimen with fuel 1842,11% higher than in a park and in the environment where there is nitrogen the body could also be exposed
to chemical substances ,ammonia,pollutants in the air.
There are also cases where the liver cannot adequately convert D3 to the D metabolite and where adipose tissue prevents conversion to the metabolite because it competes with the liver for D3.
Another confounding might be that I’d expect some of the lower paid to be in outdoors jobs like gardening and would get loads of sun exposure without actually sunbathing.
Enjoying your blogs on sun and it’s relative ‘fear’ factors. Wondering if there any studies relating the use of sunscreen to levels of skin cancer, that haven’t been paid for by those who make sunscreen?
And yet after all that time the only (?) study that showed a benefit was based in Australia, with a furious sun and half the population of Celtic descent (pale skin red hair)
If I am not mistaken Dr Malcolm Kendrick author of book The Clot Thickens or on his blog mentions some benefit from sun exposure and Vitamin D and that much skin cancer he has seen is often on the body parts less likely to be exposed to the sun.Explain that. As with everything moderation is key but sun exposure must have benefit up to a point. I feel better and sleep better after a few good hours out in the summer sun. I drag in the winter time and have purchased a sun lamp and supplement with 2,000IU of vitamin D. I think one might be more concerned with all the chemicals being absorbed into the skin from sun screen .
Vitamin D is not a vitamin but a hormone. Please do a follow-up on the “full-spectrum” impact on the body of sun exposure as opposed to taking a synthetic fractionalized supplement. Can you do some research on the possible deleterious effects of taking large amounts of synthetic vitamin D for prolonged periods?
Here is an interesting source of alternative information on this subject:
“Vitamina D. How it works. How to adjust it naturally. Why integrating it is the biggest mistake you can make.” by Angelo Rossiello and http://www.evolutamente.it
Can people in northern latitudes make vitamin D in winter, or is supplementation required in winter?
Maybe sometimes vitamin D is a hormone and sometimes a vitamin, depending on whether it is obtained via skin production or supplementation.
In Sweden, vitamin D production from the sun is only possible from late spring to early autumn.
Vitamin D isn’t really a vitamin, since the body is able to make it as long as it gets exposed to sufficient sunlight. Unfortunately our modern tendency to cover ourselves in clothes makes that impossible.
There is only one vitamin D. It is the same molecule regardless whether it is produced in the body in the presence of sunlight or whether it is consumed in food or supplements.
It is always a hormone, or at least, the biologically active downstream metabolites are. All the effects vitamin D has on the body are, as far as I am aware, due to its action as a hormone.
Dr. Rushworth,
Vitamins must be obtained from food, right? Isn’t it correct that, in some cases, vitamin D can only be obtained orally (e.g., during winter in very northern or southern latitudes)? So vitamin D is a vitamin even though it can be produced dermally during some seasons.
To be clear, vitamin D produced by the skin (or ingested orally) is really a prohormone and has no metabolic action afaik. I’ll refer to this form as D3.
So it’s clear that D3 is both a vitamin and a prohormone.
Continuing to be clear, it’s the metabolites of vitamin D which have hormonal action (i.e., calcifediol and calcitriol).
The benefits of sunshine are manifold, not just vitamin D synthesis. For example, exposure to natural light early in the day regulates the release of the hormones that regulate our sleep cycle. There is also evidence that having high levels of vitamin D (indicative of sun exposure) is correlated with many positive health outcomes – but those same outcomes are often not replicated in studies of vitamin D supplementation.
50,000 in daily is 10 times higher than what the bit d gurus recommend. There are medics who,misleadingly,. think more than 400iu daily can lead to toxicity
I think it’s a problem when you break out a certain parameter from the rest.
For example, in the summer I usually get a good tan and I don’t take vitamin supplements. I try to avoid the sun, but try to get a protective tan while cutting grass and such.
In the winter I take supplements vitamins when I haven’t eaten many fruits or vegetables, or when I haven’t consumed much calcium (dairies), I take supplements for that.
So in my case, how could you possibly make some useful statistics from my habits?
Habits vary over time.
Taking a vitamin d supplement may give you the same benefit as sun exposure but soaking up some actual sun does boost your mood which in turn reduces emotional stress.
Extremely thorough analysis; enjoyable & enlightening to read!
Are there any studies on whether sun exposure has more benefit than taking vitamin d supplements? Are there any studies about whether getting optimum vitamin levels through diet rather than supplements is better?
It is non controversial that having optimum levels of vitamin D has health benefits. It is non controversial that having low levels of vitamin D is detrimental to one’s health. Amazing how Fauci and the CDC didn’t recommend everybody getting higher vitamin D levels during Covid.
It would be great if you could also find time critically review some of the questionable claims of published work that are promoting some vegetarian/vegan diets as useful for diabetics to lower A1C etc.
Most like older suggesting papers taking cinnamon lowers blood glucose. When you read actual paper are studies used massive amounts ingested for nearly no significant drop in blood glucose. The recent papers touting on vegan/vegetarian diets and “Brewer’s yeast” shown to lower blood glucose levels seem not very useful to me.
Most have abstracts that are the focus like stating if you eat/take this XXX supplement/food it has been shown glucose is lowered say from an HBA1C of 8% to 6.5% … seems very misleading and not useful. When you go beyond the abstracts that state “XXX lowered blood glucose significantly… blah blah.” There is not such a useful outcome.
Significant would be to lower A1c into non-diabetic ranges of 4.6-4.7%. This is like one saying the Fire Department achieved “Significant” control of the fire in part of the house fire by extinguishing the fire in the attached garage… yes the rest of the house is still on fire but that is a different issue.
You will be aware that lots of correlations between Vitamin D status and various diseases exist. You will also be aware that many studies seeking to treat people with these diseases by giving vitamin D supplements have failed.
My point. Vitamin D status is an indicator of sun exposure but is not the only effect. How can it be? A ton of energy applied to the biggest organ in the body cannot be reduced to its effect on one solitary enzyme
Vitamin D trials are almost always underpowered. The meta-analysis of trials I mentioned showed a strong benefit in reducing infections in people who were deficient before they started supplementing. Infections are a common cause of death in the elderly, so from my perspective it has been shown that vitamin D supplements likely prolong life in people who are deficient to begin with.
Vitamin D (in the calcitriol form) is not an enzyme, but a hormone. There are vitamin D receptors in many different types of tissue, including bones, immune cells, intestines, kidneys, muscles, skin, neural tissue, and parathyroid glands. There are likely more tissues which express vitamin D receptors which have not yet been discovered.
The following has a variety of sources which I found helpful in extracting the preceding info.
https://www.sciencedirect.com/topics/neuroscience/vitamin-d-receptor
Does taking a vitamin D supplement as good for us as sun exposure? You seemed to imply that it is indeed. But is that true? Does the sun not generate NO (Nitric Oxide) under the skin? Is NO not a very important chemical in our body? Are the other effects of sunlight which have not been considered?
What I’m saying is that it has not been convincingly shown that sunbathing makes people live longer, and that if there is a benefit then much or all of that benefit is likely due to increasing levels of vitamin D in the body.
Yeah, I get the message that there hasn’t been any strong studies proving that sun exposure makes you live longer, however:
– are there any studies that have looked into sun exposure and specific diseases like cancer? If it protects from cancer in general (except skin cancer from excess sun) as some claim, then it’s obviously a strong argument that it can make you live longer.
– you seem to assume that sun exposure could maybe be fully substituted with taking vitamin d supplements, but what about for example serotonin production from sun exposure? Isn’t that a major, separate health benefit as well, besides from vitamin d?
I live in Malibu CA and never used sunscreen. I am 76 with perfect health. The sun is my friend. From my backyard on the cliff I see the great Pacific Ocean, and the heavenly bodies in motion on the sand. Life is good in Paradise. My grandparents came from Kongeriket Norge and Konungariket Sverige.
I have white skin(Norska and Svensk) and live in Malibu. I heard that the sun was good and acted upon those recommendations and damaged my skin from the sun. Now I am covered up when I go out in the sun(also recommended for the “ankle baiting” mosquitoes).