When health expert Tim Spector, a professor of genetic epidemiology at King’s College London and co-founder of the ZOE health app, suggested that people forego sunscreen to perhaps reduce their cancer risk, scientists were understandably perplexed by such irresponsible advice.
Spector posted on X that a pre-clinical study suggesting that vitamin D can regulate cancer immunity naturally is “another reason to stop using SPF 50 all year round, which blocks our natural defences“. After some criticism, he doubled down, adding: “There are several clinical studies supporting the mouse studies showing vit D and sunshine are helpful in melanoma survival.”
As a cancer scientist, I was concerned too. That ultraviolet (UV) light in the solar spectrum is a carcinogen has long been known. While DNA converts most incoming ultraviolet light rapidly into harmless heat energy, a small percentage of UV light photons evade this evolved defence. When unrepaired, this results in DNA lesions and, ultimately, melanoma.
Sunburn is one obvious and unpleasant warning sign of damage – but so too is our tanning response, evolved to mitigate some of the harm. Whether suntans would be a less desirable aesthetic if we saw them more clearly as warning signs of direct DNA damage, or recognised that UV exposure is highly damaging to vital collagen, is an interesting question.
Most melanomas are, however, attributable to indirect DNA damage, where UV-excited molecules react with other molecules, producing short-lived particles that are harmful to our DNA. Unlike direct damage, we have no evolved response to this.
All UV exposure is potentially harmful, and UV is present even on overcast days. The good news is that sunscreens effectively reduce the cancer risk inherent in sun exposure. Sun-protection factor (SPF) is a measure of the efficacy of sunscreens at blocking UV-rays. Roughly speaking, SPF-15 blocks 93 per cent of UV, SPF-30 97 per cent and SPF-50 98 per cent.
So what provoked Spector to forsake more than 100 years of research clearly linking sunlight to cancer, and the benefits of sun protection? A study on vitamin D in mice microbiomes. The astute reader will note, however, that humans are not mice.
Early results in rodent models rarely translate directly to people, and many seemingly exciting initial findings are eventually shown to be spurious. There are many reasons why: the vagaries of chance, confounding factors hidden in the design of experiment, experimenter bias and ineptitude, selective reporting, even outright research misconduct. For these reasons, most preliminary results do not pan out.
This renders it incredibly foolhardy to make assertions about how humans should behave based upon their often transient findings.
Spector’s error in this instance is even more egregious, because the study he cites says nothing about sunscreen, and much about the less sexy topic of faecal transplants in mice.
Spector’s unsolicited advice is thus an extrapolation built upon an extrapolation; a flimsy thread of supposition readily broken by even cursory consideration.
Firstly, despite an avalanche of supplement-pushers long evangelising vitamin D as a nigh-on-universal panacea, well-designed trials and meta-analysis have found no evidence of an impact on mortality, or indeed many of the other purported benefits that initial speculation suggested.
Nor is there any reliable evidence that application of sunscreen inhibits vitamin D production in skin, precisely because doses needed for its production are relatively minuscule.
It is also crucial to be cognisant of the pervasive nature of health misinformation. Over the past few years there have been a spate of health “influencers” of nebulous qualification and negligible ethics who have scaremongered incessantly over sunscreen.
This cacophony of claims includes utterly unfounded allegations that sunscreens do not work, are dangerous, or even that sunburn is caused not by sun but by seed oils.
Despite each of these claims being utter nonsense, they percolate online furiously, and Spector’s recent comments risk propelling their harmful consequences further.
Spector’s faux pas is yet another reminder to be cautious about making big inferences from early research, and that the mere possession of a scientific qualification is no guarantee a person is embracing best evidence.
David Robert Grimes is a cancer researcher and physicist, and the author of The Irrational Ape: Why We Fall for Disinformation, Conspiracy Theory and Propaganda