Should we be afraid of the Sun?
Sunlight and its effect on immunity and general health has been a hotly debated topic for a while now (pun intended). Most public health messages of the past century have focused on disparaging sun exposure. We’re continually told to not spend too much time out in the sun and if we are, to apply a good slathering of sunscreen for protection.
But is all this sun-fear warranted or useful? Out of all the organisms that inhabit this planet, why is it only us humans that have to worry about our 4 billion years old star?
Well for one, dermatologists will tell you that increased intermittent exposure to sunlight and increased occurrence of burning increases one's risk of suffering from malignant melanoma - the most aggressive type of skin cancer. They’d also tell you that increased exposure to sunlight also increases one’s risk of getting other forms of skin cancer too. And they’d be correct.
Sunlight is made up of ultraviolet (UV), visible, and infrared radiations. And though essential for life, light and warmth on this planet, these components of sunlight have shown to damage skin and DNA, creating free radicals in the process. Exposure to sunlight has also been shown to cause immunosuppression. It is thought that these mechanisms contribute to the formation of skin cancer.
What they fail to tell you though is that whilst the burden of disease related to overexposure to sunshine is estimated to be 50 000 deaths and 1.6 million disability-adjusted life years (DALYs) - representing 0.1% of the total global disease burden in the year 2000; disease burden due to very low exposure to UV radiation is estimated to be 9.4%. Nearly one hundred times more.
Most of us do not get enough sunshine and this increases our risk of certain cancers including breast, colon, prostate and non-Hodgkin lymphoma; and increases our risk of diseases and autoimmune disorders such as multiple sclerosis and type 1 diabetes. In fact, it has been said that sun avoidance was a risk factor for death of a similar magnitude to smoking.
Dermatologists will also forget to mention that continuous exposure to sunlight is negatively associated with malignant melanoma. And in fact that one study noted people tend to survive longer with melanomas the more sunshine they have been exposed to. The sun is not all bad, and various other factors like genetics must be considered too.
Immunosuppression after sunlight exposure is true, but it’s a little more complicated than that. Though it has been noted to occur, mostly in animal studies and men, it is thought that this mechanism keeps autoimmunity in check and gives sunlight its beneficial properties in those suffering from a wide range of autoimmune conditions.
And it's not like exposing yourself to the sun will increase your risk of skin infections. This is because other innate immune system proteins (antimicrobial peptides) are made to combat pathogens whilst sunlight shines on your skin. Light itself also helps T cell motility. And a commonly celebrated benefit of sunlight is that it increases our production of vitamin D.
Vitamin D was a furiously debated vitamin (hormone) at the start of the pandemic, and health institutions only begrudgingly began recommending it to patients many months in. Those not on board with vitamin D supplementation said that the little number of studies undertaken didn’t show much benefit in reducing the disease risk of COVID-19.
This is partly true as vitamin D supplementation is still not concretely proven to be effective. But this may be because the quality of studies on this topic was initially low, plus taking high doses of vitamin D supplementation for a few weeks is not the same as achieving good levels of this hormone naturally via the sun. Furthermore, vitamin D levels are thought to drop when we are unwell, skewing the data.
Regardless, vitamin D has been noted to play a vital role in maintaining the healthy running of our immune system and there are studies supporting its role in COVID-19. Plus there are many other studies concluding that increased sun exposure is linked to better COVID-19 outcomes. So you could say that the pandemic restrictions negatively impacted vitamin D uptake by limiting exposure to sunlight.
How much sunlight and/or vitamin D one should have is dependent on the person, what they do and where they live. That being said, most of us are both sunlight and vitamin D deficient. Sunlight deficiency being the more problematic one, and arguably the one to primarily tackle. This is because we know that food sources of vitamin D and supplements are second to UV light.
Vitamin D produced in the skin may last at least twice as long in the blood compared with ingested vitamin D. One study showed that when an adult in a bathing suit was exposed to UV radiation, giving them a slight pinkness to the skin 24 h after exposure), the amount of vitamin D produced is equivalent to ingesting between 10,000 and 25,000 IU.
At latitudes above 37°N and below 37°S, sunlight is insufficient to induce cutaneous vitamin D3 synthesis during the winter months. That sucks for most of us, but even more for those with darker skin, as increased skin pigmentation has been shown to reduce cutaneous vitamin D3 production by as much as 99.9%.
But vitamin D measurements may be used as a proxy marker of sunlight exposure and is easier to measure than time out in the sun. But more vitamin D isn’t better. With regard to sufficient levels, a prospective cohort study of 365 530 participants showed a reverse J-shape association between serum vitamin D levels and all-cause mortality.
Vitamin D levels of less than 45nmol/L and anything above 60nmol/L were associated with an increased risk of cardiovascular disease, cancer and all-cause mortality. Therefore this study suggested a serum Vitamin D ‘sweet-spot’ of 45 to 60 nmol/L.
Other similar studies have revealed a U-shape association between serum vitamin D levels and mortality rates, with similar sweet spots, but some noted figures at the higher end at 120nmol/L. An extensive review stated that “practically all persons are sufficient at levels of 50 nmol/L (20 ng/mL) and above. Serum concentrations of 25OHD above 75 nmol/L (30 ng/mL) are not associated with increased benefit.”.
But it’s not all about vitamin D. We must also remember that sunlight exposure is involved in regulating many other hormones including nitric oxide, serotonin, cortisol and melatonin. Bright exposure during the day and total darkness at night optimises melatonin levels and improves our quality of sleep. Melatonin has also been shown to act as a potent anti-inflammatory hormone.
If you have no risk factors of acquiring skin cancer, then exposing yourself to an adequate amount of sunshine throughout the year without the use of sunscreen (there is some evidence suggesting sunscreens may enhance your risk of cancer by the way), whilst taking careful precautions to not get burnt may help boost your immune system, reduce autoimmunity and improve overall health.
For those worried about burning - it has been said that the risk of burning may be reduced if your overall health is improved and if your diet is full of protective antioxidants and omega-3 fatty acids. More studies are needed in this regard, so always be careful out in the sun.
Since the Second World War developments such as cars, Netflix, computers, video games, indoor sports etc. have meant we are spending more and more time indoors. German and Danish studies revealed that indoor workers on average expose their hands and face to less than 3% of the total available amount of sunlight.
Let’s break the cycle. Go outdoors, get sun-kissed and improve your health and immunity.