WHICH STAR SIGN are you? It may not be a question that you’d expect to hear from a scientist, but your month of birth really does appear to have an influence on your life – or, at least, on your body and your health. If you’re born in the summer, for example, you’re likely to be taller than average as an adult, whereas autumn babies tend to weigh more at birth and go through puberty earlier. The effects are small (in the case of height, it’s a matter of millimetres) but significant. It’s almost as if sunlight affects human growth in the same way it does beanstalks and marrows; and, indeed, children also grow faster during spring and summer – as does the hair on your head and the hair of men’s beards.1
However, the most robust associations between your birth date and later life are to do with your risk of developing particular diseases. As far back as 1929, a Swiss psychologist called Moritz Tramer reported that individuals born in late winter were at greater risk of developing schizophrenia, an association that’s been borne out by more recent studies: in the northern hemisphere, people born between February and April are 5 to 10 per cent more likely to develop the disorder, compared with those born at other times of the year.2 Indeed, the risk is almost twice that associated with having a parent or sibling with schizophrenia. Late spring babies are at greater risk of anorexia and suicide in later life, whereas people with birthdays in autumn are marginally more likely to suffer from panic attacks, and – in men at least – alcoholism.
So, what lies behind all this? Plenty of scientists lay the blame on sunlight – in particular, the amount of sunlight mothers were exposed to during the second half of pregnancy. As we know, sunlight exposure is vital for the production of vitamin D, and vitamin D deficiency is associated with various psychiatric and immune-related disorders.
Various alternative explanations have been put forward for these month-of-birth effects, including temperature, diet and exercise levels – all of which can vary with the seasons. In the case of allergic asthma, individuals born at the end of summer and during early autumn – when there are more house-dust mites around – have a 40 per cent greater risk of developing asthma, which probably relates to when their developing immune systems are first exposed to the allergens that trigger it.3 There are also seasonal peaks and troughs in the abundance of bacteria and viruses, and how easily they spread. For instance, cold, dry weather causes the fine mist of snot and the viruses we expel when we sneeze to linger in the air for longer, making it more likely that they’ll be inhaled by someone else.4 A mother’s exposure to such infections might also influence the way her baby’s immune system develops.
Yet the sunlight exposure of the mother remains the strongest suspect in many of these birth-month-related conditions, not least because summer newborns have twice the level of vitamin D in their blood as winter ones do, which demonstrates the magnitude of the difference in sunlight exposure between these seasons. This, or some other factor in sunlight, seems to shape how the baby’s body develops, altering their future disease risk.
Sun exposure is not just an issue during pregnancy: sunlight is implicated in other medical mysteries as well. Various conditions, including type 1 diabetes,5 asthma, high blood pressure and atherosclerosis are more prevalent among people living at high latitudes – where the days are shorter and the sunlight weaker during winter months – compared with those living nearer the equator. Many of the symptoms of these conditions also tend to improve during the summer months, when there’s more sunshine around.
One of the strongest latitude associations is for multiple sclerosis (MS), which is also, interestingly, more prevalent among babies born in spring. MS is an autoimmune disease in which the insulating sheaths around nerves in the brain and spinal cord come under attack. A recent meta-analysis, which combined the results of 321 studies looking at MS prevalence, concluded that for every degree further north or south one travels from the equator, there are an extra 3.97 cases of MS per 100,000 people.6 MS is also three times more prevalent among people who were exposed to low amounts of sun during youth and adolescence.
If you’re looking for a case study on the role of sunlight in MS levels, you could look at its mysterious boom in sunny Iran, a country that, theoretically, you would expect to have relatively low levels of the disease. And historically Iran did have low rates of multiple sclerosis, in common with other Middle Eastern countries. Yet between 1989 and 2006,7 there was an eight-fold increase in cases to almost six per 100,000 people.8 Why?
The prime suspect has been a lack of vitamin D, which is increasingly being shown to have other roles in the body besides maintaining healthy bones and teeth. Vitamin D receptors are found in the heart and on pancreatic cells that synthesise insulin,9 and vitamin D deficiency is associated with both heart disease and type 1 and type 2 diabetes. It influences the development of brain cells, and their signalling and overall health.10 It is also used by various immune cells to help them fend off attack from foreign invaders and promote wound repair. Of particular relevance to multiple sclerosis, vitamin D also appears to stimulate the development of regulatory immune cells, which can prevent immune reactions from spiralling out of control.
Low vitamin D during pregnancy has been associated with an almost doubling in the risk of the baby developing multiple sclerosis in later life;11 while young adults with high vitamin D levels are at reduced risk of the disease.
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Sitting relatively close to the equator, and receiving an abundance of sunny days, there should be ample opportunity for Iranians to make enough vitamin D. And until quite recently there was. During the mid-twentieth century, Iran was a country heavily influenced by Western fashions and culture. The last Shah of Iran, Mohammed Reza, who ruled the country from 1941 until 1979, had an inclination for European sports cars, racehorses and American actresses, and he wore Western clothes; while actresses and pop singers were often photographed in miniskirts and bathing suits. All of that changed with the Islamic revolution of 1979. From then on, men were told to dress conservatively, and women were forced to wear long, loose garments, cover their hair and veil their faces – on pain of arrest by the morality police. Skin previously drenched in sunlight was suddenly covered up.
Currently, vitamin D deficiency is high among the general Iranian population, and significantly more prominent among women and children. Data from Harvard School of Public Health also implicates vitamin D in MS, showing that people with lower levels of vitamin D in their blood during the earliest stages of the disease are more likely to develop full-blown symptoms and have a poorer prognosis.12
Like circadian rhythms and melatonin, vitamin D has ancient origins; it has been estimated that phytoplankton and zooplankton in our oceans have been producing it for more than 500 million years. The inactive, precursor form of vitamin D is found in most life forms, including these tiny marine plankton: this may explain why the livers of fish – which eat plankton – are such a rich dietary source of vitamin D. In these early organisms, it helps to protect them from the more destructive aspects of the sun’s energy, by absorbing some of the UV rays that cause DNA damage.
However, the active form of vitamin D, which is so important to the human skeleton – and the apparatus needed to generate it – are only found in vertebrates.
The trouble is that at latitudes above 37°, which includes anywhere north of San Francisco, Seoul or the Mediterranean Sea, and in most of New Zealand and parts of Chile and Argentina in the southern hemisphere, the amount of vitamin D synthesised during winter months is negligible. In the UK, we can only make it between late March and September, which makes us reliant on reserves of vitamin D built up during sunnier months, as well as dietary vitamin D from sources like oily fish, egg yolk and mushrooms.
The fact that so many of us spend so much of our daytimes indoors has prompted concern that many people at high latitudes aren’t storing enough vitamin D to see them through winter – and that their bones, muscles and possibly other tissues are suffering as a result. In 2016, the UK’s Scientific Advisory Committee on Nutrition even recommended that all Brits consider taking vitamin D supplements during winter months – primarily to protect their bones. Particularly for elderly people, falls and fall-related fractures are a major source of injury and death, and a major drain on the health service – so this is good advice. Yet the list of other illnesses that have been associated with vitamin D deficiency in recent years is extensive: as well as MS, it includes cardiovascular disease, various autoimmune and inflammatory diseases, infections, and even infertility.
You might conclude, therefore, that taking vitamin D supplements would equal better health. But sadly, for many of these illnesses, this doesn’t seem to be the case. This is also true of MS: although lower levels of vitamin D are associated with an enhanced risk of developing the disease, and a more severe disease course, no study has yet shown that vitamin D supplements can improve the symptoms of MS once it has developed.13
In late 2017, a review14 of multiple trials of vitamin D supplements in patients of all ages concluded that the evidence for them preventing or being useful in non-bone-related conditions was sparse, with two exceptions: vitamin D supplements can help to prevent upper respiratory tract infections and the worsening of existing asthma. Taking them is also associated with a longer life expectancy among middle-aged and older people – but mostly for those in hospital or living in an institution, where they don’t get outside much. These things are obviously important, but as a panacea for all the health challenges of the twenty-first century, vitamin D supplements are looking decidedly lacklustre.
This isn’t necessarily the end of the story for vitamin D. It could be that we haven’t yet found the best time to give supplements, or the right dose, or that the trials haven’t lasted long enough to detect an effect on our health; also, because many trials have included people with adequate vitamin D levels, this could have concealed the benefits of vitamin D supplements to people with deficiencies. Several large trials are still ongoing, and until the results of them are in, the jury remains out.
However, it’s also worth considering whether something else in sunlight is contributing to some of the wider health benefits ascribed to vitamin D, including the reduced risk of developing MS. Vitamin D is clearly good for us, but the level in our bodies is also a strong marker of how much time we’ve spent in the sun more generally. Popping vitamin D supplements is not the same thing as spending more time outdoors; and if we rely on them to make up for inadequate sunlight exposure, then we may be missing out on something else that sunlight provides.
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Slip! Slop! Slap! As health campaigns go, the Australian Cancer Council’s SunSmart slogan, which featured a dancing cartoon seagull advising people to slip on a shirt, slop on the sunscreen and slap on a hat, ranks among the most successful in Australia’s history. Launched in 1981, the message seared itself into the collective psyche, and is widely credited with reducing the incidence of basal-cell carcinoma and squamous cell carcinoma – the two most common forms of skin cancer.
In 2007, the slogan was updated to ‘slip, slop, slap, seek and slide’, to emphasise the importance of also seeking shade and sliding on a pair of wrap-around sunglasses in order to prevent sun damage.
Australia has one of the highest rates of melanoma on the planet. On average, thirty Australians will be diagnosed with it, and three will die from it each day. With all this talk of the beneficial effects of sunlight, it’s worth stressing the downside: there is no doubt that exposure to UV light, including sunlight, is responsible for causing skin cancer.
This was recognised as early as 1928, when the fad for UV lamps and sunbaths was approaching its peak. When a British researcher called George Findlay exposed mice to daily irradiation with UV from a mercury arc light, he observed that tumours developed on their skin. Since then, many more studies have strengthened the link between UV exposure and skin cancer, as well as demonstrating that sunscreen reduces the risk of developing it.
The reason is that UV light triggers DNA mutations in our skin cells, causing them to malfunction and start growing abnormally. Increasingly, though, it seems that an additional mechanism may be at work – one that might also explain sunlight’s beneficial effects in inflammatory and autoimmune diseases. As ever, sunlight is a double-edged sword: a creator and destroyer of life.
During the 1970s, an American researcher called Margaret Kripke discovered that if she implanted skin cancers into healthy mice, they were rejected, but if she implanted them into mice that had previously been irradiated with UV light, they established themselves and grew.15 Kripke concluded that UV light must somehow be suppressing the immune system, which could help explain why immune cells – usually so good at detecting and destroying abnormal cells – sometimes fail to detect and reject early skin cancers caused by sun exposure.
In other words, as well as triggering the mutations that cause them in the first place, the reason skin cancers are permitted to grow is because the immune system is dampened by too much sun exposure.
The skin is our largest organ, covering approximately 2 square metres, and weighing some 3.6 kg. According to the Encyclopaedia Britannica, skin provides protection and receives sensory stimuli from the external environment. However, it seems that we may have grossly underestimated its function. Recent evidence suggests that our skin is also a vital part of the immune system, conveying information about outside threats to the vast immunological orchestra at its command.
The predominant cell in our outermost layer of skin, the epidermis, is the keratinocyte. As well as producing the structural protein keratin, which renders our skin almost waterproof, keratinocytes are also in constant dialogue with immune cells in nearby lymph nodes, as well as with nerve cells in the skin.
These keratinocytes are covered in receptors that can absorb UV light: they respond to it by sending chemical signals to various immune cells – particularly a subset of ‘regulatory’ cells, which help to keep the immune system in check – and if the signals are strong enough they will transmit them to the rest of the body, suppressing its immune responses.
Given that we’ve evolved as daytime creatures on this sunny planet, presumably there’s a reason for this immune suppression. One idea is that it’s a way of tolerating ‘self’; the immune system is a powerful weapon, which, left unchecked, could quickly turn on our own tissues and destroy them, so tolerating ‘self’ is essential for survival. ‘If you break tolerance, essentially the immune system will kill you,’ says Scott Byrne, an immunologist at the University of Sydney, who has been investigating this new-found role of UV light. ‘By getting sunlight, we are essentially maintaining that tolerogenic environment, which is essential for preventing autoimmune diseases.’16 On the flip side, if we get too much sunshine, our immune cells also start to tolerate cancers growing in the skin.
Prue Hart, an immunologist at the University of Western Australia, has long been fascinated by the latitude associations for autoimmune diseases such as MS, and she has been disappointed by the results of vitamin D trials, which have failed to show a benefit of supplements in slowing or stopping progression of the disease. However, the discovery that UV light suppresses certain immune responses has inspired her to start investigating UV light as a potential therapy for MS. Already she has shown that by irradiating mice with UV doses roughly equivalent to a brief stint in the midday sun she can prevent them from developing an experimental form of multiple sclerosis, called experimental autoimmune encephalomyelitis (EAE).17 Now she’s working with Byrne to investigate whether UV exposure from phototherapy lamps – more commonly used to treat inflammatory skin conditions, such as psoriasis – could slow, or even prevent, the development of MS in people showing the very first symptoms of the disease.
In a pilot study of twenty people,18 only seven of the ten who received phototherapy for two months had developed full-blown MS a year later, whereas everyone in the control group had. The UV group also reported feeling less fatigue. Importantly, vitamin D levels remained similar between the two groups, suggesting that this wasn’t the reason for the improvement. Although it is still very early days, and larger trials will need to be carried out, these results are a ray of hope for people suffering from autoimmune diseases.
And yet immune suppression doesn’t explain everything. It can’t, for instance, explain why sunbathers appear to have longer life expectancies, despite the increased risk of cancer that this activity carries.
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Richard Weller started out his career as a ‘good’ dermatologist, believing that sunlight was dreadfully bad for you, ‘because that’s what dermatologists say’. He still doesn’t dispute that it’s a major risk factor for skin cancer. Even when he discovered that the skin can produce nitric oxide – a potent dilator of blood vessels – he assumed that it would be involved in driving the growth of skin cancer rather than being beneficial to our health.
Then he discovered that we stockpile vast quantities of a storable form of nitric oxide in our skin, and that it can be activated by sunlight. That’s when the blinkers fell off: ‘Hang on,’ he thought. ‘Maybe that’s the reason why people’s blood pressure readings are lower in summer than in winter.’19 Related to this, it may also help explain the greater rates of cardiovascular disease at higher latitudes.
Subsequent experiments confirmed it. If you expose somebody to the equivalent of around 20 minutes of British summer sunlight, they will experience a temporary drop in blood pressure that continues even after they step indoors.20
It’s not only blood pressure that seems to benefit from the mobilisation of nitric oxide by sunlight. Separate studies have revealed that mice fed a high-fat diet can be protected against the usual weight gain and metabolic dysfunction through regular exposure to UV light.21 Block nitric oxide production and you block this protective effect. Nitric oxide is also implicated in wound healing, not to mention achieving and maintaining an erection in men. And nitric oxide seems to be another substance to which those regulatory cells that dampen excessive immune reactions respond.
This previously unrecognised interaction between sunlight and our skin could go some way to explaining the perplexing results of the Melanoma in Southern Sweden study.
This study was launched in 1990 to try to gain a better understanding of the risks associated with melanoma and breast cancer. Researchers recruited 29,508 women with no history of cancer, interviewing them about their health and behaviour, and then following them up at regular intervals to see how their health fared.
Among other questions, the women were asked about their sun habits. How often did they sunbathe in summer? Did they sunbathe during the winter? Did they use tanning beds? And did they go abroad to swim and sunbathe? Based on their answers, the women were placed in one of three categories: ‘avoidance of sun exposure’, ‘moderate sun exposure’ or ‘most active sun exposure’.
Twenty years after the study began, the researchers crunched some of the data, and made some surprising discoveries. The first was that life expectancy among women with active sun exposure habits was one to two years longer than that of sun avoiders. This was even after adjusting for factors such as disposable income, education level, exercise and so on, which might skew the results.
If confirmed, this would put sun avoidance on a par with smoking in terms of its effect on life expectancy, the authors said.22 Women in the sun-avoidance group had double the death rate, during the study period, of women in the high sun exposure group. Women in the moderate group fell in between.
Controversial as it sounds, the finding fits with other studies that have associated low vitamin D levels with shorter life expectancy. Of course, we now know that sunlight has other effects on our body that could help explain the link, and vitamin D may merely be a marker of overall sun exposure. Alternatively, vitamin D could be having other, unrecognised effects on our biology: ones that prevent early death.
When the Swedish researchers looked at the reason for this reduced life expectancy among sun avoiders, they discovered that it was mostly due to a greater risk of death from cardiovascular disease and other non-cancer-related illnesses, such as type 2 diabetes, autoimmune disease or chronic lung disease.
Another counter-intuitive finding of the study was that active sun-seekers who developed skin cancers other than melanoma had the highest life expectancy of all. Even so, women in the high sun exposure group were more likely to die from cancer than those in other groups, probably because they were living longer. They were also more likely to get skin cancer, including melanoma. Yet if they did get it, their survival rates were higher than those of sun avoiders with the disease.23
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All of this leaves health policymakers with a dilemma. Many Australian schools have a ‘no hat, no play’ policy, to protect children from the sun. This makes sense during the summer months, especially in a country like Australia, where the sun’s rays must travel through less atmosphere before hitting the ground, so are stronger. However, similar policies are now finding their way into schools at higher latitudes – including British schools – where the sun is often weaker.
Even Cancer Council Australia, which launched the Slip! Slop! Slap! campaign, has introduced a more nuanced message in recent years in order to reduce the risk of vitamin D deficiency. It is now emphasising the importance of the UV index – a measure of how strong the sun’s UV rays are, and therefore when we’re at greatest risk of burning – in dictating when sunlight should be avoided. Together with other Australian medical bodies, Cancer Council recommends staying out of the sun when the UV index is 3 or above – even for people who have been diagnosed with vitamin D deficiency – and following the slip, slap, slop, seek and slide message if you are outdoors for more than a few minutes.
However, in more southerly parts of the country during autumn and winter, they are actively encouraging people to head outdoors in the middle of the day with some skin uncovered in order to synthesise vitamin D.
In higher latitude countries such as Britain, the UV index rarely exceeds 3 between October and March, but it can reach 6 on a sunny day in late April, and it may climb as high as 7 or 8 during midsummer. Cancer Research UK recommends thinking about sun protection – especially between 11 a.m. and 3 p.m. – when the UV index is between 3 and 7, and using it at all times when it is 8 and above. An index of 9 or 10 is common during Mediterranean summers – and may even, if rarely, hit 11, which is where the index stops.
Most important of all is to avoid sunburn. If you compare skin cancer rates between outdoor workers and office workers, it’s those who spend their working days indoors who are at the greatest risk of deadly melanoma. Outdoor workers are at greater risk of other types of skin cancer, but these are less likely to kill. One reason is that office workers tend to engage in more ‘binge-bathing’ – that is, heading down to the beach at weekends and overindulging in sunlight, becoming burned in the process: sunburn is a major risk factor for melanoma.
Another possibility is that the difference in outcomes is related to the type of UV rays that people encounter; outdoor workers are regularly exposed to both UVA and UVB rays, whereas office workers may receive relatively high doses of UVA (which can penetrate office windows), but not UVB. Although both sorts of ray play a role in skin cancer, curiously, vitamin D – which is synthesised using UVB light – seems to afford skin cells some protection against DNA damage.
Although few people would currently advocate sunbathing as a means of avoiding skin cancer, several trials are now investigating whether applying vitamin D directly to the skin could be a way of mitigating some of the harmful effects of sun exposure.
Taken together, these new scientific findings suggest that our transition from predominantly outdoor- to indoor-based lifestyles in recent decades could be having unexpected consequences – including raising our risk of MS, as those studies in Iran have hinted. They also illustrate the pitfalls of trying to replace sunlight, which has shaped our evolution over hundreds of thousands of years, with a single supplement – vitamin D. Although vitamin D is clearly important to many aspects of our health, and supplements are one way of ensuring that those of us living at high latitudes get enough of it during the winter months,24 they are no substitute for adequate daylight exposure throughout the year (we also need bright daylight to keep our internal clocks synchronised). Too much sunlight is obviously bad for us, but too little also puts our health at risk. The sun should feature in our daily lives, as it has for millennia.
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There’s one other thing sunlight does to the skin that warrants mentioning. When sunlight hits the skin, it triggers the production of several molecules that encourage the production of melanin – the pigment that causes skin to tan and affords some protection against sun damage. One of them is ß-endorphin, a substance which triggers the same receptors as opiate drugs like morphine or heroin.
Endorphin release could be another means by which sun exposure reduces the risk of heart disease: by promoting feelings of relaxation, it may combat the negative effects of stress on the heart. Endorphins also activate the reward system, a pathway in the brain that triggers feelings of pleasure in response to specific stimuli – in this case sun exposure – encouraging us to seek them out again. Some regular sunbed users even exhibit physical withdrawal symptoms, similar to those associated with coming off heroin, if they stop tanning.
The release of ß-endorphin in response to sunlight could therefore go some way towards explaining why being in the sun feels so good, and why we so crave it when the sun grows weaker during winter.