There’s nothing quite like the feel of sunlight on your skin to make you feel glad to be alive. Yet in Australia we tend to avoid direct sunlight and use sunscreen liberally to avoid both premature skin ageing and skin cancers which are known to be caused by overexposure to ultraviolet light. It seems that Australians have been a little too diligent at keeping in the shade, since we are now a nation where 1 in 3 people are known to be vitamin D deficient, a condition caused by inadequate exposure to sunlight (1). It has been known for over 100 years that adequate levels of vitamin D are required for optimum bone health. More recently, vitamin D deficiency has been linked to an increased risk of some cancers and autoimmune diseases (2). Now it has been shown that regular sun exposure extends our lives by means other than through the effects of vitamin D (3). This newest finding has implications for those looking at oral vitamin D supplementation as a substitute for the health benefits of sunlight. In other words, vitamin D supplements are not a complete substitute for the health benefits of exposure to the sun.
Why is a low level of vitamin D a health problem?
Vitamin D promotes absorption of calcium from the intestine, and is important in regulating both calcium and phosphorus levels in the body. In children, vitamin D deficiency may result in failure to reach their genetically programmed height or, in severe deficiencies, rickets. In adults, vitamin D deficiency increases the risk of osteoporosis and fractures (2).
Cancer and other chronic diseases
As long ago as 1941, it was noted that people living at higher latitudes, i.e. further away from the equator, had a higher risk of dying of cancer. This association has since been found to occur for a range of cancers, including colon, prostate and breast cancers. Similarly, living at higher latitudes is associated with an increased risk of cardiovascular disease, multiple sclerosis and possibly other autoimmune diseases. Since many people living at higher latitudes are vitamin D deficient, it has been considered that vitamin D deficiency may be a causal factor in at least some of these cases. This link has been reinforced by the fact that the vitamin D receptor is present in most cells in the body, including activated T and B lymphocytes, and the involvement of vitamin D regulation of cell growth (2).
Exposure to sunlight reduces blood pressure
In 2014, a study demonstrated a significant reduction in blood pressure following exposure to UV A light, mediated by the production of nitric oxide in the skin. Nitric oxide is released from the skin into the blood stream where it dilates blood vessels, reducing blood pressure (3). This is the suggested mechanism for the observed higher blood pressure and greater incidence of cardiovascular disease in those living at higher latitudes. Therefore, some of the health benefits of exposure to sunlight seem to be due to mechanisms other than vitamin D (4).
What about the increased risk of skin cancer?
The most serious form of skin cancer, melanoma, is known to be sun-induced, although a good proportion of melanomas occur in non sun-exposed parts of the body, indicating that some melanomas arise from DNA mutations that are not induced by sunlight. The biggest risk factors for melanoma are considered to be a higher number of sunburn incidents, having more moles, and having red hair. There is a lower incidence of melanomas in outdoor workers than indoor, and in the untanned than the tanned, suggesting that low regular levels of exposure to the sun may be protective against melanoma, at least in the population at lowest risk (2, 4).
For non-melanoma skin cancer, basal cell tumours and squamous cell carcinomas, the major risk factors are chronic sun exposure and a higher incidence of sunburn incidents. However, these forms of skin cancer are rarely fatal, compared to melanoma which has a 10-20% risk of fatality. In fact in one study of over-40 year olds in Denmark, those with non-melanoma skin cancer were less likely to die than healthy controls, and were less likely to have a heart attack (2,4).
How do we ensure we have adequate exposure to sunshine?
It is not yet clear how much sunlight exposure is required to achieve healthy levels of nitric oxide production in the skin for reduction of blood pressure. For vitamin D, UV B rays from the sun convert a vitamin D precursor in skin cell membranes to vitamin D3, which is released into the circulation and transformed in the liver and then the kidney to the active form of vitamin D. The amount of vitamin D3 that is formed in the skin upon exposure to sunlight varies with three factors:
The intensity of sunlight – less time is required to produce the same amount of vitamin D during summer and at lower latitudes, ie closer to the equator.
The amount of vitamin D precursor present in the skin – we produce less vitamin D precursor as we age, so that after the age of 70, there is around 25% less vitamin D precursor in the skin than in a young adult. This means that older people require longer sun exposure to produce the same amount of vitamin D.
The amount of UV B that reaches the skin – melanin is a highly efficient sunscreen and blocks UVB light, which is required for vitamin D conversion. The darker the skin, the longer the sunlight exposure required to convert the same amount of vitamin D precursor. Likewise, sunscreens block most UVB light and therefore prevent the conversion of vitamin D precursor.
The Cancer Council has put out guidelines for recommended skin exposure times in different parts of Australia to achieve adequate levels of vitamin D whilst minimising sunburn risk (5). For the southern Australian cities, a fair-skinned person would need only a few minutes per day of sunshine without UV protection during summer, and as much as 2-3 hours per week during winter.
Too little sun is bad for your health
Although dermatologists are understandably reluctant to change recommendations about the dangers of sun exposure, the evidence is mounting that too little sun can kill you. In one study of 30,000 Swedish women, those who did the most sunbathing were half as likely to be dead 20 years later than those who avoided the sun altogether. And a Scandinavian study of 40,000 women found that those who went on the most sunbathing holidays were least likely to have died 15 years later (4). Some researchers say that there are potentially more undiscovered health benefits of direct sunshine on the skin. To my mind, a regular sensible dose of the real thing has to be better than the ‘sunshine substitute’ of a vitamin D supplement. And it feels better too!
(2) Holick, M.F. ( 2005) Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease Am J Clin Nutr 2004;80(suppl):1678S– 88S
(3) Liu, D, et al (2014) Journal of Investigative Dermatology 134: 1839–1846. ‘UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase.’
(4) Weller, R. (2015) The prodigal sun. New Scientist 3025, 13 June 2015, 26-27.