- Two studies from the USA and Australia have shown that lifetime sun exposure affects the risk of developing MS, as well as the course of their disease
- People with higher sun exposure had lower conversion to clinically definite MS and lower numbers of relapses
- Similar consistent results were not seen for the level of vitamin D in the blood and the course of a person’s MS
- People who significantly changed their sun exposure over the five years of the study lowered their chance of converting to MS and their chance of having a relapse
- The US study showed that lifetime sun exposure was associated with risk of developing MS in black, white and Hispanic people, whereas low vitamin D levels were a risk factor only for white people
- Another study showed that living in an area with higher sun exposure prior to the onset of disease and during childhood lowered the risk of developing MS
Two studies from the USA and Australia have shown that lifetime sun exposure affects the risk of a person developing MS as well as the course of their disease.
Previous research has shown that low exposure to ultraviolet (UV) light from the sun and low levels of vitamin D is linked to an increased risk of developing MS. However, relatively little research has looked at whether sun exposure over a person’s lifetime or vitamin D affects the severity or course of MS.
The new Australian research, published in Frontiers in Neurology, has answered this question, using information gathered as part of the Ausimmune Study. The multi-centre AusImmune study has investigated the environmental factors associated with a first attack of MS-like symptoms, known as a first demyelinating event, and has then followed these participants for many years. This longer term tracking of the participants makes the Ausimmune study uniquely powerful compared to other research designs, which only compare results from people with MS to those from people without MS at a single snapshot in time.
The new study, led by Professor Bruce Taylor from the Menzies Institute for Medical Research in Hobart, followed 145 people who had experienced a first demyelinating event for five years. Participants were asked to complete questionnaires about sun behaviour and time spent outside at different ages in order to determine their lifetime sun exposure. Researchers also took ambient UV light measurements at participants’ homes and places of work each month in order to estimate their exposure to UV light. Vitamin D levels in the participants’ blood were measured at the start of the study, at the halfway point and after the full five years.
The study showed that higher lifetime sun exposure lowered the likelihood of an individual converting to a diagnosis of MS following a first demyelinating event. For a diagnosis of MS to be made, two separate demyelinating events need to have taken place. So in this study, higher sun exposure protected people from experiencing the second demyelinating event over the following five years.
Higher UV exposure prior to experiencing a first demyelinating event, particularly during childhood and adolescence, also reduced the risk of relapse over the five years.
When researchers examined the UV exposure of the whole group following the first demyelinating event, they found that this was not linked to a reduced risk of converting to MS or experiencing a relapse. However, most participants had relatively stable sun exposure over the course of the study.
A smaller group of people who significantly increased their sun exposure over the five years following their first demyelinating event showed a reduced risk of converting to MS, which reduced their chance of having a relapse. The opposite was also true: those who significantly reduced their sun exposure increased their chance of conversion to MS and of having a relapse. This finding implies that, in some people with MS, sun exposure can have a beneficial effect on the course of the disease.
No link was found between the level of vitamin D in a person’s blood and whether or not they converted to a diagnosis of MS or had a relapse over the study period. Similarly, people whose vitamin D levels changed significantly during the study did not have any corresponding change to their risk of relapses or conversion to MS. While other research has found that vitamin D does play a role in the course of a person’s MS, this study implies that it is sun exposure that has the dominant effect, rather than vitamin D. This study provides evidence that inventions for MS that safely increase exposure to UV, such as narrow band UV radiation therapy, may be more beneficial for MS than vitamin D alone.
In a separate study, Kaiser Permanente of Southern California investigated whether lifetime sun exposure and vitamin D altered risk of MS in different racial groups. The study, published in the journal Nutrients, aimed to isolate the effects of sun exposure and vitamin D on MS risk by taking into account differences, such as genetic ancestry.
Called the ‘MS Sunshine Study’, the research looked at people with MS from white, black and Hispanic backgrounds and compared them to people without MS. The researchers found that, in people of either white or black backgrounds, higher lifetime sun exposure lowered the risk of developing MS. A similar trend was seen in the Hispanic group. This effect was independent of the level of vitamin D in their blood. Higher levels of vitamin D in the blood was associated with a lower risk of MS only in people of a white background, not in people of black or Hispanic descent.
The lack of correlation between vitamin D levels in the blood and MS development seen in these studies supports the hypothesis that sunshine is able to modulate disease course through a pathway that is independent of vitamin D.
A third study, from researchers based in Canada and the US published in the journal Neurology, has also shown that sun exposure in early life can alter the risk of developing MS. The study compared lifetime and seasonal sun exposure of people with MS to people without MS. The researchers also attempted to clearly distinguish the effects of sun exposure by taking into account other factors including genetic ancestry, smoking and vitamin D supplementation. Most of the people taking part in this study were of a white background.
The researchers found that living in an area with higher sun exposure prior to the onset of disease – in particular, the 5 to 15 years before onset – lowered the risk of developing MS. They also found childhood sun exposure was important; living in an area with higher sun exposure and experiencing high sun exposure during summer between the ages of 5 to 15 years was associated with a lower MS risk later in life. Unlike the two previous studies, this third study did not directly measure the vitamin D levels in the participants. However, this study adds further weight to the role of sun exposure in the risk of developing MS.
With thanks to MS Research Australia – the lead provider of research summaries on our website.