Tuesday July 31 2007
Exposure to sunshine as a child may protect against developing multiple sclerosis (MS), BBC News reported. According to the article, the researchers had suggested that "UV rays offer protection by altering the cell immune responses or by boosting vitamin D levels".
The story was based on a study which examined pairs of identical twins where one twin had MS. The study found that the twin with MS had less sun exposure as a child than the twin who did not develop MS. This was a relatively small study and its results should be considered as preliminary.
The actual risk for a person getting MS is small, and therefore even a 25% change in risk will mean a relatively small change in absolute risk for an individual. It should also be taken into account that exposure to the sun in childhood cannot count for all the risk of developing MS.
This potential benefit of sun exposure should be balanced with the known dangers of excessive sun exposure such as skin cancer.
Where did the story come from?
The research was conducted by Talat Islam, Thomas Mack and colleagues at the University of Southern California’s Department of Preventive Medicine in America. The study was supported by the Multiple Sclerosis Society, and several governmental agencies, including the National Institute of Neurological Disease and Stroke.
The study was published in the peer-reviewed journal Neurology, which is the journal of the American Academy of Neurology.
What kind of scientific study was this?
This was a case-control study using identical twins to look at the role of sun exposure in MS. In this type of study identical twins are used because they share exactly the same genetic make-up, and therefore any differences between them should be caused by differing environmental exposure.
Authors had access to data from a large register of twins with long-term conditions, collected between 1980 and 1992. They looked at the data on 193 identical twin pairs in which one twin had self-reported a diagnosis of MS (cases) and one twin did not report MS (controls). Surrogate measures of sun exposure were determined through a questionnaire asking the twins about their outdoor activities as children.
The 193 twin pairs who had each completed the questionnaire and both returned it before 1993 were included in the analysis. The twins were asked which of them had spent more time outdoors in different seasons and weather, sunbathing, at the beach, or playing sport.
Only the 79 twin pairs where sun exposure differed on at least one item, and whose responses were in agreement, were included in the analyses. Based on the responses, researchers calculated a score (sun exposure index) for each twin, giving a point for each item where they had more exposure than their co-twin. Scores ranged from 0 (no more exposure than twin) to 9 (more exposure than twin on all measures).
Researchers used complex statistical methods to look at whether sun exposure differed significantly between case twins and control twins. These analyses took into account other factors that might affect risk of MS, such as having had glandular fever, childhood illnesses, age at first period in women, and being a smoker.
What were the results of the study?
The researchers found that the twin who did not have MS was more likely to have spent more time outdoors in the spring, during hot days, while sunbathing and while at the beach during childhood, than the twin with MS. The difference between twins in outdoor exposure in the summer, autumn, or winter, or in exposure on cold days or during sport was not large enough to be considered statistically significant. For every item where a twin had greater outdoor exposure, their risk of MS decreased by 25%.
What interpretations did the researchers draw from these results?
They concluded that being exposed to the sun in childhood protected against developing multiple sclerosis.
What does the NHS Knowledge Service make of this study?
This study was a relatively small study, with interesting findings. However, its results should be considered preliminary, and more research would be needed to confirm these findings. There are a number of limitations to this study, including the facts that:
- Sun exposure in childhood cannot explain all of the risk for developing MS. Only about 40% of the twin pairs assessed in this study differed in their sun exposure, and therefore sun exposure did not seem to play a role in 60% of cases.
- A person’s risk of getting MS is small, and therefore even a 25% change in risk will mean a relatively small change in absolute risk for an individual.
- Although the authors adjusted their analyses for other potential risk factors for MS, it is still possible that the decrease in risk seen with outdoor exposure could be accounted for by other factors
- This research assessed sun exposure by asking about relative amounts of time the twins spent outdoors. It is not clear whether this is a reliable way to assess a person’s sun exposure. Also, because the measure is relative, it is not clear how much sun exposure (in terms of hours) the twins actually had.
- This study does not tell us whether sun exposure would have any affect on the progression of MS in people who already have the disease.
Excessive sun exposure is known to increase the risk of developing skin cancer, therefore people should take sensible precautions to avoid excessive sun exposure, and to protect their skin and their children’s skin when being exposed to the sun.