The north of Scotland has long been an important part of our quest to understand what causes multiple sclerosis (MS). Though we have known about the region’s high rate of the disease for some time, it moved centre stage after a 2012 study comprehensively revealed staggering levels in the Orkney isles which lie off the northernmost mainland.
The study appeared to heavily support the theory that low UV-B sunlight is a key initiating factor in the damage to pathways transmitting signals to and from the brain that we see in MS sufferers. People who are exposed to less of this light tend to have lower levels of vitamin D in their blood, and studies have repeatedly shown links between low vitamin D and higher MS incidence – most likely interacting with other factors like genetics and plain bad luck.
But now a more recent study has shown that Orcadians have higher vitamin D levels than the Scottish average. So what is going on? Is it back to the drawing board for understanding MS?
In fact, the authors are careful to point out that the study is not as straightforward as the headline numbers make it appear. They monitored the vitamin D levels in the blood of individuals who have lived on Orkney for at least a generation and compared them to individuals in the same age group from the rest of Scotland. They did find higher than average vitamin D levels among Orcadians, but this was entirely due to two age groups, 60 to 69-year-olds and the 70s and over.
In all other age groups, the study found that vitamin D levels were lower than the rest of Scotland. Indeed the average levels of vitamin D in Orcadians under the age of 40 was substantially lower than their Scottish equivalent, who are themselves considered to be at risk of deficiency. This age range is probably the most relevant to understanding MS, since they are more likely to be child bearing and there is a strong correlation between vitamin D levels in the womb and MS susceptibility for the offspring. The group also includes those most likely to develop MS, which are women in their late 20s and early 30s.
Vikings And Screen Addicts
There are at least two possible explanations for this difference in the age groups: it could be that Orcadians enjoy greater exposure to sun as they get older; or it could be that the younger generation is spending less time outdoors. To assess that first possibility, the study’s authors used lifestyle questionnaires. This helped them establish that the vitamin D data was being skewed by two groups – farmers and financially secure over-60s who take regular foreign holidays.
The prospect of a generational change in Orcadians is more worrying because it might mean that MS levels will rise on the islands. Indeed, there is already evidence of a rise in the past 35 years. Yet the new study hints that lifestyle changes in the older generations is actually the reason for the discrepancy. We will need to see further studies to clarify this.
‘Hay-men to that.’ Robert Scarth, CC BY-SA
So can we reinstate the lack of UV-B light and vitamin D as the prime suspect in Orcadians’ increased risk of MS? Maybe not entirely. While the incidence of MS in Orkney is 402 cases per 100,000, compared to around 200 in the rest of Scotland and 165 in England, in the even more northerly Shetlands it is 295. Yet Shetlanders should be even more sunlight-deficient than Orcadians.
Maybe the populations of the two island groups are exposed to different levels of sunlight because of differences in their culture or lifestyle, but the work has not been done in the Shetlands to take us beyond speculation here. We also know that genes play a substantial role in MS susceptibility and it cannot be ruled out that genetic difference between populations may play some role.
‘Not guilty.’ Alice, CC BY-SA
In this regard, the press has in the past wondered whether the Vikings brought MS to the Orkneys. This seems fanciful because Vikings probably had an even greater influence on the Shetlands. MS incidence in Scandinavian countries is meanwhile lower than the Scottish average. So if genetics is partly to blame, it’s more subtle than longboats and horned helmets.
In short, the study does raise a mystery, though not the one it first appeared to. It seems to support the sunlight theory of MS after all, but it leaves us asking what cultural shift is occurring in Orkney that is resulting in over 60s having higher vitamin D levels than the rest of the population. Getting to the bottom of that could be a major step towards refining our understanding of what causes the disease.
Graham Wright, Lecturer in Immunology, Edinburgh Napier University
This article was originally published on The Conversation. Read the original article.