Does Your Birthday Matter?
One of the curious findings about multiple sclerosis (MS) is that you’re more likely to develop the disease if they’re born in the spring or summer, rather than in the autumn or winter – with May being the most common month of birth for people living with MS. Several studies presented at this year’s ECTRIMS (European Committee for Treatment and Research in MS) meeting examined this phenomenon.
A study in Norway looked at everyone listed in the country’s MS database and compared them to data from the general population (Grytten Torkildsen and colleagues. ECTRIMS 2012; abstract P370). Overall, people with MS were more likely to have been born in April or May, and least likely to have been born in November or February. This “birthday effect” was least pronounced in southern Norway, but was large in the north. In northern Norway (north of the 68th parallel, which runs through Alaska, northern Canada and Greenland), people were almost 50% more likely to have been born in May compared to November.
A similar study was done across the UK (Matthews and colleagues. ECTRIMS 2012; abstract P832). Overall, people with MS were most likely to have been born in April (12% higher than average) or May (6% higher) compared to November (15% lower than average).
A number of previous studies also found an MS birthday effect in northern countries, such as Canada, Denmark and Scotland (Willer and colleagues. Br Med J 2005;330:120; free full text at www.ncbi.nlm.nih.gov/pmc/articles/PMC544426/pdf/bmj33000120.pdf).
However, it isn’t clear if this effect also applies in the southern hemisphere. A recent study in South America found no latitude effect on MS risk as you move farther away from the equator (Fragoso and colleagues. ECTRIMS 2012; abstract P709), so other more important factors may be involved that influence MS risk in these countries.
Why a person’s month of birth or geographic latitude appears to affect their risk of developing MS has been attributed to the amount of sunlight they (or their mothers) receive. If you were born in late spring, your mother had her lowest sun exposure during her winter pregnancy. MS also shows a latitude effect – with MS risk increasing as you move farther away from the equator and which corresponds to less intense sunlight.
Sunlight is important because it induces the production of vitamin D in the skin, and vitamin D has been shown to have important effects on the immune system. In fact, a connection between MS and sunlight was first suggested over a half-century ago (Acheson and colleagues. Acta Psychiatr Scand Suppl 1960;35:132-147).
Some studies have suggested that vitamin D can reduce the frequency and severity of MS relapses, but others have shown little if any benefit with vitamin D supplements (Stein and colleagues. Neurology 2011;77:1611-1618; Mosayebi and colleagues. Immunol Invest 2011;40:627-697).
Sunlight is the best source of vitamin D. Other than vitamin D-enriched foods (such as milk or bread, added almost a century ago to lower the rate of rickets), few foods naturally contain vitamin D. Food sources of vitamin D include fish, liver, egg yolks and reindeer meat (with apologies to Santa). However, these foods can’t provide all the vitamin D we need – and even sun exposure won’t provide enough during the winter months for most people in the northern hemisphere.
Although there is no strong evidence of a benefit to vitamin D, many doctors will recommend vitamin D supplements because they may provide some help (Vitamin D3 is better than vitamin D2). We’ll know more about the potential benefits of vitamin D when the results of two large studies (called SOLAR and CHOLINE) are published.
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