METHODS: This study included 264 MS patients (mean age 46.9±10 years, disease duration 14.6±10 years; 67.8% relapsing-remitting, 28% secondary progressive and 4.2% primary progressive MS) and 69 healthy controls. Subjects underwent neurological and 3 T MRIexaminations, provided blood samples and answered questions to a structured questionnaire. Information on race, skin and eye colour, supplement use, body mass index (BMI) and sun exposure was obtained by questionnaire. The vitamin D metabolites (25-hydroxy vitamin D3, 1, 25-dihydroxy vitamin D3 and 24, 25-dihydroxy vitamin D3) were measured using mass spectrometry.
RESULTS: Multivitamin supplementation (partial correlation r(p)=0.29, p<0.001), BMI (r(p)=-0.24, p=0.001), summer sun exposure (r(p)=0.22, p=0.002) and darker eye colour (r(p)=-0.18, p=0.015) had the strongest associations with vitamin D metabolite levels in the MS group. Increased summer sun exposure was associated with increased grey matter volume (GMV, r(p)=0.16, p=0.019) and whole brain volume (WBV, r(p)=0.20, p=0.004) after correcting for Extended Disability Status Scale in the MS group. Inclusion of 25-hydroxy vitamin D3 levels did not substantially affect the positive associations of sun exposure with WBV (r(p)=0.18, p=0.003) and GMV (r(p)=0.14, p=0.026) in the MS group.
CONCLUSIONS: Sun exposure may have direct effects on MRI measures of neurodegeneration in MS, independently of vitamin D.
To me, this study highlights one of the problems with examining vitamin D levels in people with MS in order to provide evidence about causation and/or prognosis - we really need to know what the vitamin D levels were pre-diagnosis. Vitamin D levels fluctuate from month to month, and your level now is not a good indication of what your level was months or even years ago.
Whilst the association of sun exposure with MRI measures of MS is interesting, this study does not tell us whether this is a direct effect of the sun, or a result of something that is a common link between the sun and MS (could be vitamin D, alternatively this could be ice-cream consumption!). There are ways of measuring long-term skin exposure to sunlight, but to work out whether there is a effect of sun exposure over and above the generation of vitamin D, what we really need is a measure of long-term vitamin D status.