Background: Multiple sclerosis (MS) is the most common chronic inflammatory and neurodegenerative autoimmune disease of the central nervous system. The cause of MS remains unknown, but both genetic and environmental predisposing factors have been identified. Vitamin D deficiency, smoking and Epstein-Barr virus (EBV) infection are the most prominent identified environmental risk factors for MS, but how early before MS onset these risk factors have impact is not known. EBV infection, smoking and vitamin D deficiency are risk factors also associated with cancer. The risk of cancer among Finnish MS patients has not been studied since 1990s.

Aims of the study: Finnish Maternity Cohort (FMC) is a serum bank collected from Finnish pregnant women since 1983. The main aims of this thesis were to study whether maternal smoking and vitamin D status during pregnancy are associated with later risk of MS among Finnish women, and to study whether maternal EBV antibodies affect the MS risk of the mother and her offspring. Further aims were to investigate the safety and efficacy of vitamin D supplementation in combination with fingolimod therapy in MS patients, and to study cancer risk and factors affecting it among MS patients in Southwest Finland from January 2004 to December 2012.

Results: Vitamin D deficiency during pregnancy was associated with an almost 2-fold risk of MS in both the Finnish mothers and their offspring. Smoking Finnish mothers had a 45% higher future MS risk compared to non-smokers. Highest titers of maternal EBV antibodies in comparison with the lowest tripled the MS risk in the mother and doubled it in her offspring independently of vitamin D and cotinine levels. Use of vitamin D supplements as an add-on to fingolimod showed beneficial effects on magnetic resonance image (MRI) outcomes and depression. Risk of cancer among MS patients in Southwest Finland was equal to controls, but age at breast cancer diagnosis was significantly higher among the MS patients. Smoking history of the patients was incompletely documented.

Conclusions: Correcting for vitamin D deficiency during pregnancy can be beneficial in MS prevention for both the mother and her child. Smoking is a risk factor for MS in Finnish women and should be a target for lifestyle intervention in young women. Prevention of EBV infection by vaccination could hold potential for decreasing the risk of both numerous cancers and MS, but the risk of increasing MS predisposition by postponing the timing of the EBV infection is a potential risk that must be taken into consideration.