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BACKGROUND/OBJECTIVES: To investigate whether vitamin-B density in the diet 2-8 years before diagnosis is associated with olfactory function at the time of diagnosis.

SUBJECTS/METHODS: This prospective nested case-control study included patients with Parkinson's disease (PD), multiple system atrophy and progressive supranuclear paralysis identified between 2004 and 2009 in the county of Västerbotten in northern Sweden. The case database (NYPUM study; Newly Diagnosed Parkinson in Umeå; n=147) was cross-linked to the Northern Sweden Health and Disease Study (NSHDS). Identified patients (n=96) and controls (n=375) were matched for sex, age, year of health survey, sub-cohort and geographical area. Dietary intake was assessed by a food frequency questionnaire, and the brief smell identification test (B-SIT) was used to measure olfactory function at the time of diagnosis.

RESULTS: There was no difference in vitamin-B or any other macro- or micro-nutrient densities, energy intake or body mass index (kg/m(2); BMI) between patients and controls at baseline at the time of the healthcare survey. A lower thiamin and folate density, amount per 1 megajoule, was reported in patients who scored below median on B-SIT (<7) when compared with that in patients who scored ⩾7 at the time of diagnosis. After adjusting for age, sex and BMI using linear and logistic regressions, an even stronger association was found between thiamin density and olfactory function.

CONCLUSIONS: A low thiamin and folate density in the reported diet, 2-8 years before PD diagnosis, was significantly associated with olfactory dysfunction at the time of PD diagnosis.European Journal of Clinical Nutrition advance online publication, 5 October 2016; doi:10.1038/ejcn.2016.181.