The first nationally representative study of prevalence of vitamin D deficiency based on serum 25(OH)D data for Irish adults (n=1500) showed that 40.1% of Irish adults had year-round serum concentrations <50nmol/L. The prevalence rose to 55.0% in the winter months^1*†‡

* There was a significant difference between winter and summer for all age and sex categories (P<0·01). † There were no significant differences between males and females (P>0·1 in all cases; unpaired t test).

• The study showed that 17.5% of Irish adults are using vitamin D supplements¹
• Vitamin D supplement use was associated with a higher mean serum 25(OH)D level in winter (15.7nmol/L) and to a lesser degree in summer (6.8nmol/L)¹

* Mean values were significantly different from no supplement use within same season (P<0·05; unpaired t test). † Mean values were significantly different from no supplement use in winter (P<0·001; unpaired t test). ‡ Mean values were significantly different from supplement use in winter (P<0·05; unpaired t test); there was no significant difference between summer and winter for supplement users (P>0·1 in all cases).