‘This supplementary information is presented as submitted by the corresponding author. It has not been copy-edited by NTvG.’
Insufficient sunlight exposure is a common cause for vitamin D deficiency. The existing vitamin D deficiency studies in The Netherlands have been performed in high risk groups or in an elderly population. Moreover, little is known about regional differences in vitamin D deficiency in The Netherlands. We therefore investigated the prevalence of vitamin D deficiency in random primary healthcare patients from urban-, suburban- and rural areas during two successive seasonal periods. Furthermore, we retrospectively assessed all vitamin D blood-tests from our laboratory information system to investigate seasonal variety.
In random blood samples from SHL-Groep, a primary healthcare laboratory, vitamin D levels were measured during the winter- (21st January – 11th March 2013) and summer period (1st August until 30th of August 2013). Patient’s mean vitamin D levels were categorized based on age, and subdivided in gender and postal code area in order to study regional differences. Additionally, we retrospectively examined all vitamin D blood-tests ordered from January 2010 until August 2012 (50,441 patients) from our laboratory information system. The mean vitamin D levels were plotted against time to evaluate seasonality.
Differences in vitamin D levels between urban- (The Hague), rural- (Zeeland) and suburban area (West-Brabant) were tested with ANOVA and differences in percentage vitamin-D deficiency with chi-square.
The 25(OH)D was measured in serum with an electrochemical luminescence immunoassay (Vitamin D Total Assay) with a Cobas 8000 (Roche Diagnostics Nederland B.V)) platform. Serum tubes were centrifuged directly after complete clotting and vitamin D measurements were performed on the day of phlebotomy. Statistical analyses were performed with Excel (version 2003), Analyse-IT and SPSS (version 20)
During the winter period vitamin D was measured in 2503 patients randomly. Mean age was 57 years (range 0-98) and 41.3% was male. The mean vitamin D level was 48.4 nmol/l. We found a vitamin D level below 50 nmol/l and 30 nmol/l in 58.8% (61.2% for men and 57.1% for women) and 29.9% (28.8 for men, 30.6 for women) of the tested population, respectively. A vitamin D level below 30 nmol/l was found in 38.2%, 25.7% and 25% of men younger than 50 years, between 50-70 years and older than 70 years. For women this was 28.2%, 26.3% and 40.2%, respectively.
The mean vitamin D level differs significantly between regions; during the winter period 43.6 nmol/l in an urban-versus 52.4 nmol/l in a rural population. A vitamin D level below 50 nmol/l was found in 65.6% of the urban- and in 50.9% of the rural population.
During the summer period vitamin D was measured in 1910 random patients. Mean age was 57 years (range 2-100) and 40.0% was male. The mean vitamin D level was 61.6 nmol/l. A vitamin D level below 50 nmol/l and below 30 nmol/l was found in 35.4% (men 30.9%, women 38.5%) and 11.6% (men 8.1%, women 13.9%), respectively. A vitamin D level below 30 nmol/l was found in 8.3% of men younger than 50 years, 6.9% of men between 50 and 70 years and 10.5% of men older than 70 years. For women younger than 50 years, between 50-70 years and older than 70 years this was 10.1%, 9.4% and 25.4%, respectively. During the summer period the mean vitamin D level in the urban population was lower (54 nmol/l) than in a rural population (62 nmol/l). A vitamin D level below 50 nmol/l was found in 43.7% and 33.5% respectively.
From January 2010 to August 2012, 63.917 vitamin D tests were ordered for 50.441 patients. The mean age was 51.0 (range 0-111) years and 29.7% was male. The mean vitamin D level is 44.0 nmol/l (range 5 - 375 nmol/l). From these patients 62.7% has a vitamin D level below 50nmol/l; (63.9% for men and 62.1% for women). The mean vitamin D-level varies seasonally. Conclusion During the winter period low vitamin D-levels are highly prevalent in Dutch primary health care patients (58.8% <50nmol/l; 29.9% <30nmol/l). Moreover, patients younger than 50 years often have vitamin D levels below 30 nmol/l (women 28.2 %; men 38.2%). Even during the summer period a low vitamin D level is common; 55.1% of the women older than 70 years in the study population had a vitamin D level below 50 nmol/l and 25.4% below 30 nmol/l. Vitamin D levels below 50 nmol/l during the winter period are more common in urban- than in rural areas (65.6% and 50.9%, respectively). Mean vitamin D levels in primary health care patients vary seasonally.