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Seasonal variation of serum vitamin D and the effect of vitamin D supplementation in Irish community-dwelling older people.
Age Ageing. 2010 Nov 3.
Romero-Ortuno R, Cogan L, Browne J, Healy M, Casey MC, Cunningham C, Walsh JB, Kenny RA.
Trinity College Dublin-TRIL Clinic, Hospital 4, Top Floor, St James's Hospital, James's Street, Dublin 8, Co. Dublin, Ireland.
Background: Ireland is at 53°N, and its population risk of vitamin D deficiency is high. Previous Irish studies suggested a significant seasonality of serum 25-hydroxyvitamin D 25(OH)D and a beneficial effect of supplementation in raising 25(OH)D levels. However, in Irish older people, little is known about the magnitude of the supplementation effect and whether supplementation affects 25(OH)D seasonality. Design: cross-sectional observational. Setting: outpatient clinic. Subjects: five hundred and forty-six community-dwelling subjects (mean age 73.0, SD 7.4; 68.5% females) were assessed between September 2007 and May 2009.
METHODS: for supplemented and non-supplemented: 'cosinor' analysis (Pulse_XP(®)) of monthly 25(OH)D. Period global solar radiation (GSR) and solar elevation angle (SEA) data were collected as proxy markers of ultraviolet-B radiation exposure. Multivariate linear regression was conducted to investigate the independent effect of GSR and SEA on 25(OH)D, controlling for confounders. Results: supplemented group (N = 183): 89.1% were on cholecalciferol 800 IU/day.
Mean 25(OH)D = 64.1 (95% confidence interval: 52.2-75.8) nmol/l, with no significant seasonality; regression: neither GSR nor SEA predicted 25(OH)D.
Non-supplemented group (N = 363): mean 25(OH)D = 40.3 (35.5-45.0) nmol/l, with significant seasonality (55.5% variance remaining), peak in August, amplitude = 6.0 (3.1-8.8) nmol/l; regression: both GSR (P = 0.002) and the interaction GSR * SEA (P = 0.018) predicted 25(OH)D.
Conclusions: vitamin D supplementation was associated with a mean serum 25(OH)D increase of 23.8 nmol/l. Interestingly, supplementation seemed to blunt seasonality. In the supplemented group, 72.1% had individual 25(OH)D levels below the recommended 75 nmol/l. There is a case for universal supplementation in Irish older people, probably at a higher dose. Further research is needed to establish the optimum dose. PMID: 21051444