Increased vitamin D supplementation recommended during summer season in the gulf region: a counterintuitive seasonal effect in vitamin D levels in adult, overweight and obese Middle Eastern residents.

PURPOSE: Seasonal variations in circulating vitamin D levels provide vital information as to the most appropriate time to either start or increase vitamin D supplementation to maintain optimal vitamin D levels. In this follow-up study, we determined seasonal differences in serum 25(OH)-vitamin D (25(OH)D) levels, as well as parallel changes in metabolic parameters, in a cohort of adult, overweight and obese Saudis. METHODS: A total of 121 adult, overweight, obese, and consenting Saudis aged 18-70 years were randomly recruited from four Primary Health Care Centers in Riyadh, Saudi Arabia. They were divided according to the season when baseline measurements were made [74 summer (April-October); 47 winter (November-March)]. Anthropometrics were obtained, and fasting blood samples were taken at baseline and every 3 months for 1 year. Fasting blood glucose, corrected calcium levels, and lipid profiles were measured routinely. Serum 25(OH)-vitamin D was quantified using a specific enzyme-linked immunosorbent assay (ELISA). RESULTS: Age- and BMI-matched mean 25(OH)-vitamin D levels from the winter group were significantly higher than those of the summer group (P < 0·001). In both groups, HDL-C levels improved significantly as 25(OH)-vitamin D levels increased with subsequent follow-ups, even after adjusting for age, gender and BMI (P < 0·001). CONCLUSION: Seasonal differences in serum 25(OH)-vitamin D levels in Saudi Arabia are counterintuitive, with circulating levels being higher during the winter than the summer season. Increased vitamin D supplementation is thus recommended to maintain optimal serum 25(OH)-vitamin D levels during the summer season.