title: 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.
abstract: OBJECTIVE
  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).
  
  
  CONCLUSIONS
  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.
authors: Biomarkers Research Program, Biochemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia. aldaghri2000@hotmail.com
authors: Nasser M
authors: NM
authors: Al-Daghri
authors: Omar S
authors: OS
authors: Al-Attas
authors: Majed S
authors: MS
authors: Alokail
authors: Khalid M
authors: KM
authors: Alkharfy
authors: Emad
authors: E
authors: El-Kholie
authors: Mansour
authors: M
authors: Yousef
authors: Abdulaziz
authors: A
authors: Al-Othman
authors: Yousef
authors: Y
authors: Al-Saleh
authors: Shaun
authors: S
authors: Sabico
authors: Sudhesh
authors: S
authors: Kumar
authors: George P
authors: GP
authors: Chrousos
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001786:Blood Glucose / Q000378:metabolism; D015992:Body Mass Index; D002118:Calcium / Q000097:blood; D015331:Cohort Studies; D019587:Dietary Supplements / Q000592:standards*; D004797:Enzyme-Linked Immunosorbent Assay; D005215:Fasting / Q000097:blood; D005260:Female; D006801:Humans; D008055:Lipids / Q000097:blood; D008297:Male; D008875:Middle Aged; D012044:Regression Analysis; D012529:Saudi Arabia; D012621:Seasons; D013472:Sunlight; D014807:Vitamin D / Q000008:administration & dosage* / Q000097:blood*; D014815:Vitamins / Q000008:administration & dosage; D055815:Young Adult
chemical_list: D001786:Blood Glucose; D008055:Lipids; D014815:Vitamins; D014807:Vitamin D; D002118:Calcium