title: Vitamin D deficiency and seasonal variation over the years in São Paulo, Brazil.
abstract: Brazil is a tropical/subtropical geographic area with elevated ultraviolet (UV) radiation. We report very high prevalence of vitamin D deficiency in a large database of Brazilian subjects and show seasonal and reciprocal relationship between vitamin D and parathyroid hormone (PTH) over the years in this tropical area.
  
  
  
  We aim to examine the prevalence of vitamin D deficiency, characterize the temporal relationship between 25-hydroxyvitamin D levels (25(OH)D) and intact PTH (iPTH) according to seasons, and investigate potential associations between 25(OH)D levels and extra-skeletal outcomes in a Brazilian population.
  
  
  
  We retrospectively determined population weekly mean concentrations of unpaired 25(OH)D and iPTH using 39,004 laboratory results of Brazilian individuals of both genders aged 2 to 95 years. The 25(OH)D and iPTH distributions were normalized, and the means fit with a sinusoidal function. Potential associations between 25(OH)D serum levels and inflammatory markers, fasting glucose, HbA1c and Homeostasis Model Assessment index (HOMA) were examined.
  
  
  
  Of the samples, 33.9 % had 25(OH)D serum concentrations lower than 20 ng/mL, while the vast majority (70.7 %) were found to be vitamin D deficient or insufficient (<30 ng/mL). Vitamin D deficiency was significantly higher during the winter as compared to the summer (38.4 % <20 ng/mL and 75.5 % <30 ng/mL versus 23.3 % <20 ng/mL and 62.5 % <30 ng/mL, respectively; p < 0.001). Seasonal variation was observed for both 25(OH)D and iPTH. 25(OH)D peaks occurred in March and troughs in September. iPTH levels showed an inverted pattern of peaks and troughs with a delay of 1 ± 5 week. 25(OH)D was significantly associated with inflammatory markers but not with glucose homeostasis.
  
  
  
  A sinusoidal interrelationship has been detected between vitamin D and PTH in this tropical population. A large percentage of the individuals showed vitamin D deficiency. Public health strategies are needed to better understand and manage this very high and apparently contradictory prevalence of vitamin D deficiency.
authors.affiliation: Rheumatology Division, Universidade Federal de São Paulo/Escola Paulista de Medicina (Unifesp/EPM), São Paulo, Brazil.|CURA Imagem and Diagnóstico, São Paulo, Brazil.
authors.forename: M
authors.initials: M
authors.lastname: Eloi
authors.affiliation: Rheumatology Division, Universidade Federal de São Paulo/Escola Paulista de Medicina (Unifesp/EPM), São Paulo, Brazil.|CURA Imagem and Diagnóstico, São Paulo, Brazil.
authors.forename: D V
authors.initials: DV
authors.lastname: Horvath
authors.affiliation: Rheumatology Division, Universidade Federal de São Paulo/Escola Paulista de Medicina (Unifesp/EPM), São Paulo, Brazil.
authors.forename: V L
authors.initials: VL
authors.lastname: Szejnfeld
authors.affiliation: Rheumatology Division, Universidade Federal de São Paulo/Escola Paulista de Medicina (Unifesp/EPM), São Paulo, Brazil.|CURA Imagem and Diagnóstico, São Paulo, Brazil.
authors.forename: J C
authors.initials: JC
authors.lastname: Ortega
authors.affiliation: CURA Imagem and Diagnóstico, São Paulo, Brazil.
authors.forename: D A C
authors.initials: DA
authors.lastname: Rocha
authors.affiliation: CURA Imagem and Diagnóstico, São Paulo, Brazil.|Imaging and Diagnosis Department, Universidade Federal de São Paulo/Escola Paulista de Medicina (Unifesp/EPM), São Paulo, Brazil.
authors.forename: J
authors.initials: J
authors.lastname: Szejnfeld
authors.affiliation: Rheumatology Division, Universidade Federal de São Paulo/Escola Paulista de Medicina (Unifesp/EPM), São Paulo, Brazil. cheldan@uol.com.br.|CURA Imagem and Diagnóstico, São Paulo, Brazil. cheldan@uol.com.br.
authors.forename: C H M
authors.initials: CH
authors.lastname: Castro
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001938:Brazil / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood*; D012189:Retrospective Studies; D012621:Seasons; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000453:epidemiology*; D055815:Young Adult
chemical_list: D010281:Parathyroid Hormone; D014807:Vitamin D; C104450:25-hydroxyvitamin D
keywords: 25(OH)D; Brazilian population; Glucose homeostasis; Inflammation; PTH; Vitamin D