Vitamin D status in a rural postmenopausal female population.
BACKGROUND
Inadequate vitamin D nutritional status is increasingly recognized as common in North American and European populations, but the extent of the shortfall and the parameters of the distribution for populations of interest remain uncertain.


OBJECTIVE
To report the distribution of values for serum 25-hydroxyvitamin D [25(OH)D] in a population of rural postmenopausal women, together with quantification of factors related to vitamin D status.


METHODS
Nine largely agrarian counties in eastern Nebraska (approximately 41 degrees N).


METHODS
A population-based sample of 1,179 women 55 years of age and older recruited into a four-year trial of calcium and vitamin D supplementation.


METHODS
Baseline biochemical, dietary, and anthropometric measurements obtained on entry into trial.


RESULTS
Serum 25(OH)D concentration at baseline varied cyclically with season, with the solar cycle explaining 2.9% of the total variance (P < 0.001). Mean seasonally adjusted 25(OH)D concentration was 71.1 nmol/L. Serum 25(OH)D also exhibited the expected inverse curvilinear relationship with serum parathyroid hormone (PTH), with the inflection point of the curve located at approximately 80 nmol/L. Supplements containing vitamin D were regularly taken by 59% of the cohort (median dose: 200 IU/d). Nevertheless, approximately 4% of all women had values below the laboratory reference range and more than two-thirds fell below 80 nmol/L. Seasonally adjusted serum 25(OH)D concentration was positively correlated with the size of daily vitamin D supplement dose, and negatively with age, weight, and body mass index (P < 0.01 for all). In stepwise multiple linear regression models, weight, age, and supplement dose were independently correlated with seasonally adjusted serum 25(OH)D, and together explained 19% of the total variance of adjusted 25(OH)D concentration. Women taking supplements had only one-sixth the chance of having a 25(OH)D value below the reference limit of the assay, compared to women who did not use supplements.


CONCLUSIONS
Approximately two-thirds of this rural population fell below 80 nmol/L, a value considered to be the lower end of the optimal range. Based on the slope of 25(OH)D on supplement dose observed in these women, it would require an additional vitamin D input of nearly 2000 IU/d to reach the goal of an RDA for vitamin D, i.e., to bring 97.5% of the cohort to levels of 80 nmol/L or higher.
Lappe
Joan M
JM
Creighton University Medical Center, Osteoporosis Research Center, 601 North 30 Street-Suite 4820, Omaha, NE 68131, USA. jmlappe@creighton.edu
Davies
K Michael
KM
Travers-Gustafson
Dianne
D
Heaney
Robert P
RP
D000368:Aged; D019540:Area Under Curve; D050071:Bone Density Conservation Agents / Q000008:administration & dosage / Q000097:blood*; D002136:Calcium, Dietary / Q000008:administration & dosage*; D015331:Cohort Studies; D019587:Dietary Supplements; D005260:Female; D006801:Humans; D008875:Middle Aged; D009329:Nebraska / Q000453:epidemiology; D018673:Nutrition Policy; D009751:Nutritional Requirements; D009752:Nutritional Status; D010281:Parathyroid Hormone / Q000097:blood; D017698:Postmenopause / Q000097:blood; D012621:Seasons; D014807:Vitamin D / Q000008:administration & dosage* / Q000031:analogs & derivatives / Q000097:blood*; D014808:Vitamin D Deficiency / Q000097:blood / Q000188:drug therapy / Q000453:epidemiology*
D050071:Bone Density Conservation Agents; D002136:Calcium, Dietary; D010281:Parathyroid Hormone; D014807:Vitamin D; C104450:25-hydroxyvitamin D