Monica Nelson

Date of Award

12-2007

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Food and Nutrition Sciences

Advisor

Susan Sullivan

Second Committee Member

James Blum

Third Committee Member

Richard Cook

Abstract

The purpose of this research was to measure the serum 25-hydroxyvitamin D [25(OH)D] response to daily supplementation with 800 IU vitamin D3 during winter in premenopausal women living in Maine, and to examine the effects of body composition and hormonal contraceptive use on baseline serum 25(OH)D levels and the response to supplementation. One hundred twelve women (22.2±3.7 years old) received placebo from March 2005 until September 2005 when they were randomized to receive either placebo or 800 IU vitamin D3 through February 2006. Eighty-six women completed the study. Body composition was measured by dual-energy x-ray absorptiometry. Actual vitamin D3 content of the supplements averaged 885 IU per capsule. In February 2005 the mean±SD serum 25(OH)D was 62.0±23.4 nmol/L in all subjects. Twenty-nine percent of subjects had optimal serum 25(OH)D levels (=75 nmol/L). Serum 25(OH)D levels were significantly higher (p < 0.0005) in the 58 hormonal contraceptive users (68.6±24.0 nmol/L) than in the 28 non-users (48.3±14.8 nmol/L). The 25(OH)D concentration increased with estrogen dose. Subjects in the highest tertile body fat (>33%) had significantly lower serum 25(OH)D levels (47.8±17.3 nmol/L) than subjects in the middle and lowest tertiles (69.4±23.8 and 69.0±22.2 nmol/L). Estrogen dose, percent body fat, and alcohol consumption were significant predictors of February 2005 serum 25(OH)D levels. Serum 25(OH)D levels increased by 35.3±23.2 nmol/L from February 2005 to February 2006 in the treatment group, compared to 10.9±16.9 nmol/L in the placebo group. Treatment group, magnitude of summer increase in 25(OH)D, estrogen dose, and baseline serum 25(OH)D levels, but not body fat, were significant predictors of the one-year change in 25(OH)D levels. Daily supplementation with 800 IU vitamin D3 during winter achieved optimal 25(OH)D levels (=75 nmol/L) in 80% of subjects, indicating that this dose is too low to optimize vitamin D status in the population as a whole. Body fat does not appear to influence the serum 25(OH)D response to supplementation with vitamin D3, except through its influence on the baseline serum 25(OH)D level. Further research is needed to determine whether there is a health benefit to the higher serum 25(OH)D levels in oral contraceptive users.

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