Date

June 2010

Document Type

Thesis

Degree Name

M.S.

Institution

Oregon Health & Science University

Abstract

Background: Vitamin D deficiency is more prevalent in overweight and obese individuals than in normal weight individuals. Reasons for this difference are not well understood but are attributed to reduced sun exposure and therefore reduced endogenous synthesis of vitamin D, lower intake of dietary vitamin D, and greater sequestration of vitamin D in adipose tissue in the those who are overweight or obese compared to their normal weight peers. Objectives: The primary aim of this study was to determine how serum 25-hydroxyvitamin D [25(OH)D] concentration, a marker of vitamin D status, changes with weight loss among individuals who are overweight and obese. A secondary aim was to determine whether demographic and lifestyle factors influence the change in circulating 25(OH)D concentrations with weight loss. Method: A randomized controlled trial was used to assess the impact of a six-month weight loss intervention on change in mean serum 25(OH)D concentration in individuals adhering to either a high- (n=60) or low-carbohydrate (n= 59) diet. Weight, body composition, energy and nutrient intake, and fasting serum 25(OH)D and parathyroid hormone (PTH) concentrations were measured before and after the intervention. Results: In this study, 92% of participants had serum 25(OH)D concentrations <30 ng/ml and were considered vitamin D insufficient. Furthermore, 40% of participants had serum 25(OH)D concentrations <20 ng/ml and were considered vitamin D deficient. These frequencies of vitamin D insufficiency and deficiency were seen before and after weight loss and regardless of dietary assignment. Mean weight loss was 12 ± 6 kg and 6 ± 6 kg in the low-carbohydrate and high-carbohydrate diet groups, respectively. Fat mass decreased 6 ± 5 kg and 5.5 ± 5 kg in the low-carbohydrate and high-carbohydrate diet groups, respectively. Mean serum 25(OH)D concentration was 22 ± 7 at baseline and the mean change in 25(OH)D concentration was -2 ± 6 ng/ml after weight loss. The mean change in serum 25(OH)D concentration was not different between diet groups nor was it influenced by weight loss. Significant correlations were observed between serum 25(OH)D concentration and weight (r = -0.2), BMI (r = -0.2) and PTH concentration (r = -0.2) before and after weight loss. However, no significant correlations were observed between the change in serum 25(OH)D concentration and weight loss, fat mass, or BMI. Conclusions: The prevalence of vitamin D deficiency and insufficiency was high among the overweight and obese individuals participating in this study. Mean serum 25(OH)D concentrations did not improve despite significant weight loss and loss of fat-mass. These results suggest that vitamin D supplementation may be necessary to achieve and maintain optimal vitamin D concentrations in overweight and obese individuals, especially those living in the Pacific Northwest. Additional research is needed to confirm these results and to better understand the physiological mechanisms that contribute the high risk of vitamin D deficiency and insufficiency among those who are overweight and obese.

Identifier

doi:10.6083/M48P5XH0

Division

Graduate Programs in Human Nutrition

School

School of Medicine

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