Understanding the relationship between obesity/ fat distribution/metabolic profiles and vitamin D status in young women (the safe-D study)
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2020-10-01.
© 2018 Dr Marjan Tabesh
Vitamin D deficiency is highly prevalent and associated with an increased risk of many chronic health conditions including obesity, cardiovascular disease and diabetes. Vitamin D deficiency affects millions of Australians, causing considerable suffering, economic loss and mortality. The aim of this study was to evaluate the associations of vitamin D and obesity/fat distribution/metabolic profiles and examine the effects of behavioural intervention and vitamin D supplementation on weight / fat distribution and metabolic profiles in young women. This study was run in two parts. In Part A, we evaluated the associations of obesity/fat distribution/metabolic profiles and 25 hydroxyvitamin D (25 OHD) in a cross-sectional study of 407, 16-25 year-old women. In Part B in a controlled randomised clinical trial over a period of 12 months we examined the effects of behavioural intervention and vitamin D supplementation on weight, fat distribution and metabolic profiles in 123 young women with mild-to-moderate vitamin D deficiency (25 OHD between 25-75 nmol/L). Data were collected at baseline, 4 months and 12 months follow-ups using an online survey and when participants attended a site visit in a fasted state. Parameters including blood pressure, anthropometry, metabolic profiles, serum 25 OHD levels, and body composition (using dual energy X ray absorptiometry) were measured. For the cross-sectional analysis, we combined 407 Safe-D Part A data with 150 participants from another study, the Young Female Health Initiative (YFHI). Cross-sectional analyses showed that after adjustment for covariates, higher 25 OHD levels were associated with greater high density lipoprotein (HDL) levels and greater triglyceride levels. They were also associated with lower body mass index (BMI), total fat percentage, visceral fat percentage, visceral fat to total fat ratio and lower trunk fat to total fat ratio. Although these associations were statistically significant, they were very small in magnitude and of uncertain clinical significance. In the clinical trial part of the project, after four months of intervention, both vitamin D supplementation and the use of a mobile-based application (app), resulted in a significant increase in 25 OHD levels. There were no significant differences between the three groups in lipid profiles, BMI or glucose metabolism after 4 months intervention. However, vitamin D supplementation resulted in a reduction in BMI, and behavioural intervention resulted in a significant reduction in haemoglobin A1c (HbA1c) when compared to baseline. Both the per protocol and intention to treat analyses were used to evaluate the effects of vitamin D improvements on cardiovascular risk factors after 12 months follow-up. In total, 102 participants were included in the 12 months follow-up intention to treat analysis. There were no significant differences in glucose metabolism, lipid profiles, systolic and diastolic blood pressure, anthropometric measures, high sensitivity C-reactive protein (hs-CRP) levels and body composition among the three groups after 12 months of follow-up. Results showed that the improvement in 25 OHD levels by either taking vitamin D supplements or increased safe sun exposure did not affect cardiovascular risk factors in healthy young women. In chapter 7 we evaluated the effects of behavioural intervention over one year follow-up on sun related behaviours. Time spent in the sun did not change following behavioural intervention but sun risky behaviours decreased significantly in summer time in this group. Moreover, in this study no significant associations were observed between ultraviolet radiation (UVR) exposure and skin type, ethnicity or skin sensitivity to sun light. However, significant positive association was observed between sun exposure and physical activity.
Keywordsvitamin D; obesity; fat distribution; metabolic profile; young women
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- Medicine (RMH) - Theses