Vitamin D Status: Determinants of 25-hydroxyvitamin D concentration and its changes in response to vitamin D supplementation

Abstract

Aims The primary aim of this thesis was to determine which variables, such as certain personal characteristics (age, sex, ethnicity and BMI) and ingested substances (alcohol consumption and smoking status), are associated with changes in a person’s vitamin D status, as measured by changes in 25-hydroxyvitamin D (25(OH)D) concentrations, in response to vitamin D supplementation. The secondary aim of this thesis was to identify variables associated crosssectionally with 25(OH)D concentrations. Methods The study participants came from a sub-sample of the Vitamin D Assessment (ViDA) Study, which is a large randomised double-blind controlled trial of 5110 participants aged 50-84 years, recruited in 2011-2012, to determine whether monthly 100,000 IU doses of vitamin D3 given until July 2015 prevent cardiovascular disease (primary aim). This thesis analysed data collected from a sub-sample of 854 participants, who comprised a randomly selected annual sample (n=441) from whom blood samples were collected at baseline, 6, 12, 24 and 36 months, and a bone-density sample (n=413) from whom blood was collected at baseline and 24 months. The blood was stored and at a later date 25(OH)D concentrations were measured. Multiple linear regression was used to determine the relationship that certain personal characteristics and ingested substances had with 25(OH)D concentrations at baseline, and with changes in 25(OH)D concentrations at each period. Results Mean (SD) baseline 25(OH)D concentration was 59 (24) nmol/L, with 39% of participants having baseline 25(OH)D concentrations of less than the minimum (50 nmol/L) recommended by the Ministry of Health. Larger body fat mass was associated with lower 25(OH)D concentrations at baseline, and with smaller changes in 25(OH)D concentrations in response to vitamin D supplementation. Compared to Europeans, Maori, Pacific and South Asian people had lower baseline 25(OH)D concentrations but changes in 25(OH)D concentrations in response to supplementation were similar amongst these ethnicities, such that South Asians still had lower 25(OH)D concentrations than Europeans at the study end. Conclusions People with higher BMIs and of Maori, Pacific and South Asian descent, as well as other darker skinned people, are more likely to need an increased intake of vitamin D. A fortification strategy has the potential to increase the vitamin D levels of these people to acceptable levels.