| Summary: | In addition to its established role in bone health, vitamin D (2S(OH)D) may also have a role in modulating immune function and early life development. Despite recent advances, there is a lack of consensus with regards to the optimal vitamin D cut-offs for multiple health outcomes and this uncertainty is further compounded by the wide measurement variability for the vitamin. Consequently, the work described in this thesis aimed to explore these areas of controversy. Using data from ongoing studies at the University of Ulster, a comparison study (n 131), of vitamin D status in the two most widely used methods (liquid chromatography mass spectrometry (LC-MS/MS) and enzyme immunoassay (ELISA)) of measurement was undertaken. Significant variation in definition of status was observed, with overestimation of vitamin D concentrations by ELISA >2S% compared to LC-MS/MS. In a second study, using LC-MS IMS, the vitamin D status and markers of bone health of a sample of older Irish adults (n 1936) form the Trinity, Ulster Department of Agriculture (TUDA) study was assessed. A total of 16% were vitamin D deficient «2Snmolll) and 42% were deemed to be insufficient (2S-S0nmolll). These levels of nonoptimal vitamin D concentrations were coupled with high rates of impaired bone health (31 % classified as osteopenic and 18% osteoporotic from BMD measures). A higher prevalence of impaired bone health and vitamin D inadequacy was observed in females compared to males while individuals who were vitamin D deficient or insufficient were significantly more likely to be osteoporotic than those who were sufficient (>SO nmol/l). These data provide additional evidence to support the recent 10M recommendation of a 2S(OH)D concentration of 50 nmol/l for optimal bone health. In a third study, the association between vitamin D status, immune markers of inflammation and the ratio of pro: anti-cytokines was investigated in a sub-sample of TUDA participants (n 998). Vitamin D was significantly correlated with pro-inflammatory markers and a 25(OH)D concentration >75nmol was associated with an improved inflammatory (profile as determined by the pro:anti cytokine ratio) compared to individuals with a 25(OH)D status <25 or 25-75nmol/l. In a fourth study, vitamin D status was assessed within a sample (n 260) of pregnant women from a sunny equatorial country (5degS) (Seychelles). Maternal vitamin D status was observed to be >75nmol/l through all sample periods of pregnancy and was significantly associated with higher birth weight and length with no apparent upper limit of effect. These results demonstrate the importance of optimal vitamin D status during pregnancy and the need for adequate dietary recommendations in order to achieve this level within far latitude populations that are exposed to low UVB sun light. In conclusion, the results within the current thesis suggest concentrations of vitamin D greater than recently recommended cut-offs for bone health (50nmol/l) are associated with extra-skeletal health benefits. Furthermore, consideration needs to be given to the current vitamin D dietary recommendations within the UK and Ireland in order to address the high level of deficiency observed in the older adult population and to achieve the optimal vitamin D concentration in terms of benefits for bone health, immune function and neonatal health outcomes for the whole population
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