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In recent years there has been a resurgence in vitamin D research. These studies have used numerous methods to estimate vitamin D, such as 25-hydroxyvitamin D (25 (OH) D) measurement, UV dose estimation or dietary estimates from food questionnaires. A large number of these studies have found a beneficial relationship between vitamin D and the risk and survival of many diseases and conditions, such as osteoporosis, cardiovascular diseases and cancer [1-6]. There has been some strong evidence supporting an association between vitamin D and the decreased risk and improved survival of various cancers, such as prostate, breast and colorectal cancers [5, 7]. However, the relationship between vitamin D and oesophageal and gastric cancer occurrence and mortality has been scarce and mixed [6, 8-10]; only a handful of studies have been carried out to date. Different study designs, confounder adjustment and follow-up times from these studies lead to significant differences between studies. This may be one reason why there has been no consistent relationship found. Additionally, the studies which have been undertaken, suffer from a number of limitations such as small sample sizes, study design flaws or not adequately accounting for the various sources of vitamin D, for example, supplementation use. Moreover, as each published study used different vitamin D estimations, comparisons between these are difficult. Therefore, there is a need to explore this relationship further.

This thesis aimed to explore the relationship between vitamin D and oesophageal or gastric cancer risk and survival. It utilised vitamin D concentration in the …