Linxian, People’s Republic of China is a semi-arid mountainous area in the center of the country (36° N latitude) with a mainly rural population. This population has very high rates of esophageal squamous cell carcinoma (ESCC) and gastric cardia adenocarcinoma, with age standardized rates for both men and women of about 100/100,000/year for the two tumors combined (5). Residents of this area have a limited diet, and a nutritional deficiencies are common (6). Prospective epidemiologic studies in Linxian have shown increased risk of ESCC or gastric cardia adenocarcinoma in subjects with lower concentrations of serum selenium (7), serum α-tocopherol (8), and tissue zinc (9). For some other nutrients, such as β-carotene, β-cryptoxanthin, and retinol (10), deficiency is common but is not associated with increased risk of these cancers. We recently reported that higher vitamin D status, as measured by serum 25-hydroxyvitamin D (25(OH)D), was associated with an increased risk of ESCC in a prospective study in this population (11).

The overall median serum 25(OH)D concentration was 33.1 nmol/L and the medians for men and women were 47.0 and 28.5, respectively (Table 1). Subjects with squamous dysplasia had significantly higher median serum 25(OH)D concentrations than those without, 36.5 and 31.5 respectively (Wilcoxon Two-sample test p = 0.0004).

We examined several different potential predictors of serum 25(OH)D concentration using multiple linear regression including variables previously shown to be associated with esophageal squamous dysplasia or of interest a priori (Table 1). We used multivariate linear regression to examine predictors of serum concentration using log transformed 25(OH)D. The total r² for the model was 23% with the strongest predictors being sex (male, β= 0.52, p=<0.0001); height (+1 cm, β= −0.0077, p= 0.018); weight (+1 Kg, β= 0.0037, p= 0.078); and the fourth category of tooth loss, 12–31 teeth lost (β= −0.086, p= 0.082). Smoking tobacco and drinking alcohol were not significantly associated with serum concentration and neither was number of people in the household nor household income.

We have now shown, in two independent data sets, that higher serum 25(OH)D concentration is associated with increased risk of esophageal neoplasia in this population. Two previous prospective studies have demonstrated that men with higher vitamin D status are at increased risk of pancreatic (17) or prostate (18) cancer. This is the first study to find that higher serum vitamin D is associated with increased risk in women, albeit not for cancer but for a pre-neoplastic lesion.

Confounding is always a potential explanation for associations reported in observational epidemiologic studies. It is possible that vitamin D could be correlated with intake of an environmental contaminant that co-occurs with a vitamin D food source. However, this seems unlikely since the typical diet in Linxian provides little vitamin D: fatty fish and liver are rarely consumed (6) and median egg intake is also low (19). A second possibility is that our result is due to some unmeasured confounder that is correlated with outdoor activity, which is the major contributor to vitamin D status other than constitutional differences between individuals. The majority of the Linxian population and the members of this cohort are subsistence farmers however, and all spend large amounts of time outdoors. Finally, although there is some variation in SES, and household income is associated with risk of esophageal squamous dysplasia (13), we saw no association between household income and serum 25(OH)D status.

The association between higher vitamin D status and increased risk of ESCC and its precursor lesion may be due to an interaction with another factor rather than simply due to vitamin D itself. For example, the population of Linxian relies on coal for both cooking and heating, and previous studies have demonstrated that the people are heavily exposed to polycyclic aromatic hydrocarbons (PAHs)(20). A recent study showed that in vitamin D deficient rats, an injection of the active vitamin D metabolite 1,25(OH)D induced the expression of both Phase I and Phase II enzymes (21) in the intestine, which metabolize PAHs. Although Phase II enzymes participate only in the detoxication of these pro-carcinogens, Phase I enzymes create reactive intermediates which could be detoxified or could form adducts with DNA or protein and increase the risk of cancer. The balance of activation and detoxication in this system can depend on many factors, including the amount and type of PAH or other chemical exposure, genetic variation in the genes encoding the metabolizing enzymes, the availability of conjugating factors necessary for the Phase II enzymes (e.g. glutathione), etc. If higher vitamin D status leads to more Phase I enzyme activity in humans, this may have an adverse effect in a PAH-exposed group such as the population of Linxian (22). Therefore, it is plausible that some other factors in this population may explain the unexpected direction of the observed association between vitamin D and esophageal squamous dysplasia.

Our study has several strengths and weaknesses. We studied the association between serum 25(OH)D concentration and the established pre-neoplastic lesion for ESCC in a large, asymptomatic group of subjects from a high-risk region for ESCC. We used the best marker for vitamin D status, serum 25(OH)D concentration. We used the gold-standard diagnostic technique, endoscopy with Lugol’s iodine staining and biopsy (14), to establish the presence or absence of esophageal squamous dysplasia. Our study is limited by the use of a pre-neoplastic lesion, which confers a significantly increased risk of ESCC (12) but is not itself cancer. Also, our study is cross-sectional, so it is possible that the higher level of serum 25(OH)D is somehow secondary to the dysplasia, although this seems unlikely. Our similar finding in a prospective study (11) from this same population suggests that reverse causation is an unlikely explanation for our findings.