Prospective epidemiologic data on the association between vitamin D and all-cause and cause-specific mortality risk are limited, particularly in Asian populations. In the present study, among a nested cohort of subjects from the General Population Nutrition Intervention Trial in Linxian, China, we aimed to test whether baseline serum 25(OH)D concentration was prospectively associated with all-cause mortality or mortality due to cerebrovascular accidents, all cancers, upper gastrointestinal cancers, or cardiovascular disease. We found that serum 25(OH)D concentration was not associated with all-cause mortality or any of the cause-specific mortality categories.
Two recent reviews of observational studies concluded that increasing circulating 25(OH)D levels were associated with a decrease in all-cause mortality rates (2
). However, previous observational and supplementation studies mostly focused on populations in the United States and Europe, and there is limited epidemiologic evidence in Asian populations. We present what is to our knowledge the first prospective study of the association between serum 25(OH)D and all-cause and cause-specific mortality risks in a Chinese population. In addition, except for 1 study conducted in Finland with 27 years of follow-up (15
), all previous studies had 13 or fewer years of follow-up time, substantially less than our follow-up time of over 24 years. Ecologic studies in China have suggested that through the production of vitamin D, solar ultraviolet B radiation may be correlated with reduced mortality rates for some types of cancers (16
), particularly cancers of the esophagus and stomach (17
). However, we did not find associations between serum 25(OH)D concentrations and all cancer mortality or upper gastrointestinal cancer mortality in our Chinese population. Differences between our results and those of other analyses may be due to the narrow range of serum vitamin D levels and to population characteristics, so generalizability to other populations may be limited.
Although most of the Linxian population and members of this cohort are subsistence farmers (>98%) and spend large amounts of time outdoors, more than three quarters of the cohort members had inadequate levels of vitamin D by previously defined standards (18
). The typical diet in Linxian provides little vitamin D; fatty fish and liver are rarely consumed (22
), and egg consumption is also low (23
). In addition, concentrations of 25(OH)D may be genetically determined to some extent (24
). Our blood samples were drawn in the spring, a likely nadir for serum 25(OH)D concentrations, so this single measurement may not reflect year-round exposure.
Our study has several strengths and weaknesses. We used serum 25(OH)D concentration, which is the best marker for vitamin D status, from blood collected during a single season (3 months) in 1 year. We also had a long follow-up time of more than 24 years, so reverse causation is unlikely. Over this follow-up time, we had substantial numbers of deaths and very reliable cause-of-death information (10
). However, a weakness of our study is the relatively narrow distribution of serum 25(OH)D concentrations in our cohort. Furthermore, we had only a single measurement of serum 25(OH)D concentrations, and this may not adequately rank exposure status over long follow-up, although analyses stratified at the midpoint of follow-up (12 years) showed no difference in risk estimates (data not shown). In summary, we found no association between serum 25(OH)D concentrations and all-cause mortality or cause-specific mortality risks in a Chinese population with low levels of vitamin D.