The research was conducted by Taiki Yamaji, of the Center for Public Health Sciences of the National Cancer Center in Japan, and her colleagues.
Vitamin D is an essential vitamin for our bodies. It not only helps to maintain calcium levels for good bone health; it also plays significant roles in immune system functioning, neuronal communication, and muscle functioning.
The National Institutes of Health (NIH) say that adults should aim to get around 600 International Units of vitamin D every day.
The body's main source of vitamin D is sunlight, which is why it is often referred to as the "sunshine vitamin." We can also get vitamin D from certain foods, including salmon, tuna, and cheese, as well as from dietary supplements, which are available to purchase online.
However, Yamaji and colleagues point out that most research on vitamin D and cancer risk to date has focused on white populations.
"Given that vitamin D concentrations and metabolism vary substantially by race/ethnicity," note the researchers, "whether similar associations would also be observed in non-Caucasian populations remains to be clarified."
With this in mind, the researchers sought to determine how vitamin D levels influence the risk of cancer in Japanese adults.
Overall cancer risk reduced by a fifth
The researchers analyzed the data of 33,736 Japanese people who were a part of the Japan Public Health Center-based Prospective Study. They were between the ages of 40 and 69, and they were followed-up for an average of 16 years.
Blood samples were taken from each participant at study baseline. These were assessed for levels of 25-hydroxyvitamin D, which is the circulating form of vitamin D.
The participants were divided into four groups based on their vitamin D levels, ranging from the lowest to highest. Information was also gathered on the subjects' medical history, as well as on their dietary and lifestyle factors.
Over the 16-year follow-up period, a total of 3,301 new cancer cases were identified among the study participants.
Compared with subjects who had the lowest vitamin D levels, those with a higher level had a 20 percent lower risk of cancer overall, the team reports.
This finding persisted after accounting for myriad possible confounding factors, including age, body mass index (BMI), levels of physical activity, smoking status, and alcohol intake.
On looking at specific cancers, the researchers found that a higher vitamin D level was associated with a 30–50 percent lower risk of liver cancer, with this reduced risk being more prominent in men.
Higher vitamin D levels were not linked to a lower risk of lung cancer or prostate cancer, the team reports, and no association was found between higher vitamin D levels and an increase in cancer risk.
A possible 'ceiling effect'
Yamaji and colleagues caution that their findings are solely observational, so no conclusions can be made about the link between vitamin D and cancer risk.
Furthermore, the study has some important limitations. For example, they point out that there were only a small number of organ-specific cancers included in their analysis.
Additionally, they note that it is possible that some cancer risk factors that were not accounted for in this study might have influenced the findings.
Still, the researchers say that their findings "support the hypothesis that vitamin D has protective effects against cancers at many sites."
That being said, the results indicate that there is a "ceiling effect" for vitamin D and cancer risk. In other words, there is an optimal level of vitamin D that protects us against cancer, but going beyond this level offers no further benefit.
"Future studies are needed," the researchers conclude, "to clarify the dose-response pattern and the optimal [vitamin D] concentrations for cancer prevention."