Higher vitamin D levels may have a protective effect against breast cancer, according to researchers in China.
Mounting evidence suggests that vitamin D plays a role in the prevention of several cancers, breast cancer being one of them. To date, there have been many case-control studies from Western societies, examining the vitamin D levels of breast cancer patients when they are first diagnosed and comparing them to similar population controls of people without breast cancer. Often times, the vitamin D levels are lower in the newly diagnosed breast cancer patients.
However, to date, these same types of case-control studies are lacking from Eastern societies. In this study, Peizhan Chen and colleagues conducted a population case-control study on vitamin D and breast cancer to assess the relationship among Chinese women. Unlike many western populations, women in China rarely use supplements, and there are also not as many fortified foods. These factors may contribute to lower vitamin D levels when compared to western women.
The researchers included 593 newly diagnosed breast cancer patients aged 30 to 87 years old. Blood samples were collected before any treatment was provided. Five hundred-eighty female control participants were randomly selected from a breast cancer screening project at the same hospital in Shanghai.
They found the following:
- Mean 25(OH)D level for controls was 15.67 ng/ml compared with 11.31 ng/ml for breast cancer patients.
- Among controls, 80% had severe deficiency (<20 ng/ml), 15% were deficient (20-30 ng/ml) and 5% were sufficient (>30 ng/ml).
- Among breast cancer patients, 96% were severely deficient, 3% were deficient, and only 0.7% were sufficient.
- Women with the highest 25(OH)D status showed a significant decreased breast cancer risk compared to women in the lowest quartile (<10 ng/ml). For every 1 ng/ml increase in vitamin D status, there was a 16% decreased risk in breast cancer.
The researchers then took their results and pooled them with past case-control studies of similar design and performed a meta-analysis. Like their study and past research, the meta-analysis showed that the highest vitamin D levels had a significant protective effect for breast cancer risk.
There are obvious limitations in case-control studies, namely that they are not designed to distinguish whether the relationship between the factor and condition is an association or whether there is causation. Here, we don’t know if low vitamin D is just associated with increased breast cancer risk or if low vitamin D actually increases your risk of breast cancer.
Also, the researchers point out that given the high level of different results in all of the studies in their meta-analysis, there may be selection bias in the control groups of some of the studies. They conclude that since vitamin D deficiency is common in all populations, women will likely benefit from supplementation. To solidify the research on vitamin D and breast cancer, they call for further large scale trials.
Chen P, Li M, Gu X, Liu Y, Li X, Li C, Wang Y, Xie D, Wang F, Yu C, Li J, Chen R, Zhu J, Ou Z, Wang H. Higher blood 25(OH)D level may reduce the breast cancer risk: evidence from a Chinese population based case-control study and meta-analysis of the observational studies. PLOS ONE, 2013.