A new study suggests that, the more vitamin D you have in your system, the less likely you are to develop breast cancer.
Where some studies have revealed that overall cancer risk is lower in people with higher levels of this vitamin, others have suggested that vitamin D has no impact on a person's vulnerability to the disease.
Still, the case for ensuring that you get enough vitamin D is fairly strong, as low blood levels of this nutrient have been associated with a raised risk of bladder cancer and, in a study that was published earlier this year, an elevated risk of bowel cancer.
In a pooled analysis of a prospective cohort study and two randomized clinical trials, researchers at the University of California, San Diego School of Medicine in have now investigated whether and to what extent high levels of vitamin D in the blood were associated with a lower risk of developing breast cancer.
Their analysis — which was conducted in collaboration with specialists from Creighton University in Omaha, NE, the Medical University of South Carolina in Columbia, and the nonprofit organization GrassrootsHealth in Encinitas, CA — suggests that certain levels of vitamin D correlate with a "markedly lower" risk of breast cancer.
These results are now published in the journal PLOS ONE.
High vitamin D levels may lower risk
Principal investigator Cedric F. Garland and team analyzed the data of two randomized clinical trials with a total of 3,325 participants between them, and those of a prospective cohort study numbering 1,713 participants.
All of these participants were women — aged 63, on average — who were cancer-free at baseline. The data used in the analysis were collected in 2002–2017, and the participants' health was followed for a mean period of 4 years.
More specifically, the team looked for associations between the risk of developing breast cancer in women and volunteers' serum (a part of the human blood, minus the red blood cells) concentration of 25-hydroxyvitamin D (25(OH)D), a prominent vitamin D biomarker.
During the period in which the three analyzed studies were conducted, a total of 77 new cases of breast cancer were noted. "The age-adjusted incidence rate of breast cancer was 512 cases per 100,000 person-years in the pooled cohort," the researchers specify.
The analysis revealed that people with higher blood concentrations of the vitamin D biomarker were exposed to a significantly lower risk of breast cancer.
"We found that participants with blood levels of 25(OH)D that were above 60 nanograms per milliliter (ng/ml) had one-fifth the risk of breast cancer compared to those with less than 20 ng/ml."
Cedric F. Garland
Moreover, the higher the levels of vitamin D in the system, the lower the breast cancer risk, explain the scientists.
These findings are particularly noteworthy when considering that how much vitamin D we should have in our systems to be healthy is a matter that is still largely up for debate.
Garland and his team estimated that the minimum healthy level of 25(OH)D in blood should be about 60 nanograms per milliliter, which is a lot more than the 20 nanograms per milliliter concentration recommended by the National Academy of Medicine.
'Strongest association yet'
The findings reported by the research team held true even after the analysis results were adjusted for potential influencing factors, such as the participants' age, body mass index (BMI), smoking habits, and the consumption of calcium supplements.
"Increasing vitamin D blood levels substantially above 20 ng/ml," explains first study author Sharon McDonnell "appears to be important for the prevention of breast cancer."
These findings are especially exciting for Garland, who has been studying the association between serum levels of vitamin D and the risk of cancer for many years, and who has always maintained that this nutrient plays an important role in keeping the disease at bay.
The principal investigator explains that the current study builds on and strengthens the findings of previous epidemiological studies with similar results.
Epidemiological research focuses on calculating the incidence of a disease in a certain population, and the factors that might influence the occurrence of that condition. However, these kinds of studies do not offer clear proof of cause and effect, so the results must be read accordingly.
Another possible shortcoming of the current study, which is reported by Garland, is that "[t]his study was limited to postmenopausal breast cancer."
"Further research is needed," he goes on to say, "on whether high 25(OH)D levels might prevent premenopausal breast cancer."
"Nonetheless," Garland concludes, "this paper reports the strongest association yet between serum vitamin D and reduction in risk of breast cancer."