Researchers recently presented groundbreaking findings that vitamin D reduces the risk of all cancers at the American Public Health Association’s Annual Meeting. The randomized controlled trial (RCT) was the first to evaluate the effects of vitamin D supplementation on cancer as the primary outcome.

These results have arrived with much anticipation due to the consistent evidence suggesting that higher vitamin D levels and increased sun exposure are linked to lower cancer risk. In fact, over 15 types of cancer have been associated with low sun exposure.

Animal studies have proposed various mechanisms to explain these associations. For instance, vitamin D has been shown to promote cellular differentiation, decrease cancer cell growth, stimulate cell death and decrease tumor blood vessel formation in studies of cancer cell and tumors in mice.

Now, the gold standard of research, a randomized controlled trial, has investigated the direct effects of vitamin D on cancer, and it followed a near flawless study design:

  • Large sample population: The study included 2,302 healthy menopausal women ages 55 and older.
  • Long duration: the experiment lasted four years.
  • Adequate vitamin D3 dose: While the dosage did not quite meet the recommendations of the Vitamin D Council of 5000 IU per day, it provided enough vitamin D to cause a significant increase in vitamin D levels at 2,000 IU per day.
  • Use of a control group: Researchers compared the difference in cancer incidence between the vitamin D group and a control group. The control group received placebo.

After four years of supplementing with 2000 IU of vitamin D3 daily, the average vitamin D levels of the vitamin D group increased from 32.8 ng/ml to 43.9 ng/ml. The control group’s average levels decreased slightly from an average of 32.8 ng/ml to 31.6 ng/ml.

Throughout the four years, 106 of the 2,300 women developed at least one type of non-skin cancer. The researchers discovered that cancer incidence (43 treatment and 63 controls) was significantly lower in the treatment group (p < 0.05). Furthermore, the analyses revealed that both vitamin D supplementation and vitamin D status were significant predictors of cancer risk after one year of intervention.

The researchers stated,

“Supplementing with 2000 IU/day of vitamin D3 and 1500 mg/day of calcium substantially reduced risk of all cancers combined. This finding provides great impetus for improving vitamin D status through advances in vitamin D nutritional policy.”

This study presented a groundbreaking discovery for public health. Cancer is the number two cause of death in the United States with approximately 40% of men and women developing cancer in their lifetime. This upsetting percentage could decrease through the identification of modifiable risk factors, such as vitamin D and calcium supplementation.

Further RCTs are needed to confirm the findings of this study.


Tovey, A. & Cannell, JJ. Vitamin D supplementation directly reduces cancer risk, according to RCT. The Vitamin D Council Blog & Newsletter, February 2, 2017.