Cancer is the second leading cause of death in the United States and a major public health burden. Previous studies have suggested that vitamin D may lower the risk of developing cancer, so a new randomized trial tests this hypothesis in healthy, older women.

Cancer only comes second to heart disease as a leading cause of death in the U.S., accounting for almost 600,000 deaths in 2014.

The disease is a major public health concern, as approximately 40 percent of U.S. individuals are estimated to receive a cancer diagnosis at one point in their lives. Additionally, the total healthcare cost of cancer is predicted to reach $156 billion by the year 2020.

Recently, scientists have suggested that vitamin Dmay have a protective role against cancer. Its active component, calcitriol, has been investigated and some studies propose that it causes cancer cells to die and therefore stops them from proliferating.

A new clinical trial carried out by Joan Lappe, Ph.D., of the Creighton University Schools of Nursing and Medicine in Omaha, NE, tests the effects of dietary vitamin D and calcium supplementation on healthy postmenopausal women.

The findings were published in the journal JAMA.

Studying the effect of vitamin D and calcium on cancer risk

Lappe and team carried out a double-blind, randomized clinical trial in 31 rural counties. The trial lasted for 4 years, from June 2009 to August 2015.

During the trial, the scientists randomly divided a total of 2,303 women aged at least 55 years into two groups: 1,156 were assigned to the treatment group, and 1,147 were assigned to the placebo group.

The mean baseline level of 25-hydroxyvitamin D serum (vitamin D) in these participants was 32.8 nanograms per milliliter.

Participants in the treatment group were administered 2,000 international units of vitamin D-3 per day and 1,500 daily milligrams of calcium. Those in the placebo group received the placebo equivalent of this dosage.

Throughout the 4 years, the scientists followed the patients and looked at the incidence of all types of cancer, with the exception of nonmelanoma skin cancer.

The scientists used Cox proportional hazards regression to analyze cancer incidence while also adjusting for factors that might have influenced the results - such as age, body mass index (BMI), smoking, or family history.

Trial finds no significant effect of vitamin D supplementation

Of all the participants, 109 were diagnosed with cancer during the study: 3.89 percent in the treatment group, and 5.58 percent in the placebo group.

The 4-year incidence of a cancer diagnosis was calculated at 0.042 in the vitamin D and calcium supplementation group, and 0.060 in the group that received a placebo.

Overall, the researchers did not find a "significantly lower risk" of developing cancer in the treatment group throughout the 4 years. Additionally, the team did not find any "statistically significant difference" between the two groups in terms of breast cancer incidence.

The authors speculate that their study might not have detected any significant effect of the supplements because their sample had higher vitamin D levels than the average population of the United States might have. This was because the older women were at risk of osteoporosis, so limiting their intake of vitamin D and calcium - which improve skeletal strength - would have been unethical.

Lappe and colleagues also note some other limitations of the study. For instance, the fact that it focused only on predominantly non-Hispanic white women means that the results cannot be widely generalized.

The team also noted some adverse effects that might be related to the study. For example, 16 participants in the treatment group and 10 participants in the placebo group were diagnosed with kidney stones. Additionally, six women in the treatment group and two in the placebo group developed high serum calcium levels.

Finally, the authors conclude that "further research is necessary to assess the possible role of vitamin D in cancer prevention."