A four-year, randomized study conducted by Creighton University School of Medicine followed 1,179 healthy, postmenopausal women from rural Eastern Nebraska. Research participants were all 55 years and older and free of known cancers for at least 10 years prior to entering the Creighton study. Subjects were randomly assigned to take daily dosages of 1,400-1,500 mg supplemental calcium plus 1,100 IU of vitamin D3, or placebos. This was a 4-y, population-based, double-blind, randomized placebo-controlled trial.
On the Undoctored forum, we know that is better to get calcium from food than from calcium tablets because of the risk of kidney stones and calcium deposits in arteries.
Over the course of four years, women in the calcium/vitamin D3 group experienced a 60 percent decrease in their cancer risk than the group taking placebos.
On the premise that some women entered the study with undiagnosed cancers, researchers then eliminated the first-year results and looked at the last three years of the study. When they did that, the results became even more dramatic with the calcium/vitamin D3 group showing a startling 77 percent cancer-risk reduction.
In multiple logistic regression models, both treatment and serum 25-hydroxyvitamin D concentrations were significant, independent predictors of cancer risk. These amazing effects were achieved with what many nutritionists consider to be a low dose of vitamin D3. The vitamin D from exposure to sunlight was not estimated in this study. In the three-year analysis, there was no statistically significant difference in cancer incidence between participants taking placebos and those taking just calcium supplements.
Until 2009, this was the only randomized controlled trial on cancer incidence that used a sufficiently high dose of vitamin D3.
William Grant said approximately 35% reduction in risk was attributed to vitamin D alone. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2715207/
Undoctored Inner Circle members take a higher dose of vitamin D3 than that used in the study and would not have to take a calcium supplement. Vitamin D3 increases calcium absorption.