In this randomized, double-blind, placebo-controlled trial, daily supplementation with 1000 mg of elemental calcium combined with 400 IU of vitamin D3 had no effect on breast cancer incidence. Thus, the main findings do not support a causal relationship between calcium and vitamin D supplement use and reduced breast cancer incidence, despite the association observed in some epidemiological studies.
As reviewed (20
), although some observational studies (4
) support an inverse association between higher vitamin D intakes and lower breast cancer risk in postmenopausal women, others (8
) do not. A recent meta-analysis of observational studies including premenopausal and postmenopausal women reported that a modest reduction of breast cancer incidence was associated with higher intake of vitamin D (≥400 IU/day) of borderline statistical significance (24
). Similarly, an association between both higher calcium intake (7
) and higher calcium levels (25
) and lower breast cancer risk in postmenopausal women has been observed in some, but not all (10
), reports. Studies in postmenopausal women of the relationship between circulating 25-hydroxyvitamin D levels and breast cancer risk have also had mixed results. In three studies (26
), higher circulating 25-hydroxyvitamin D levels were associated with lower breast cancer risk. However, in a similar number of reports, either no association (11
) or only a borderline association (31
) was seen between 25-hydroxyvitamin D levels and breast cancer risk. In the current nested case–control study, with 895 case patients and 898 control subjects, no statistically significant association between higher baseline 25-hydroxyvitamin D and subsequent lower breast cancer risk was seen in analyses that adjusted for BMI and physical activity. In the nested case–control analysis that looked at the interaction effects within the trial design, no statistically significant interaction between baseline 25-hydroxyvitamin D levels, random assignment to supplement use, and breast cancer risk emerged, suggesting that even at the highest baseline 25-hydroxyvitamin D level, supplementation with 400 IU/day of D3
together with calcium was not associated with lower breast cancer risk.
Based on the relatively high threshold of about 75 nmol/L of 25-hydroxyvitamin D, which has been associated with low breast cancer risk in some reports (26
) and is estimated to require about 1700–2000 IU of supplement daily to achieve (27
), a higher vitamin D dose than used in this trial has been recommended by some as needed to influence the risk of breast cancer (28
). Although the dose of vitamin D used in this trial remains an issue (33
), when baseline vitamin D intakes were examined across 25-hydroxyvitamin D quintiles in the current trial, the distribution of vitamin D intake in each quintile showed substantial overlap. The difference in reported total vitamin D intake between women with the lowest mean and highest quintiles values for 25-hydroxyvitamin D was, surprisingly, only 238 IU/day of vitamin D, yet the mean ± SD 25-hydroxyvitamin D level in the upper quintile was 81.9 ± 13.2 nmol/L. (The IU refers to the intake of vitamin D by participants, the nmol/L refers to the serum 25-hydroxyvitamin D levels associated with the reported intakes.) A technology assessment of 16 prospective trials of the influence of vitamin D supplementation (with or without calcium) on 25-hydroxyvitamin D levels (34
) also describes considerable heterogeneity between supplement dose and subsequent magnitude of change in 25-hydroxyvitamin D levels. Such results suggest that factors other than dietary and supplement intake of vitamin D likely influence 25-hydroxyvitamin D levels. In fact, although sunlight exposure is a recognized influence on 25-hydroxyvitamin D levels, a substantial genetic influence on such levels has also been reported (35
The relative contribution of factors influencing 25-hydroxyvitamin D levels remains to be defined, especially for higher concentrations found in individuals who do not report high-dose supplement use. Although this mechanism is speculative, a genetic predisposition to both high 25-hydroxyvitamin D levels and low breast cancer risk could appear as a protective effect of vitamin D on breast cancer. Before future clinical trials of high-dose vitamin D regimens to reduce breast cancer risk are implemented, it will be important to demonstrate that the selected vitamin D dose can definitively increase circulatory 25-hydroxyvitamin D levels to the projected target level. Definitive assessment of factors that influence the relationship between vitamin D supplement use and subsequent changes in circulating 25-hydroxyvitamin D levels are therefore a research priority.
Levels of 25-hydroxyvitamin D at baseline were statistically significantly higher among lean women and/or those with more recreational activity than overweight or obese or less active women. Based on the associations of these breast cancer risk factors with 25-hydroxyvitamin D, 25-hydroxyvitamin D could be a potential mediator of lifestyle influence on breast cancer. Alternatively, lifestyle choices could have led to more sunlight exposure, with higher 25-hydroxyvitamin D levels and lower breast cancer risk as independent processes. The finding that analyses adjusted for BMI and physical activity did not identify an association between baseline 25-hydroxyvitamin D levels and breast cancer risk suggests that the association between 25-hydroxyvitamin D and breast cancer seen in some observational studies could be confounded to some degree by such factors.
Only one of the 22 subgroup analyses we performed demonstrated a statistically significant interaction between randomization group and the selected participant characteristics and suggested a differential calcium plus vitamin D supplement effect on breast cancer incidence according to baseline vitamin D intake. However, in the nested case–control analyses, no statistically significant interactions were observed.
The vitamin D dosage in this trial generally followed recommendations from the Institute of Medicine (37
). We cannot assess whether a higher dosage would have changed the outcome of the current study. However, our findings provide some evidence against that hypothesis. Because approximately half of the women were taking an additional 400 IU of nonprotocol vitamin D supplement daily, actual vitamin D supplement intake was greater than 800 IU daily for a substantial number of participants in the supplement group. Nonetheless, no effects on risk of breast cancer overall or in sensitivity analyses that were adjusted for study adherence were observed. Although further study of relationships among calcium plus vitamin D supplement use and breast cancer can be considered, current evidence does not support their use in any dose to reduce breast cancer risk.
Breast cancers in the supplement group were somewhat smaller (P = .05) but were of similar stage to those in the placebo group. Because the frequency of abnormal mammograms and rates of mammography screening were similar in the two randomization groups, differential influence on mammographic breast cancer detection does not explain the cancer findings.
Study strengths include the large, diverse study population, the double-blind placebo-controlled design, comprehensive breast cancer risk assessment, serial mammography monitoring, and central adjudication of breast cancers via pathology report review. Although a limitation is that discontinuation rates were higher than optimal, study supplement adherence was comparable to that in most chronic disease prevention trials, with 76% of participants still taking study pills at the end of the trial. Nonetheless, discontinuation of study pills would decrease the difference between the placebo and treated group in vitamin D intake. Because hip fracture was the primary study endpoint, the intervention included calcium as well as vitamin D. The inclusion of calcium could be considered a limitation, given the more modest information supporting calcium's potential relation to breast cancer.
Given the latency of breast cancer, the 7-year duration of the trial also could be questioned. However, raloxifene and tamoxifen, the two agents with a Federal Drug Administration label indication for breast cancer risk reduction, had efficacy demonstrated in trials of about 5-year duration (38
). In addition, given difficulties in maintaining long-term drug adherence (40
), any putative pharmacologic intervention that requires decades-long and continuous exposure would likely have limited public health implications.
Allowing nonprotocol calcium and vitamin D supplement use during the trial represents another limitation. However, nonprotocol supplement use was closely comparable in the two randomization groups and only about 15% of placebo group women could be considered drop-ins (with nonprotocol vitamin D supplement use increased by >400 IU daily during the study course). In addition, the difference in calcium and vitamin D dose between randomization groups was sufficient to statistically significantly increase bone mineral density in the whole population and statistically significantly decrease hip fracture in women older than 65 years (12
). Finally, this trial cannot separate calcium and vitamin D influence on breast cancer because these agents were used together in the study.
In summary, calcium and vitamin D supplementation in the dosage provided in this trial did not reduce the incidence of invasive breast cancer in postmenopausal women. In addition, 25-hydroxyvitamin D levels were not associated with subsequent breast cancer risk. These findings do not support a relationship between total vitamin D intake and 25-hydroxyvitamin D levels with breast cancer risk.