Vitamin D intake and breast cancer risk in postmenopausal women: the Iowa Women’s Health Study

Abstract

Vitamin D, a prosteroid hormone with anti-proliferative and pro-differentiation activity, is thought to act as a cancer chemopreventive agent. This study evaluated the association between vitamin D intake and breast cancer risk among women in a large prospective cohort study. A total of 34,321 postmenopausal women who had completed a questionnaire that included diet and supplement use were followed for breast cancer incidence from 1986 to 2004. Adjusted relative risks (RR) for breast cancer were calculated for dietary, supplemental, and total vitamin D intake among all women. The adjusted RR of breast cancer for women consuming >800 IU/day versus <400 IU/day total vitamin D was 0.89 (95% CI: 0.77-1.03). RRs were stronger among women with negative than positive ER or PR status. The association of high vitamin D intake with breast cancer was strongest in the first 5 years after baseline dietary assessment (RR = 0.66; 95% CI: 0.46-0.94 compared with lowest-intake group), and diminished over time. Changes in vitamin D intake over time might have contributed to the diminished association observed in later years. Vitamin D intake of >800 IU/day appears to be associated with a small decrease in risk of breast cancer among postmenopausal women. Studies evaluating all sources of vitamin D, especially sun exposure, are needed to fully understand the association between vitamin D and breast cancer risk.

Plasma 25‐hydroxyvitamin D and premenopausal breast cancer risk in a German case‐control study

“…Variables were entered as covariates in the logistic regression model if (i) including the covariate considerably changed the OR for the main variables of interest (>5% change in the ORs), (ii) if the covariate is a known or potential breast cancer risk factor or, (iii) if the covariate was likely to be associated with the main variable of interest (biological plausibility). OR estimates were both adjusted for time of blood collection in four categories, January-March, April-June, July-September, October-December only and with additional adjustment for first-degree family history of breast cancer (yes/no), number of births (0, 1-2, 3), duration of breast feeding (continuous, in months), age at menarche (<13, [13][14]15), BMI (continuous, in kg/m 2 ) and alcohol consumption (0, 1-18, 18 g ethanol/day). Age at first birth, smoking and education did not affect the estimates considerably and were therefore not included in the final model.…”

“…[1][2][3][4][5][6] Most of the epidemiologic studies regarding breast cancer risk have assessed the effects of vitamin D only for dietary intake yielding inconsistent results. [7][8][9][10][11][12][13][14][15] However, beside dietary intake, the main source of vitamin D is cutaneous production via sun exposure. Studies assessing the association of breast cancer risk and measures of sun exposure, as a proxy of endogenous vitamin D synthesis, have observed inverse associations.…”

Vitamin D in health and disease: an insight into traditional functions and new roles for the ‘sunshine vitamin’

“…Cohort studies undertaken by Shin et al (122) , McCullough et al (123) , John et al (124) and Robien et al (125) all found some inverse associations between vitamin D intake and/or exposure and breast cancer risk. Shin et al (122) , as part of the Nurses' Health Study (n 88 691), reported no significant associations between total (dietary and supplemental) vitamin D intake and breast cancer incidence in postmenopausal women but did report significant associations when comparing the lowest total vitamin D intake (# 3·75 mg) with the highest vitamin D intake (.…”

“…Robien et al (125) reported a weaker non-statistically significant relationship with a RR of 0·89 (95 % CI 0·77, 1·03; P ¼ 0·12) for women with a vitamin D intake of $ 20 mg in the Iowa Women's Health Study. All of these studies are notable due to their large size and prospective cohort design; however, as these were observational studies a causal relationship cannot be confirmed and both studies did not adequately measure sun exposure, which is the main source of vitamin D for most populations.…”

Epidemiology of vitamin D in health and disease

“…From a large prospective cohort study of 34 321 postmenopausal women followed for 18 years in the Iowa Women's Health Study, women with vitamin D intake . 800 IU/d had an adjusted risk for breast cancer of 0·89 (weak association) compared with those with vitamin D intake , 400 IU/d (134) . From a population-based casecontrol study in Canada involving 972 newly diagnosed invasive breast cancer and 1135 controls, reduced breast cancer risk was associated with increased sunlight exposure from age 10 to 19 years (135) .…”