Dairy-Food, Calcium, Magnesium, and Vitamin D Intake and Endometriosis:
A Prospective Cohort Study
Holly R. Harris*, Jorge E. Chavarro, Susan Malspeis, Walter C. Willett, and Stacey A. Missmer
* Correspondence to Dr. Holly R. Harris, Ob/Gyn Epidemiology Center, Brigham and Women’s Hospital, 221 Longwood Avenue, Boston,
MA 02115 (e-mail: email@example.com).
Initially submitted January 23, 2012; accepted for publication May 2, 2012.
The etiology of endometriosis is poorly understood, and few modifiable risk factors have been identified. Dairy
foods and some nutrients can modulate inflammatory and immune factors, which are altered in women with endo-
metriosis. We investigated whether intake of dairy foods, nutrients concentrated in dairy foods, and predicted
plasma 25-hydroxyvitamin D (25(OH)D) levels were associated with incident laparoscopically confirmed endome-
triosis among 70,556 US women in Nurses’ Health Study II. Diet was assessed via food frequency questionnaire.
A score for predicted 25(OH)D level was calculated for each participant. During 737,712 person-years of follow-
up over a 14-year period (1991–2005), 1,385 cases of incident laparoscopically confirmed endometriosis were
reported. Intakes of total and low-fat dairy foods were associated with a lower risk of endometriosis. Women con-
suming more than 3 servings of total dairy foods per day were 18% less likely to be diagnosed with endometriosis
than those reporting 2 servings per day (rate ratio=0.82, 95% confidence interval: 0.71, 0.95; Ptrend=0.03). In
addition, predicted plasma 25(OH)D level was inversely associated with endometriosis. Women in the highest
quintile of predicted vitamin D level had a 24% lower risk of endometriosis than women in the lowest quintile (rate
ratio=0.76, 95% confidence interval: 0.60, 0.97; Ptrend=0.004). Our findings suggest that greater predicted
plasma 25(OH)D levels and higher intake of dairy foods are associated with a decreased risk of endometriosis.
dairy foods; diet; endometriosis; magnesium; phosphorus; vitamin D
Abbreviations: BMI, body mass index; CI, confidence interval; FFQ, food frequency questionnaire; NHS II, Nurses’ Health Study
II; 25(OH)D, 25-hydroxyvitamin D; OR, odds ratio; RR, rate ratio.
Endometriosis is a disorder characterized by the presence
of endometrial tissue outside the uterine cavity. It has an
estimated prevalence of 10% (1) and is the third-leading
cause of gynecological hospitalization in the United States
(2). Signs and symptoms vary in severity and include dys-
menorrhea, dyspareunia, infertility, dysuria, and dyschezia
(3). Despite its prevalence and associated morbidity, the
etiology of endometriosis is poorly understood, and few
modifiable risk factors have been identified. One factor that
may influence endometriosis is diet, which can act through
multiple pathways, including effects on inflammation,
smooth muscle contractility, immune function, and estro-
Recent studies have suggested that vascular inflammation
is present among persons with endometriosis (4–6). Intakes
of dairy foods and dietary calcium have been inversely
related to inflammatory stress (7), suggesting that dairy
foods and nutrients concentrated in these foods may influ-
ence endometriosis risk. Magnesium relaxes smooth
muscle (8, 9) and may thus be related to endometriosis
through its influence on retrograde menstruation.
Endometriosis risk may also be influenced by dietary vita-
min D intake and plasma 25-hydroxyvitamin D (25(OH)D)
concentration. Beyond its role in calcium and bone homeo-
stasis, vitamin D has been shown to influence immune
function (10). Women with endometriosis exhibit changes
in cell-mediated immunity, with altered T-helper cell:T-
D may influence the development of endometriosis through
its immunomodulatory effects.
Am J Epidemiol. 2013;177(5):420–430
American Journal of Epidemiology
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Few studies have examined the relationship between diet
and endometriosis, and only 2 have examined associations
with dairy foods or nutrients. In the first human study that
examined dietary intake, Parazzini et al. (14) found no as-
sociation between milk or cheese intake and endometriosis
risk. Most recently, in a population-based case-control
study, Trabert et al. (15) reported a nonsignificant inverse
association between dairy-food and calcium consumption
and surgically confirmed endometriosis. In this study, we
investigated whether intake of dairy foods, nutrients con-
centrated in dairy foods (calcium, vitamin D, magnesium,
and phosphorus), and predicted plasma 25(OH)D levels
were associated with incident laparoscopically confirmed
endometriosis in a prospective cohort study over a 14-year
follow-up period. We also examined whether the associa-
tion between these factors and endometriosis varied accord-
ing to the fertility status of endometriosis patients and
whether this association was modified by body mass index
(BMI; weight (kg)/height (m)2), parity, or smoking.
MATERIALS AND METHODS
Nurses’ Health Study II (NHS II) is an ongoing prospec-
tive cohort study that was established in 1989 when
116,430 US female registered nurses aged 25–42 years
completed a baseline questionnaire that collected informa-
tion on demographic and lifestyle factors, anthropometric
variables, and disease history. Follow-up questionnaires are
sent biennially to participants, with questions requesting
updated information on incident disease risk factors. Ques-
tionnaire response rates through 2005 were approximately
90%. Further details on the study have been provided else-
Follow-up for the current analyses began in 1991, when
NHS II participants (n=97,807) returned the dietary as-
sessment, and concluded in 2005. We excluded participants
who had an implausible total energy intake (<800 kcal/day
or >4,200 kcal/day) or left more than 70 food items blank
on the 1991 food frequency questionnaire (FFQ). Partici-
pants were also excluded if they reported a diagnosis of en-
dometriosis, a history of infertility, or a cancer diagnosis
(other than nonmelanoma skin cancer) prior to June 1991.
The analytical cohort was limited to women who were pre-
menopausal and had intact uteri, since endometriosis rarely
occurs incidentally among postmenopausal women or sub-
sequent to a hysterectomy. After these exclusions, 70,556
premenopausal women with dietary information remained.
Implied consent was assumed upon return of the completed
questionnaire. This study was approved by the institutional
review boards of the Harvard School of Public Health and
Brigham and Women’s Hospital, Boston, Massachusetts.
Diet was assessed in 1991, 1995, 1999, and 2003 using
an FFQ listing over 130 food items, including 11 individual
dairy foods. Participants were asked how often, on average,
they had consumed each type of food or beverage during
the previous year. Nine responses were possible, ranging
from never or less than once per month to 6 or more times
per day. Intakes of the nutrients of interest (vitamin D,
calcium, magnesium, and phosphorus) were calculated by
multiplying the portion size of a single serving of each
food by its reported frequency of intake, then multiplying
the total amount consumed by the nutrient content of the
food, and summing the nutrient contributions of all food
items using US Department of Agriculture food composi-
tion data (17), while also taking dietary supplements into
account. The reproducibility and validity of the NHS II
FFQ have been reported elsewhere (18–20). The FFQ has
been shown to provide valid estimates of dairy-food and
nutrient intake, with deattenuated correlation coefficients
for dairy foods between the FFQ and 1-week diet records
ranging from 0.57 for hard cheeses (18) to 0.94 for yogurt
(18), 0.75 for calcium intake (21), 0.63 for phosphorus
(22), and 0.71 for magnesium (22). Vitamin D intake has
been validated using plasma concentrations of 25(OH)D,
with reported correlations of 0.25 (P < 0.001) (23, 24).
Intakes of all nutrients were adjusted for total energy intake
using the residual method (21).
Predicted plasma 25(OH)D level
A model for predicting plasma 25(OH)D levels was de-
veloped using 1,498 NHS II participants with no prior
history of cancer who gave blood samples between 1996
and 1999 and had served as controls in previous nested
case-control studies. Plasma 25(OH)D concentration was
measured by means of an enzyme immunoassay, and a
linear regression model was then developed to predict
plasma 25(OH)D levels on the basis of age, season of
blood draw, race/ethnicity, geographical region, dietary
vitamin D intake, BMI, alcohol intake, and physical activi-
ty, as previously described (25, 26). R2for the prediction
model ranged from 0.25 to 0.33 (26). From the predictors’
regression coefficients, a predicted-25(OH)D score was cal-
culated for each cohort member.
Ascertainment and definition of endometriosis
Starting in 1993, participants were asked on each biennial
questionnaire if they had “ever had physician-diagnosed en-
dometriosis,” and if so, the date of diagnosis and whether it
had been confirmed by laparoscopy. The validity of self-
reported endometriosis in this cohort has been described
previously (27). Briefly, a diagnosis of endometriosis was
confirmed by medical records in 96% of women who re-
ported laparoscopic confirmation. However, a review of the
medical records of women without laparoscopic confirma-
tion indicated a clinical diagnosis of endometriosis in only
54%. In addition, a diagnosis of endometriosis at the time
of hysterectomy was confirmed in 80% of the cases, but
endometriosis was the primary indication for hysterectomy
in only 6% of those for whom an indication was available.
Therefore, to minimize the magnitude of misclassification
and prevent confounding by indication for hysterectomy, we
restricted our definition of incident diagnosis of endometriosis
Dairy-Food and Vitamin D Intake and Endometriosis
Am J Epidemiol. 2013;177(5):420–430
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