Table 2.

Being Breastfed in Infancy and its Duration and Colorectal Cancer Risk Among Participants From the Nurses’ Health Study (1992–2014) and the Nurses’ Health Study II (1991–2015) and Among Those With Reports of Their Mothers From the Nurses’ Mothers’ Study (2001)

				Duration of being breastfed
	Not Breastfed	Breastfed	P	≤3 months	4–8 months	≥9 months	P trend a
Nurses’ Health Studies
Person-years	1,914,584	1,572,628		307,384	315,079	242,186
No. of cases	525	965		100	142	193
HR (95% CI)b	1 (reference)	1.21 (1.08–1.36)	.001	1.13 (0.91–1.40)	1.14 (0.94–1.38)	1.35 (1.13–1.61)	.001
HR (95% CI)c	1 (reference)	1.23 (1.10–1.38)	<.001	1.14 (0.92–1.42)	1.17 (0.97–1.42)	1.36 (1.14–1.63)	<.001
HR (95% CI)d	1 (reference)	1.23 (1.09–1.38)	<.001	1.14 (0.92–1.42)	1.16 (0.96–1.41)	1.35 (1.13–1.62)	.001
Nurses’ Mothers’ Study
Person-years	320,088	295,305		150,783	107,676	34,604
No. of cases	55	94		41	35	17
HR (95% CI)b	1 (reference)	1.48 (1.05–2.11)	.03	1.33 (0.87–2.02)	1.52 (0.98–2.35)	2.03 (1.14–3.64)	.009
HR (95% CI)c	1 (reference)	1.58 (1.11–2.24)	.01	1.41 (0.92–2.15)	1.57 (1.01–2.45)	2.31 (1.28–4.19)	.003
HR (95% CI)e	1 (reference)	1.57 (1.10–2.25)	.01	1.39 (0.90–2.13)	1.58 (1.00–2.48)	2.43 (1.33–4.44)	.003

Note: Participants with unknown duration of being breastfed were excluded from the duration analysis.

CI, confidence interval; HR, hazard ratio.

^aCalculated by entering the duration category (not breastfed, ≤3 months, 4–8 months, or ≥9 months) as an ordinal variable.

^bFrom Cox proportional hazards regression conditioned on age in months and calendar year of the survey cycle and adjusted for race/ethnicity (white or non-white), history of colorectal cancer screening (yes or no), and history of lower endoscopy for symptomatic reasons (yes or no).

^cAdditionally adjusted for adult height (in quintiles), family history of colorectal cancer (yes or no), history of inflammatory bowel disease (yes or no), history of diabetes (yes or no), regular aspirin use (yes or no), menopausal status and hormone therapy use (premenopausal, postmenopausal never user, postmenopausal ever user, or unknown), body mass index (in quintiles), physical activity (in quintiles), pack-years of smoking (0, 1–4, 5–19, 20–39, or ≥40), alcohol intake in g/d (0, 0.1–14.9, or ≥15.0), intakes of total calories, red meat, dietary fiber, total calcium, total folate, and total vitamin D (all in quintiles), and Alternate Healthy Eating Index-2010 (in quintiles).

^dAdditionally adjusted for preterm birth (yes or no) and birth weight in pounds (<5.5, 5.5–6.9, 7.0–8.4, or ≥8.5).

^eAdditionally adjusted for preterm birth (yes or no), birth weight in pounds (<5.5, 5.5–6.9, 7.0–8.4, or ≥8.5), delivery mode (cesarean or vaginal birth), mother’s age at the nurse’s birth (continuous), mother’s usual body mass index before pregnancy (continuous), mother gaining ≥40-lb weight during pregnancy (yes or no), mother smoking cigarettes during pregnancy (yes or no), mother drinking alcohol during pregnancy (yes or no), mother using vitamins during pregnancy (yes or no), mother receiving higher education (yes or no), father receiving higher education (yes or no), and highest parental occupation (professional, managerial, clerical, craftsman, service, laborer, farmer, military, or housewife/don’t work).