Table 1.

Details of key studies included in the present review

Study; country	Study design	Analytic sample	Lactation status	Cardio-metabolic measures
45 and Up (42); Australia	Retrospective study of baseline data of the 45 and Up cohort.	74,785 women aged ≥45 years, selected from the Australian Medicare Database. Recruited from 2006–2009.	Total breastfeeding duration was assessed by the question, “For how many months, in total, have you breastfed?” Average breastfeeding duration was calculated as total duration divided by reported number of births.	Blood pressure: self-reported treatment for high blood pressure in the past month.
Coronary Artery Risk Development in Young Adults (CARDIA) (24, 25); United States	Prospective cohort with 20-year follow-up. Women were reexamined at 7, 10, 15, and/or 20 years.	1,399 black and white women from 4 geographic areas of the US. Baseline data were collected in 1985–1986 when participants were aged 18–30 years.	At each exam, women reported births since the previous exam. For each pregnancy, women reported lactation history as none, <6 weeks, 6–11 weeks, 3–6 months, or >6 months. The midpoint of each interval was used, or a value of 210 days for >6 months, to sum total duration of lifetime lactation.	Anthropometry: weight, height, and waist circumference measured by trained technicians. Blood pressure: average of the second and third measures taken by trained technicians. Lipids: fasting measured total cholesterol, HDL, and triglycerides. LDL calculated from Friedwald equation. Glucose: fasting measured glucose and insulin.
Korean Women’s Cohort (38); South Korea	Prospective cohort study with 6-year follow-up.	177,749 premenopausal women ≥20 years (mean age 30.1 years) insured by the Korean Medical Insurance Company. Baseline data were collected in 1994.	Lifetime duration of lactation: the sum of reported lactation for each child. Lactation history was asked for each child, up to a maximum of 5 children.	Blood pressure: elevated blood pressure or current use of hypertensive medication. A health-care professional measured blood pressure a single time.
Million Women Study (5); United Kingdom	Retrospective study of baseline data of the Million Women Study.	740,628 postmenopausal women (mean age 57.5 years) invited to the National Health Service Breast Screening Program in England and Scotland between 1996 and 2001.	Lifetime duration of lactation: the sum of reported breastfeeding, in months, for each child.	Anthropometry: self-reported current height and weight.
Nord-Trøndelag Health Study (HUNT2) (51, 52); Norway	Prospective cohort study with 15-year follow-up. Additional retrospective analyses using baseline data.	21,889 women aged 20–85 years living in Nord-Trøndelag, Norway. HUNT2 baseline was 1995–1997.	Lifetime duration of lactation: the sum of responses to the question, “For how many months did you breastfeed?” asked for each live birth.	Anthropometry: measured height, weight, and waist circumference. Blood pressure: mean of the second and third measures taken by trained technicians. Lipids: nonfasting measured total cholesterol, HDL, and triglycerides. LDL calculated from Friedwald equation. Glucose: nonfasting measured glucose. Cardiovascular mortality: assessed prospectively from linked death records.
Nurses’ Health Study (NHS) and NHS II (71, 73, 74); United States	Prospective cohort studies, with questionnaires sent every 2 years.	83,585–89,326* parous women in NHS. Women were from 11 states and aged 30–55 years at enrollment, which began in 1976. 44,198–73,418* parous women in NHS II. Women were from 14 states and aged 25–42 years at baseline (1989).	Lactation history was assessed once in NHS (1986), when the youngest woman was age 40 years. Women reported total duration of lactation for all pregnancies. Lactation history was assessed in NHS II in 1993, 1997, and 2003. In 1993, women were asked about lifetime duration of lactation. In 1997, a more detailed questionnaire asked about lactation history, including timing of introduction of formula and solid foods, for up to 4 children; women with ≥5 children reported total additional months of breastfeeding. Women who gave birth after 1997 completed a similar questionnaire in 2003.	Incident myocardial infarction (MI): women reported nonfatal MI, which was confirmed by medical records. Fatal MIs were assessed from death records or family members (NHS). Blood pressure: self-report of physician’s diagnosis of high blood pressure, excluding during pregnancy (NHS II). Glucose: self-reported diabetes diagnosis, if also supported by report of classic symptoms, elevated plasma glucose, or diabetes treatment (NHS and NHS II).
Project Viva (70); United States	Prospective cohort study with 3-year follow-up.	570 women (mean age 36.0–38.8 years across lactation groups) recruited during their first prenatal visit from obstetric practices in eastern Massachusetts. Sample size was smaller for fasting blood samples (n = 175).	At 6- and 12-month follow-up visits, women were asked if they were breastfeeding and if they had introduced formula or solid foods. Timing of stopping breastfeeding and introducing solid foods was reported in months, weeks, or days. These data were used to calculate duration of any and exclusive breastfeeding.	Anthropometry: measured weight, height, waist circumference, and subscapular and triceps skinfolds. Lipids: measured total cholesterol, HDL, LDL, and triglycerides. Glucose: fasting measured glucose and insulin. Hemoglobin A1c was assessed on all samples. Insulin resistance was calculated from a homeostasis model using fasting insulin and glucose.
Study of Women’s Health Across the Nation (SWAN) (64) and SWAN-Heart (47, 67); United States	Retrospective study of baseline data of the SWAN cohort. SWAN-Heart is an ancillary study.	2,516 women aged 42–52 years, recruited from 7 clinical sites across the US. Enrollment was 1996–1997. SWAN-Heart (2001–2005) participants were aged 45–58 years (n = 297–351).*	Lactation duration: participants reported lactation duration in months for each live birth. Duration was truncated at one year per pregnancy.	Anthropometry: height, weight, and waist circumference. Blood pressure: measured. Lipids: measured fasting total cholesterol and HDL. Glucose: measured fasting glucose and insulin. Subclinical cardiovascular disease (SWAN-Heart): coronary artery calcification, aortic calcification, common carotid artery intima-media thickness, and adventitial diameters. Anthropometry (SWAN-Heart): visceral adiposity assessed by computed tomography.
Women’s Health Initiative (68); United States	Prospective cohort study with 7.9-year follow-up. Additional retrospective analyses using baseline data.	139,681 postmenopausal women aged 50–79 years at enrollment.	Lifetime duration of lactation: assessed from responses to the question, “Did you breastfeed or nurse any children for at least one month?” Those who responded yes were asked, “Thinking about all the children you breastfed, how many months total did you breastfed (your best guess)?”	Anthropometry: measured height and weight. Blood pressure: self-reported history of treatment for hypertension and measured blood pressure. Lipids and glucose: self-reported need to take medication for “cholesterol” or “sugar diabetes.” Nurses validated medication use. Prevalent cardiovascular disease: self-reported. Incident cardiovascular disease: confirmed by physician.

^*Sample sizes varied depending on outcomes examined.