Heart disease is the leading cause of death in U.S. women.1 The importance of weight control, healthful dietary habits, and adequate physical activity are recognized as key components of cardiovascular disease prevention. Lactation history has been linked with reduced risk of myocardial infarction, hypertension, noninsulin-dependent diabetes mellitus, and the metabolic syndrome in women during mid- to late life.2–5 However, studies have never measured cardiovascular disease risk factors proximate to pregnancies and rely on recall of lactation and self-report of cardiovascular disease events or risk factors. Reverse causation (ie, favorable cardiometabolic profiles and lower body mass index [BMI, calculated as weight (kg)/[height (m)]2] cause longer lactation duration) remains a potential explanation for retrospective study findings.
Carotid artery intima-media thickness is a measure of subclinical atherosclerosis and a strong predictor of future heart disease and stroke, particularly in women.6 Cross-sectional studies of lactation and carotid artery intima-media thickness have not revealed an association between the two.7,8 However, the odds of aortic and coronary artery calcification were higher,7 and lumen and adventitial sections of the carotid arteries8 were smaller in women who never breastfed compared with women with 3 or more months of cumulative breastfeeding.
No studies of cardiovascular disease have considered risk factors that delay lactogenesis9 such as insulin resistance, obesity, and gestational diabetes mellitus (GDM) and may be subject to reverse causation. One exception is the 20-year Coronary Artery Risk Development in Young Adults study that measured prepregnancy risk factors through biochemical testing at 2- to 5-year intervals before and after pregnancies.5 In Coronary Artery Risk Development in Young Adults, lactation duration was associated with lower incidence of the metabolic syndrome independent of prepregnancy cardiometabolic risk factors, sociodemographics, lifestyle behaviors, and weight gain.5 Thus, we hypothesized that longer lactation duration would show a graded protective association with subclinical atherosclerosis in midlife independent of prepregnancy cardiometabolic status, perinatal outcomes, lifestyle behaviors, and follow-up characteristics.
MATERIALS AND METHODS
The Coronary Artery Risk Development in Young Adults study is a population-based, multicenter, longitudinal, observational study examining the trends and determinants of coronary heart disease risk factors in young black and white men and women. In 1985–1986, 5,115 participants (2,787 women) aged 18–30 years (52% black, 48% white) were recruited from four geographic areas in the United States: Birmingham, Alabama, Chicago, Illinois, Minneapolis, Minnesota, and Oakland, California. Participants attended in-person examinations every 2–5 years for measurements of blood pressure (BP), anthropometry, biochemical parameters, sociodemographics, medical conditions and medications, and lifestyle behaviors. In women enrolled in Coronary Artery Risk Development in Young Adults, we also assessed reproductive history, detailed pregnancy course, and outcomes at each examination. Retention rates at follow-up examinations 7, 10, 15, and 20 years later (2005–2006) were 81%, 79%, 74%, and 72% of the surviving cohort, respectively. Institutional review boards at each participating study center approved the study. Written, informed consent was obtained from participants for all procedures and for this current analysis.
Of 2,787 women enrolled in 1985–1986 (baseline), 2,014 (72%) attended the examination 20 years later in 2005–2006. After exclusions (Fig. 1), the final analytic sample included 846 women without heart disease or overt diabetes before pregnancy, who delivered at least once after baseline (total of 1,535 births from 1986–2006), had common carotid intima-media thickness measured in 2005–2006, and reported lactation duration. Those excluded had less education, higher BMI, and higher percentage of black race.
Methodologies for data collection and venipuncture are described elsewhere.10 Briefly, women fasted before each examination and reported the number of hours since their last intake of food or beverages before the blood sample draw. Procedures for collection and storage of plasma and serum samples, laboratory quality control procedures, and methodology for analysis of plasma lipids lipoproteins, glucose, and insulin11 and calculation of the homeostatic model assessment of insulin resistance have been previously described.12 Prepregnancy risk factors were obtained at baseline. Homeostatic model assessment of insulin resistance=(G0 X I0)/22.5 and G0=fasting glucose and I0=fasting insulin.
After an initial 5-minute rest, BP was measured three times at 1-minute intervals and the second and third values averaged. From year 0 to 15, BP was measured using the Hawksley random-zero sphygmomanometer; the first and fifth phase Korotkoff sounds were recorded. At year 20, BP was measured with an automated sphygmomanometer with a standardized protocol. Omron values were recalibrated to corresponding random zero values based on measurement techniques in 903 participants, as estimated random zero systolic value=(3.74+0.96×Omron systolic value) and estimated random zero diastolic value=(1.30+0.97×Omron diastolic value).
Certified technicians measured weight, height, and waist circumference at each examination according to a standardized protocol using calibrated research equipment as previously described.10 Body mass index was computed as weight in kilograms divided by squared height in meters.
The metabolic syndrome was ascertained by the National Cholesterol Education Program criteria: the presence of three of five characteristics (waist girth greater than 88 cm, fasting triglycerides 150 mg/dL or greater, high-density lipoprotein cholesterol (HDL-C) less than 50 mg/dL, BPs 130 or greater or 85 mm Hg or greater or treatment with antihypertensive medication, fasting glucose 100 mg/dL or greater, or treatment with diabetes medication), and incident diabetes was assessed by fasting serum glucose 126 mg/dL or greater, 2-hour serum glucose 200 mg/dL or greater, self-report of diabetes and medication treatment in examination years 0, 7, 10, 15, and 20, or all of these.
The common carotid intima-media thickness was measured at 20 years postbaseline (June 2005 to August 2006) when women were between the ages of 38 and 50 years. High-resolution B-mode ultrasonography was used to acquire a longitudinal image of the common carotid arterial wall thickness, two images of the carotid artery bulb, and two images of the internal carotid artery above the bulb on the right and left sides.13 These images of the common carotid artery were obtained according to a standard protocol using the GE-Logiq-700 with a high-resolution M12L transducer operating at a frequency of 13 MHz. Measurements of the maximal carotid intima-media thickness were made at a central reading center by readers blinded to all clinical information. The maximum intima-media thickness of the common carotid (mm) was defined as the mean of the carotid intima-media thickness of the near and far walls on both the left and right sides with one to four measurements available for the common carotid and one to eight for the carotid artery bulb and internal carotid artery. The common carotid intima-media thickness measure was analyzed as a continuous measure. This measure has shown the strongest correlation with cardiovascular disease risk factors in Coronary Artery Risk Development in Young Adults.13 Any atherosclerotic plaque (measured in year 20) was included as part of the intima media and a note was made about the extent of stenosis that existed anywhere in the right or left carotid artery.13
At each examination, participants reported whether they were currently pregnant or lactating, number of pregnancies and births since their last examination, and how they ended (abortion, miscarriage, and live births or stillbirths), dates of delivery(ies), pregnancy complications (ie, hypertensive disorders of pregnancy [hypertension during gestation with or without proteinuria], GDM, preterm birth at less than 37 weeks of gestation), perinatal outcomes including gestational age, neonatal birth weight, and cesarean delivery. Parity is defined as number of births beyond 20 weeks of gestation. Time (years) from baseline to the first pregnancy and from the last birth to the year 20 examination were calculated from dates of delivery. Validation of self-report of GDM based on prenatal medical record abstraction had a sensitivity for self-report of 100% and specificity of 92%.14 Self-report of preterm births had sensitivity and specificity of 84% and 89%, respectively. Hypertensive disorders of pregnancy were overreported by women: low sensitivity (40%) with high specificity (90%).
Women reported lactation duration for each birth at examination years 7, 10, 15, and 20 based on the following categories: none, less than 6 weeks, 6–11 week, 3–6 months, or greater than 6 months. To calculate total lactation duration across all postbaseline births, we assigned the midpoint for each lactation category: 21 days for less than 6 weeks, 66 days for 6–11 weeks, 135 days for 3–6 months, and 210 days as the upper limit for greater than 6 months. We summed the number of days of lactation across all births to estimate the overall duration of lactation for each woman. The overall duration (months) was next divided into four categories (n, women) to evenly distribute the analytic sample as well as represent clinically relevant periods of lactation: 0 to less than 1 month (n=262), 1 to less than 6 months (n=210), 6 to less than 10 months (n=169), and 10 months or greater (n=205).
Sociodemographics and behavioral data (alcohol intake [mL per day], cigarette smoking [pack-years], education, marital status, oral contraceptive use, physical activity score) were collected at each examination using self- and interviewer-administered questionnaires. The Coronary Artery Risk Development in Young Adults Physical Activity History15 provided physical activity scores that correlate positively with symptom-limited graded treadmill exercise test duration. Women also reported menopausal status, medication use, and medical history (hypertension, heart disease, diabetes, and medications [diabetes, lipid-lowering, hormone replacement, or hormonal contraceptives]). Family history of diabetes and heart disease for one or more first-degree relatives (father, mother or siblings) was reported at examinations in years 0, 5, 10, and 20.
Differences in characteristics at baseline and follow-up among lactation categories were assessed using χ2 statistics for categorical variables (clinic site, race, education, perinatal outcomes, medication use, medical history) and by comparison of means for continuous variables using F-tests (fasting plasma lipids and glucose, age, BMI, homeostatic model assessment of insulin resistance, systolic and diastolic BPs). Median and interquartile ranges were reported for alcohol intake, physical activity, age at first birth, and time since last birth to account for skewing in the data. All P values are for two-sided tests with statistical significance at <.05. Trend P values were obtained by ordering lactation categories from shortest to longest duration.
Linear regression models evaluated unadjusted and adjusted mean (95% confidence interval [CI]) maximum common carotid intima-media thickness among lactation categories using procedures from SAS for Windows 9.1.3. Evaluation of potential confounders was based on a priori hypotheses for prepregnancy measures (BMI, HDL-C, BP, homeostatic model assessment of insulin resistance), parity, education, age, number of postbaseline births, race, smoking, and time since last birth. Covariates were not included if they were not associated with common carotid intima-media thickness independent of the other model covariates (statistical significance level P>.05). Adjusted models were devised by stepwise addition of prepregnancy risk factors and then addition of other covariates. We evaluated change in weight (BMI at year 20) and BP as potential mediators (ie, on the causal pathway) in the association. Effect modification of the lactation duration and common carotid intima-media thickness association by race, number of births, and time since last birth were evaluated by introduction of crossproduct terms for additive interaction (significance P<.10). None of the interaction terms reached statistical significance.
The sample of 846 women in the Coronary Artery Risk Development in Young Adults study (46% black) had a mean age of 24 years (range 18–30 years); 72% were nulliparous at baseline (1985–1986) and gave birth to 1,535 children during the 20-year follow-up period. Crude mean (95% CI) for maximum common carotid intima-media thickness (mm) was thicker for black than white women: 0.801 (0.791–0.812) and 0.729 (0.719–0.738), respectively (P<.001). Shorter lactation duration was associated with black race, nulliparity, younger age, higher prepregnancy BMI, and homeostatic model assessment of insulin resistance and lower prepregnancy plasma HDL-C and physical activity score (Table 1; all P<.05). By the 20-year follow-up examination (Table 2), lactation duration was positively associated with plasma HDL-C and inversely associated with BMI, diastolic and systolic BP, fasting serum glucose, homeostatic model assessment of insulin resistance, incident type 2 diabetes mellitus, hypertension, and the metabolic syndrome. Lactation duration was also positively associated with attained education and physical activity levels. The presence of atherosclerotic plaques at year 20 (Fig. 2) was associated with shorter lactation duration in crude and covariate adjusted analyses (prepregnancy BMI, HDL-C and systolic BP, age, smoking, parity; P trend=.050).
In multivariable linear regression models (Table 3), lactation duration displayed a graded inverse association with mean maximum common carotid intima-media thickness (mm); group differences compared with referent (0–1 month) in unadjusted models ranged from −0.034 to −0.062 (P trend<.001). Adjustment for covariates that met the study criteria as confounders (age, race, baseline parity, number of postbaseline births) resulted in attenuation of group differences to −0.021 to −0.033 (P trend=.002). Addition of other risk factors (prepregnancy systolic BP, BMI, HDL-C and homeostatic model assessment of insulin resistance, education, smoking) resulted in minimal attenuation of these estimates by 10%, although the graded association became less pronounced (P trend=.010).
Adjustment for pregnancy complications, history of hypertension outside of pregnancy, medical conditions, time interval since last birth, oral contraceptive use, and fasting blood lipids had minimal effect on model estimates. However, stepwise addition of potential mediators (BMI and systolic BP at year 20) of the lactation association with common carotid intima-media thickness resulted in modest attenuation of the group differences that remained statistically significant or was near statistical significance independent of attained BMI and systolic BP during the 20-year follow-up (P trend=.019 and .050, respectively).
Our findings from this longitudinal study support the hypothesis that greater lactation duration has persistent effects that reduce the risk of early subclinical atherosclerosis in women during midlife. Most importantly, the graded inverse association between lactation duration and early atherosclerosis remained after adjustment for prepregnancy cardiometabolic risk factors (ie, systolic BP, BMI, HDL-C, homeostatic model assessment of insulin resistance), and smoking habit, providing evidence against reverse causation. We also found that changes in risk factors mediated the lactation association with subclinical atherosclerosis, including elevations in BP and weight gain from baseline to the year 20 examination (average 12 years from last delivery). These robust findings provide insight into the pathways through which lactation may affect the maternal vasculature and influence cardiovascular disease risk.
Our findings contrast with null findings from two cross-sectional studies of 297 peri- or postmenopausal women aged 45–58 years7 and 607 premenopausal women within 4–12 years postdelivery.8 These studies categorized lifetime lactation history across all births (ie, breastfeeding all children for at least 3 months, some, or never), but did not assess overall duration. A consistent pattern of breastfeeding across all births was associated with lower risk of coronary artery calcification and larger lumen and adventitial mean diameters of the vasculature compared with never breastfeeding, but no difference in common carotid intima-media thickness.7,8 The absence of longitudinal biochemical data collection during the perinatal period may contribute to unmeasured confounding from preexisting maternal risk profiles that resulted in null findings.
Our study is consistent with previous reports of lower risk of self-reported hypertension and cardiovascular disease related to longer lactation, including 23% reduction in risk of myocardial infarction.3 Previous studies had limited power to examine graded associations with common carotid intima-media thickness, lacked perinatal risk factor measurements, self-reported cardiovascular disease events, and had low rates of extended breastfeeding (ie, 70% reported duration less than 6 months) that may limit their relevance to contemporary cohorts.
Physiological effects of lactogenesis on maternal cardiovascular function include the release of the neuropeptide, oxytocin, associated with decreased maternal BP and stress responses.16,17 In humans, oxytocin has both antistress and BP-lowering effects in some but not all studies.18,19 One cross-sectional study reported higher maternal BP but lower heart rate during breastfeeding compared with bottle-feeding.20 Our findings that higher BMI and systolic BP at year 20 mediated the association provide evidence that lactation may reduce atherosclerosis through favorable effects on adiposity and blood vessels.
Previous epidemiologic studies21–24 report mixed findings with regard to the longer-term effects of breastfeeding on BP levels or hypertensive disorders in women. These findings include inverse associations with average systolic and diastolic BP22 or no association.23 Two studies of hypertension outcomes21,24 reported inverse associations with lifetime lactation. However, these studies obtained a single measurement of BP many years postdelivery and did not assess gestational hypertensive disorders or perinatal risk status.25 These studies did not assess cardiometabolic risk factors before, during or soon after pregnancy.
Strengths of our analysis include the prospective design with repeated measurements of cardiometabolic risk factors before pregnancy and recall of pregnancy complications and lactation duration within 3 months to 4 years postpartum (maximum of 6 years) as well as measurements of prepregnancy cardiometabolic risk factors (BP, obesity, and metabolic status) that may delay lactogenesis9 to minimize reverse causation. We controlled for confounding from social and cultural determinants of breastfeeding by adjustment for age, race, education, and smoking that cluster with healthful lifestyle. History of GDM and preterm birth were very accurately reported by women in the Coronary Artery Risk Development in Young Adults study.
Limitations of our study include the variable time intervals for measurements of BP and other risk factors in relation to pregnancies, missing common carotid intima-media thickness measurements for 8% of women who attended the year 20 examination, and the fact that hypertensive disorders of pregnancy were overreported by women as noted in other epidemiologic studies.26 However, time interval before the first birth or after the last birth did not confound or modify the lactation and common carotid intima-media thickness association. Although this study measured common carotid intima-media thickness only once at 20 years postbaseline, the young age at baseline (mean 24 years), control for prepregnancy cardiometabolic risk factors, and lifestyle behaviors minimized confounding resulting from preexisting metabolic risk factors. Lactation duration per birth was limited to a maximum of 6 months as a result of the method of data collection.
Pregnancy imposes greater demands on the cardiovascular system, including persistent effects on cardiovascular disease risk factors and vascular remodeling in women. Lactation may be crucial to return of maternal physiologic and metabolic systems to the prepregnancy state. Lactating women exhibit less atherogenic blood lipid profiles, greater insulin sensitivity, and less inflammation compared with nonlactating women.5,27,28 Longer-term effects of lactation that persist postweaning have been reported for various cardiometabolic risk factors.5,28 For example, we previously reported 6 mg/dL higher HDL-C levels with 3 months or more of lactation28 and lower incidence of the metabolic syndrome among women in the Coronary Artery Risk Development in Young Adults study with and without a history of GDM independent of prepregnancy metabolic syndrome components and other risk factors.5 Given the strong inverse association between plasma HCL-C and cardiovascular disease risk,29 the biochemical evidence supports lactation's role in preventing atherosclerosis and carotid artery plaque formation.
Suboptimal lactation may adversely affect maternal health and increases health care costs.30 U.S. breastfeeding rates have increased dramatically during the past 50 years, but are still well below current recommendations that breastfeeding should continue for at least 1 year. Currently, 79% of U.S. women initiate lactation, but by 6 months, the rate drops to 49% and by 1 year to 26%.31 Thus, improving lactation duration has great potential for a positive effect on women's health and health care cost savings. The Coronary Artery Risk Development in Young Adults study provides strong evidence that lactation reduces future subclinical atherosclerosis by accounting for biochemical and clinical risk factors that preceded pregnancy and lifestyle behaviors. Higher common carotid intima-media thickness in our study corresponds to 3–5 years of vascular aging32 for suboptimal lactation.
If the persistent beneficial effects of lactation are substantiated, breastfeeding may not only be seen as an important behavior to preserve child health, but may represent a unique opportunity for prevention of cardiovascular diseases in women.
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