Among women with and without GDM pregnancies, a longer cumulative duration of lactation was strongly protective, even after controlling for parity and baseline covariates, including components of the metabolic syndrome before pregnancy. For women with non-GDM pregnancies, there was a threshold effect with lactation >1 month conferring protection compared with 0–1 month. Among women with GDM pregnancies, we found a strong, graded inverse association of lactation with incidence of the metabolic syndrome, those with longest lactation approaching the non-GDM incidence rate of the metabolic syndrome. The associations remained after controlling for mediators such as changes in physical activity or weight gain during follow-up, with a stronger protective association among the GDM group.
Associations were similar within race and BMI groups, especially for women with a history of GDM. However, black women had much shorter duration of lactation that limited our ability to fully assess the association of extended duration of lactation with disease risk. Overweight status was closely related to black race, and our sample size was too small to fully assess the separate effects. We did not observe significant interactions by number of singleton pregnancies because most women (84%) delivered only one or two pregnancies.
Epidemiologic studies examining lactation history and prevalence of the metabolic syndrome or cardiovascular disease (CVD) risk have not measured preconception risk factor levels or stratified by GDM status. Cross-sectional studies of primarily perimenopausal and postmenopausal women reported that lactation >1 month (9
) or any lactation (10
) was associated with 21–22% lower prevalence of the metabolic syndrome, and that lactation >12 months was associated with 9–20% lower prevalence of CVD risk factors, but not incidence of CVD (11
). In studies of lactation and type 2 diabetes after GDM and non-GDM pregnancies, glucose levels before pregnancy were not measured (6
). Longer lifetime lactation ≥4 months was associated with a 25% lower incidence of type 2 diabetes among white women, but not those with a history of GDM (13
). In Latinas with previous GDM, findings on lactation and future diabetes risk were inconclusive, and duration was not assessed (26
Our findings for women of reproductive age show a much stronger protective association for >1 month of lactation: lower incidence of the metabolic syndrome by 39–56% for non-GDM and by 44–86% for GDM groups. Because our sample included only nulliparas at baseline and preconception measurements of all metabolic syndrome components, we minimized confounding by preexisting conditions before pregnancy and lactation.
Our study's unique strengths include prospective collection of “preconception” measurements of the metabolic syndrome components to confirm that women were free of the metabolic syndrome before pregnancies, and stratification by GDM status. Metabolic syndrome components were measured at 3- to 7-year intervals before and after pregnancies over a 20-year period, thereby maintaining the temporality of the exposure (pregnancy and lactation duration) to new onset of metabolic syndrome. We also modeled lactation as a time-dependent main effect and controlled for multiple potential confounders including age, time-dependent parity, secular trends, sociodemographics, and behavioral attributes. The validity of our findings is enhanced by the population-based sample, high cohort retention rate over 20 years of follow-up, and measurement of all five metabolic syndrome components at baseline for 100% of the sample and for three or more follow-up visits for 72% of the sample. We also examined associations separately for black and white race groups, and found consistent associations.
Limitations include no data on lactation intensity, ascertainment of GDM by self-report, variable time intervals to conception and from delivery relative to CARDIA exams, and few higher order births. Our GDM validation study showed high sensitivity and specificity of GDM by self-report, and nondifferential misclassification would bias our findings toward the null. Women who lactated may have had healthier lifestyles than women who did not lactate, but we accounted for these traits as well as weight gain. Despite control for various behavioral and other potential confounders, residual confounding is always possible in observational studies.
One proposed mechanism through which lactation may influence cardiometabolic health is through greater weight loss. Although milk production increases maternal total energy expenditure by 15–25% (27
), evidence for greater postpartum weight loss is equivocal (29
). Prospective studies that measured maternal weights (not self-reported) before or during early pregnancy have generally reported lower postpartum weight retention at 1 year postpartum (30
), more rapid loss approaching pregravid weight (31
), or 1–2 kg greater weight losses within 3–6 months postpartum among lactating women (32
). Yet, weight change did not explain the protective association between lactation and the metabolic syndrome in our study.
Another possibility is that lactation affects body composition and regional fat distribution. Well-nourished lactating women lose about 2 kg in total fat mass by 6 months postpartum based on magnetic resonance imaging (33
) or mass spectrometry (34
) and tend to mobilize more fat from the thigh than the trunk (33
). In lactating American women, skinfold thickness was reduced in the suprailiac and subscapular regions, but increased in the triceps region (35
). Yet, studies that compared lactating versus nonlactating women using dual-energy X-ray absorptiometry found greater declines in total body fat mass within 3–6 months postpartum (36
) but no differences in mobilization of fat from leg, arm, and trunk regions (36
). In 26 women with previous GDM, visceral fat mass via computed tomography at 3 months postpartum did not differ by lactation status (38
). Longitudinal studies are needed to examine postpartum visceral fat changes in larger samples.
Acutely, lactogenesis has favorable effects on maternal cardiometabolic blood profiles, but few studies have adjusted for BMI, or measured postweaning levels. Lactating women had lower plasma TGs (39
) and higher plasma HDL cholesterol levels and HDL cholesterol/total cholesterol ratios at 3–6 months postpartum unadjusted for BMI (4
). They also had elevations in respiratory quotient and carbohydrate utilization consistent with preferential use of glucose (28
). Moreover, lactating versus nonlactating women exhibited greater insulin sensitivity among GDM and non-GDM groups (5
). In postweaning studies, plasma HDL cholesterol levels were higher with longer duration of lactation after accounting for preconception levels and weight gain (8
), but lactation was not associated with plasma total cholesterol or other lipids (8
In summary, longer duration of lactation was associated with lower incidence of the metabolic syndrome years after delivery and after weaning among women with non-GDM as well as GDM pregnancies. Lifestyle behaviors did not explain these associations. Lactation may ameliorate the increased risk of the metabolic syndrome associated with higher parity (30% per birth in non-GDM, 150% for GDM) (3
). By contrast, other studies have not demonstrated clear benefits of lactation on future health of women with a history of GDM. Our data provide strong evidence that lactation may have lasting favorable effects on metabolic risk profiles among women with a history of GDM who are most susceptible to developing metabolic diseases, as well as women without GDM. Further investigation is needed to elucidate the mechanisms through which lactation may influence women's cardiometabolic risk profiles, and whether lifestyle modifications, including lactation duration, may affect development of coronary heart disease and type 2 diabetes, particularly, among high-risk groups such as women with a history of GDM.