This analysis included 139,681 women with ≥1 live birth and no missing information on lactation (59,769 women who participated in the WHI CT, 79,912 who participated in the WHI OS). Sociodemographic characteristics of the post-menopausal study participants are shown in . The majority of patients (58%) reported some history of lactation, but only 6% had a cumulative history of lactation greater than 24 months. Few women reported 12+ months of lactation per live birth. On average, 35 years had passed since women had lactated when they enrolled in the WHI.
Characteristics of Women enrolling in the WHI Observational Study or Controlled Trial who reported one or more live births (N=139,681)
On enrollment in the WHI, 30% of women were obese (BMI≥30). In age-adjusted analyses, we found parous women were more likely to be obese or hypertensive than nulliparous women. However, parous women who had lactated were less likely to be obese or have hypertension than parous women who had not lactated. In similar, age-adjusted models, we found the prevalence of diabetes and hyperlipidemia was not significantly different among nulliparous or parous women who had lactated. However, parous women who had not lactated were more likely to have diabetes or hyperlipidemia than parous women who had lactated.
shows the relationship between duration of lactation and obesity among parous women. Increasing duration of lactation was associated with a lower prevalence of obesity in the univariable model and in the model adjusted for sociodemographic variables. After additional adjustment for lifestyle and family history variables, however, we did not find a significant relationship between 6 or less, or 24+ months lactation and prevalence of post-menopausal obesity; with 7-23 months of lactation there was a trend towards less obesity (p=0.07).
Association between months of lactation and obesity on enrolling in the Womens Health Initiative Observational Study or Controlled Trial
Increasing duration of lactation was associated with a reduced prevalence of cardiovascular risk factors including hypertension, diabetes, and hyperlipidemia, even after adjustment for sociodemographic variables, lifestyle variables, family history, and BMI category (p<0.01 for all tests for trend, ). Similarly, increasing duration of lactation was associated with a lower prevalence of CVD prior to enrolling in the WHI (p<0.01 for all tests for trend, ). Women who reported a lifetime history of more than 12 months of lactation were less likely to have hypertension (OR=0.88, p<0.001), diabetes (OR= 0.80 , p<0.001), hyperlipidemia (OR=0.81, p<0.001) or cardiovascular disease (OR= 0.91, p=0.008 ) than women who never breastfed. When compared to women who had never breastfed, women who reported a cumulative lifetime duration of lactation of ≥13 months were less likely to have developed CVD prior to enrolling in the WHI (OR=0.91, 95% CI= 0.85 to 0.98, p=0.008). Based on multivariate adjusted prevalence ratios from generalized linear models, we estimate that among parous women who did not breastfeed compared to those who breastfed for more than 12 months, 42.1% vs. 38.6% would have hypertension, 5.3% vs. 4.3% would have diabetes, 14.8% vs. 12.3% would have hyperlipidemia, and 9.9 vs. 9.1% would have developed cardiovascular disease, although 30% of each group would be obese when postmenopausal.
Association between duration of lactation and self-reported history of Hypertension, Diabetes, or Hyperlipidemia on enrolling in the Womens Health Initiative Observational Study and Controlled Trial
Association between Duration of Lactation and Cardiovascular Disease (CVD) among participants in the Womens Health Initiative Observational Study and Controlled Trial
When examining fully-adjusted models of the duration of lactation and prevalence of CVD, we found no significant interactions between age at last lactation (p for interaction=0.58), race/ethnicity (p for interaction=0.35), or history of tobacco use (p for interaction=0.65). However, there was a significant interaction with respect to age (p for interaction= 0.02). In fully-adjusted analyses stratified by age, we found the cardiovascular benefits of lactation decreased as women aged; women who were 50-59 years of age on enrolling in the WHI who reported a cumulative history of lactation of ≥7 months were significantly less likely to have CVD than similarly-aged women who had never breastfed (OR (95% CI)=0.84 (0.71to 0.99), 0.80 (0.65 to 0.97), and 0.75 (0.58 to 0.96) for women who had breastfed for 7-12 months, 13-23 months, and 24+ months, respectively). Among women aged 60-69, only those who reported 13-23 months of lactation were significantly less likely than women who had never breastfed to have developed CVD, OR=0.85 (0.75 to 0.96), and among women aged 70-79 there were no significant relationships between duration of lactation and prevalent CVD.
The majority of participants in the WHI OS (58%) reported they had been breastfed as an infant; when we added this variable to analyses in which we adjusted for weight at age 18 and/or weight at birth with and in place of BMI category on enrollment in addition to sociodemographic, family history, and other lifestyle variables, we saw similar relationships between duration of lactation and risk of hypertension, diabetes, hyperlipidemia (data available on request). However, the association with CVD was somewhat attenuated and not statistically significant in the smaller population of OS participants.
When we used Cox models to look at incident CVD over the 7.9 years WHI participants had been followed, we found duration of lactation was associated with a decrease in incident CVD in univariable but not adjusted models (). However, again, there was an interaction with age (p for interaction= 0.04, without adjusting for BMI category; p for interaction=0.06, with adjustment for BMI category); when we stratified the adjusted model by age, we found more cardiovascular benefits of lactation among younger women. Among women who were 50-59 on enrollment, when compared to women who never lactated, women with a lifetime duration of 7-12 months of lactation were less likely to develop CVD (HR=0.79, 95% CI 0.66 to 0.94, without adjusting for BMI category; HR=0.80, 95% CI 0.67 to 0.95, with adjustment for BMI category) as were women with 24+ months of lifetime lactation (HR=0.66, 95% CI 0.50 to 0.86 without adjusting for BMI category; HR=0.68, 95% CI 0.52 to 0.89, with adjustment for BMI category), p for trend among women aged 50-59 on enrollment=0.001. Among women who were older than 60 when they enrolled in the WHI, duration of lactation was not associated with incident CVD.
When we stratified the Cox models by parity (p for interaction=0.06 without adjusting for BMI category, p=0.07 with adjustment for BMI category), we found that when compared to women who never lactated, women with one live birth who breastfed for 7-12 months were significantly less likely to develop CVD (HR 0.72 (0.53 to 0.97), while among women with 2 or 3 live births, lactation was only associated with a significant reduction in incident CVD if the woman reported 24+ months of lactation (for 2 live births, HR=0.58 (0.35 to 0.95), for 3 live births, HR=0.78 (0.63 to 0.98)).
In analyses of the subgroup of women who had not developed CVD prior to enrolling in the WHI, duration of lactation was not associated with incident CVD.