Infant Feeding in America: Enough to Break a Mother's Heart?

Raising Awareness to Prevent Heart Disease

Cardiovascular disease (CVD) is a leading cause of death of women around the world. The American Heart Association's “Red Dress” campaign has sought to raise awareness of women's risk of heart disease, ¹ but, unfortunately, a red dress alone does little to protect a woman from heart disease. Although many women are now aware that heart disease affects many in their communities, far fewer are clear on what they can do to protect themselves from heart disease. Diet, exercise, smoking cessation, and blood pressure control are all recognized as key elements of preventing cardiovascular disease. Infant feeding has received less attention, but a growing body of literature indicates that lactation plays an important role in determining women's future risk of both hypertension and heart disease.

Infant Feeding and Postpartum Weight Retention

Various studies have indicated that mothers who breastfed might lose more weight than those who did not.^2–4 Recently, the Million Women Study reported a relationship between lactation and mothers' obesity in later life. ⁵ At all levels of parity, that is, no matter how many times a woman gave birth, mothers who never breastfed were the most obese, whereas mothers who breastfed as recommended, for more than 6 months after the birth of each child, were the thinnest; the weight of mothers who breastfed for less than 6 months per child was in between those who breastfed for more than 6 months and those who didn't breastfeed at all. It is interesting that, as long as mothers breastfed as recommended, they were (on average) no more obese than women who had never been pregnant, unless they had four or more children. Of note, these data are adjusted for the woman's age, region of residence, socioeconomic status, tobacco use, and physical activity. Similarly, a Finnish study recently found maternal weight to vary by duration of lactation. ⁶ This study is an important one because it had data on women's age, obesity, and blood pressure prior to their first pregnancy. Again, although there were not significant differences in any of these variables between groups before pregnancy, after their first birth, differences were seen between mothers who breastfed for less than 6 months and those who breastfed for more than 6 months. It is notable that these differences persisted as women had their second, third, and even fourth pregnancy.

Although we often think of obesity primarily in terms of how much someone weighs, or his or her body mass index (BMI), it is worth noting that not all fats are created equal. Visceral fat, or belly fat, is the most metabolically active fat, and it has been shown to accumulate during pregnancy. ⁷ Thus, breastfeeding may have important effects on women's visceral adiposity in later life. Two studies have now examined the relationship between lactation and visceral adiposity.^8,9 In both of these studies, computerized tomography scans were used to assess women's abdominal adiposity. The first used data collected from 363 women as part of the SWAN Heart study; these women were free of clinical CVD at the time they enrolled, between 2001 and 2003. ⁸ The second study used data from 89 mothers who were assessed 4–12 years after they gave birth to a single infant at a hospital in Pittsburgh, PA. ⁹ In both of these studies, premenopausal mothers who had not lactated had the most visceral adiposity. In contrast, mothers who had lactated for 3 or more months after each birth had no more visceral fat than women who had never been pregnant. More explicitly, mothers who did not breastfeed had waist circumferences that were on average 6.5 cm larger than mothers who breastfed for 3 or more months after every birth. Similarly, they had waist-to-hip ratios that were on average almost 5% larger. On computerized tomography, mothers who had not breastfed had 28% more visceral adiposity and 20% greater visceral to total abdominal fat ratios. Compared with mothers who breastfed for 3 or more months after every birth, those who never breastfed had on average 37 cm² more visceral fat at the level of their umbilicus. ⁹ These studies adjusted for the mother's diet, activity, socioeconomic status, and current BMI. Even after controlling for known risk factors for CVD (diet, activity, and BMI), these mothers' body composition was significantly different years later based on how they had fed their infants.

Infant Feeding Affects Maternal Blood Pressure in Later Life

Lactation also appears to affect women's risk of hypertension in later life (that is, mothers who do not breastfeed are more likely to develop hypertension^6,10–15a). This benefit appears to persist into menopause, to be independent of BMI, and to reduce risk of hypertension by about 10%. Mothers who breastfeed for as little as 1–3 months after giving birth significantly reduce their risk of developing hypertension, although the longer a mother breastfeeds, the better. The first study published on this topic involved over 100,000 Korean mothers who were on average 32 years of age on enrollment. ¹² This study excluded women with preexisting hypertension. Of note is that 85% of these study participants had a BMI of <23 kg/m², which makes this population quite a bit thinner than the typical U.S. population. It is surprising that only about half of these mothers reported ever breastfeeding; of those who breastfed, 81% reported less than 1 year of lifetime lactation. In this study, normal weight mothers who had ever lactated were the least likely to have developed hypertension (that is, a blood pressure >140/90 mm Hg or need for medications), whereas heavier mothers were more likely to develop hypertension, particularly if they never breastfed. Of note is that this was true even in models adjusted for the woman's age, parity, smoking, alcohol intake, exercise, and age at first pregnancy. When these investigators looked at similarly adjusted models examining the effect of mean lactation per birth on risk of hypertension, they found that those who had breastfed for as little as 1–3 months or as much as an average of 9 months per birth had a reduced risk of developing hypertension. However, mothers who breastfed more than an average of 9 months per birth were not seen to have less risk of developing hypertension.

The next study to examine the relationship between lactation and blood pressure had the benefit of being able to additionally adjust for the mothers' daily caloric intake. When Ram et al. ¹¹ studied 2,516 women, they compared mothers who ever versus never breastfed and found significant differences in both abdominal obesity and hypertension, with mothers who had not breastfed being more likely to have developed hypertension. When they examined the effect of each year of lactation on the mother's risk of developing hypertension, they found a 10% reduction in the adjusted odds of a mother's developing hypertension. Similarly, data from the Women's Health Initiative (WHI) found that mothers who breastfed for only 1–6 months had significantly lower odds of developing hypertension. ¹⁰ It is important that this WHI study was able to adjust not only for women's caloric intake, but also for sodium intake, which has been linked to hypertension. In addition, this WHI study adjusted for a large number of sociodemographic factors, other lifestyle variables, and whether or not the mother was obese. Even after adjusting for all of these factors, the longer a woman had breastfed over the course of her lifetime, the less likely she was to have developed hypertension before enrolling in the WHI. This trend was highly statistically significant, with mothers who breastfed for as little as one to 6 months showing about a 5% reduction in their odds of developing hypertension, whereas mothers who had breastfed for more than 7 months had a 11–13% reduction in their odds of developing hypertension. ¹⁰

However, one of the problems with the first three studies to examine the relationship between lactation and maternal hypertension was that they all used mothers who never breastfed as their referent group. One hypothesis is that the body prepares for lactation during pregnancy and that lactation serves, among other purposes, as a way of “resetting” maternal physiology after pregnancy. ¹⁶ Thus, one of the more recent studies to examine this question ¹⁵ modeled the hazard a woman faces if she breastfeeds her first child for less than the 1 year recommended by the American Academy of Pediatrics, ¹⁷ rather than looking for the benefit lactation might offer in terms of preventing hypertension. This study, which used data from over 55,000 U.S. nurses who participated in the 14-year Nurses Health Study (NHS) 2 (NHS2), examined the effect of the way a mother fed her first child (as this likely has a large effect on both whether she has more children and how she feeds her subsequent children). In addition, this study used propensity scores to try to adjust for how likely a mother was to breastfeed, given her sociodemographic characteristics, her BMI at 18 years of age, and whether or not she experienced pregnancy complications. In models adjusted for multiple covariates that affect risk of hypertension, including dietary sodium intake and alcohol use, ¹⁵ this study found that the earlier a mother ceased lactation after her first birth, the greater the hazard that she would develop hypertension, with mothers who breastfed for less than 3 months having hazard ratios similar to those of mothers who never breastfed, ranging from 27% to 29% greater than those for mothers who breastfed as recommended. In models that considered exclusive lactation instead of any lactation, the results were very similar. Compared with mothers who exclusively breastfed as recommended for the first 6 months of their infant's life, mothers who never breastfed had a 29% greater hazard of developing hypertension, in models adjusted for both lifestyle and family history variables. ¹⁵

Lactation and Maternal Risk of Heart Disease

Studies have also shown that mothers who do not breastfeed face a higher risk of diabetes^18–20 and hyperlipidemia,^11,21 both of which are known to increase risk of CVD. Thus, it is not surprising that studies have recently shown that mothers who do not breastfeed are more likely to experience heart attacks and CVD than those who do.^10,22 In the WHI, women with a single live birth who breastfed for 7–12 months were 28% less likely to have developed CVD than women who never breastfed. ¹⁰ In the NHS, compared with parous women who had never breastfed, women who had lactated for 2 or more years had a 23% lower risk of coronary heart disease, after adjusting for age, parity, lifestyle factors, family history, early-adult adiposity, and history of stillbirth. ²² Studies have also looked at more objective measures of vascular disease, using electron beam tomography or ultasonography to examine women's coronary and carotid arteries. These measures of subclinical CVD have also been associated with infant feeding practices.^23,24 Compared with mothers who had breastfed all of their children for at least 3 months, mothers who had not breastfed were over five times as likely to have aortic calcification, in models adjusted for measures of socioeconomic status and lifestyle and family history variables, BMI, and traditional risk factors for CVD. ²³

The Limits of Observational Studies

All of the studies published to date have been observational studies and thus, despite efforts to adjust for important covariates, may be subject to residual confounding. However, an unobserved confounding factor to account for the effects on risk of hypertension that have been reported would have to be quite large. For instance, the NHS2 study estimated that this unobserved confounder would have to have an effect on blood pressure similar in magnitude to vigorous physical activity 7 days a week or to a DASH diet score in the highest versus lowest quintiles. ¹⁵ In analyzing the WHI data, we estimated that if a randomized trial had shown similar effect sizes, that is, if it were possible to package into a pill the effects on a mother's health of 1 or more years of lactation, the number needed to treat to prevent one case of maternal hypertension would be 33. For the same low price, this magic pill would also prevent one of every 40 moms from needing medication for high cholesterol, one of 88 from diabetes, and one of every 126 from a heart attack or stroke. ¹⁰ Given the magnitude of these effects, publication of the PROBIT trial's ²⁵ maternal health data is eagerly awaited.

Potential Mechanisms and Population Effects

We know that lactation affects multiple hormones that affect blood pressure, including oxytocin, ²⁶ prolactin,^27,28 cortisol, ²⁹ estrogen, and progesterone. Recently, we modeled the population-level effects that might be expected if the United States moved from our current, suboptimal rates of lactation to having 90% of mothers able to meet infant feeding recommendations. ³⁰ If the United States succeeded in allowing 90% of mothers to breastfeed for 1 year after every birth, we would prevent almost 54,000 U.S. women from requiring treatment for hypertension and almost 14,000 from having heart attacks each year. ³⁰ Reducing this disease burden might prevent over 4,000 premature maternal deaths before the age of 70 years. Clearly, these types of changes could represent some impressive healthcare savings; it is estimated that suboptimal breastfeeding currently costs the United States $859 million in maternal healthcare costs and over $17.4 billion in societal costs from maternal deaths before the age of 70 years.

Breastfeeding Is Vital to Maternal Health

Although many think of breastfeeding as important to promoting child health and well-being, the maternal health burden of suboptimal breastfeeding appears to be even larger than the pediatric health burden.^30,31 In fact, recent studies indicate that the maternal deaths attributed to suboptimal breastfeeding outnumber pediatric deaths by about 5 to 1.^30,31 If mothers do not get to breastfeed, they face increased risk of heart disease and, potentially, premature death. This means that hospitals that are not “Baby Friendly” are also a risk to women's health. Breastfeeding is not always easy, but neither is dieting, exercise, smoking cessation, or treating hypertension. If we are serious about fighting heart disease, we need to commit to promoting all aspects of a healthy lifestyle, which for women includes breastfeeding their babies.