Gut Check is a periodic look at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?
Breastfeeding may reduce a mother’s chances of developing heart disease or suffering a stroke by about 10 percent, says a study published on Wednesday in the Journal of the American Heart Association.
Tell me more:
Lots of research has shown that being breastfed is good for a baby’s immune system. As for mom, lactation can postpone when she resumes ovulating and reduce her risk of ovarian cancer and breast cancer. But the effect on her cardiovascular health — including heart attacks and strokes — is less clear.
The latest study, based on 289,573 women in China (average age 51), finds that those who said they had breastfed had a 9 percent lower risk of heart disease and an 8 percent lower risk of stroke over the eight years they were followed compared to mothers who said they had never breastfed.
That conclusion jibes with some earlier studies but not others; previous research has reached conflicting conclusions. A famous U.S. study found a statistically significant reduction in coronary heart disease only in women who breastfed for at least two years (total for all their children); for women who breastfed for shorter times, the risk was also mostly lower but more likely due to chance. The China study finds no such threshold effect.
In contrast, a Norwegian study found that mothers who breastfed for seven to 12 months had half the risk of dying of cardiovascular disease compared to women who breastfed for two years or more — that is, more was worse. But the China study finds that more is better: Each additional six months of breastfeeding was associated with 4 percent less risk of heart disease and 3 percent less risk of stroke, so that mothers who breastfed for two years or more had an 18 percent lower chance of heart disease risk and a 17 percent lower chance of stroke compared to those who never breastfed.
One drawback of the China study is that it relied on women’s recollections of whether and how long they breastfed, sometimes decades earlier. Memories are fallible, and sometimes people give the answers they think scientists want, making this study design less rigorous than, say, real-time health records.
The biggest caveat, however, is that the study was observational. Women weren’t assigned to breastfeed or not; they made their own choices. That always means the reasons for what they did, not their actual behavior, might have caused the outcome.
For instance, women with high BMIs are less likely to start breastfeeding and to stick with it, said Dr. Alison Stuebe, of the University of North Carolina School of Medicine, who was not involved in the China study but has co-authored research reporting a cardiovascular benefit with breastfeeding. Women who are anxious or depressed, or who have not bonded with their newborn, are also less likely to breastfeed. Obesity, anxiety, depression, and low levels of oxytocin (the “love hormone”) all raise the risk of cardiovascular disease. “Healthier women find breastfeeding easier and less healthy women find it harder,” Stuebe said.
Those confounding factors can be so powerful that what seems like a maternal benefit from breastfeeding might not be. For instance, research has suggested it reduces the risk of type 2 diabetes, hypertension, and cardiovascular disease, but Stuebe showed in a 2015 analysis that it’s quite possible that women with such underlying diseases (or even risks for them) are less likely to breastfeed. That is, disease causes non-breastfeeding, rather than non-breastfeeding causing disease.
Because of such potential confounding, the China study could not prove cause and effect, said epidemiologist Sanne Peters of the University of Oxford, the co-first author: “Women who breastfeed may be more likely to engage in other beneficial health behaviors that lower their risk of cardiovascular diseases than women who do not.”
She and her colleagues tried to take account of sociodemographic, lifestyle, and biological differences among women who breastfed for long periods, short periods, or not at all, but the findings might nevertheless be explained by factors they didn’t measure. For instance, they did not collect data on cardiovascular risk factors such as gestational diabetes or BMI before, during, or after pregnancy, she said, though those might affect both a women’s likelihood of breastfeeding “as well as her future risk of cardiovascular diseases,” Peters said.
Such caveats notwithstanding, there is some physiological validity for a cardiovascular benefit from breastfeeding. The resulting release of oxytocin can lower the stress response, Stuebe said, while lactation itself can lower cholesterol, blood pressure, and glucose levels after pregnancy — all good for cardiovascular health — and quickly get rid of stored fat. “If the breast isn’t pouring fat into the baby, it’s staying in the mom,” she said.
With so many reasons why mothers who breastfed had a lower chance of heart disease and stroke than mothers who didn’t, the evidence that breastfeeding itself deserves the credit is weak.