Investigators examined data from four previous studies looking at the connection between lactation and diabetes in a total of about 206,000 women. They also analyzed data from five previous studies on the link between lactation and high blood pressure in a total of about 255,000 women.
Overall, mothers who breastfed for more than 12 months were 30% less likely to develop diabetes and 13% less likely to develop high blood pressure than women who didn't nurse babies that long.
This may be because breastfeeding burns a lot of calories and helps reverse metabolic problems that can develop during pregnancy like higher cholesterol, more fats circulating in the blood, and a diminished ability to process sugars, said Dr. Haitham Ahmed, senior author of the study and chair of cardiology at AdvantageCare Physicians in Brooklyn, New York.
"In many ways it can be a reset to the adverse metabolic profile in pregnancy," Ahmed said by email. "Many women are not able to breastfeed, but for those who are, that may be an excellent way to improve long term cardiovascular and metabolic health of new mothers."
Pediatricians recommend that mothers exclusively breastfeed infants until they're at least six months old and continue to nurse for at least one year as they introduce some foods because it can reduce babies' risk of ear and respiratory infections, sudden infant death syndrome, allergies, obesity and diabetes. For mothers, breastfeeding for at least one year has been linked to a lower risk of depression, obesity, and certain cancers.
In the current study, breastfeeding appeared to have a protective affect against high blood pressure and diabetes even after researchers accounted for other factors that can impact the risk of developing these conditions like obesity, smoking, and family medical history.
One drawback of the analysis is that none of the smaller studies were gold-standard controlled trials, so they couldn't prove that breastfeeding protects against diabetes or high blood pressure.
Researchers also didn't look at other factors that may impact women's health after pregnancy including the total number of births they have, race, age, or pregnancy complications, said Erica Gunderson, a researcher in cardiovascular and metabolic conditions at Kaiser Permanente Northern California. The study focused primarily on older women in populations with high levels of breastfeeding, and results might be different for other populations, Gunderson, who wasn't involved in the study, said by email.
Even so, the results should give women one more reason to breastfeed as long as possible, said Jennifer Yourkavitch of the Center for Women's Health and Wellness at the University of North Carolina at Greensboro.
"It takes energy to make milk -- lactation burns calories," Yourkavitch, who wasn't involved in the study, said by email. "And that could spur post-pregnancy weight loss and prevent excessive weight gain, which is a risk factor for cardiovascular and metabolic diseases."
Cardiovascular disease remains the leading cause of death in women, researchers note in JAMA Network Open.
While the study suggests breastfeeding might help protect against this fate, it's not the only defense women have, said Yukiko Washio, a researcher at RTI International who wasn't involved in the study.
Mothers who can't breastfeed or choose not to breastfeed "can still work on preventing diabetes and hypertension by abstinence from tobacco use and harmful drinking, as well as proper physical activity and nutrition," Washio said by email.