Women who breast-fed for at least 6 months across all births reduced their risk for type 2 diabetes throughout their childbearing years by almost half compared with those who did not breast-feed, a 30-year study has found.
"We found a very strong association between breastfeeding duration and lower risk of developing diabetes, even after accounting for all possible confounding risk factors," lead author Erica P. Gunderson, PhD, senior research scientist at the Kaiser Permanente Division of Research in Oakland, California, said in a news release.
"The incidence of diabetes decreased in a graded manner as breastfeeding duration increased, regardless of race, gestational diabetes, lifestyle behaviors, body size, and other metabolic risk factors measured before pregnancy, implying the possibility that the underlying mechanism may be biological," she explained.
Dr Gunderson and colleagues published their findings online January 16 in JAMA Internal Medicine.
The researchers analyzed data from the 30-year Coronary Artery Disease Risk Development in Young Adults (CARDIA) study, a study of black and white women treated at one of four geographically diverse centers in the United States. The analysis included 1238 women (n = 615 black and 623 white) without diabetes prior to pregnancy who delivered at least one live singleton birth (n = 2302) after baseline.
During a mean follow-up of 24.7 years, participants underwent systematic testing that included "multiple assessments of glucose tolerance and other risk factors" up to seven times during the study, the authors write. A total of 155 (12.5%) developed gestational diabetes (GD); this group was analyzed separately for subsequent development of type 2 diabetes.
Among the women who did not develop GD, the adjusted relative hazard (aRH) for incident diabetes was 0.75 (95% confidence interval [CI], 0.51 - 1.09) for women who breast-fed for 0 to 6 months, 0.52 (95% CI, 0.31 - 0.87) for those who breast-fed for more than 6 months to fewer than 12 months, and 0.53 (95% CI, 0.29 - 0.98) for women who breast-fed for 12 months or more compared with women who did not breast-feed (P for trend = .01).
"For the full sample of 1238 women, models with continuous months of lactation yielded consistent lower RHs of incident diabetes in unadjusted and adjusted race-stratified models and combined models," the researchers write.
During 27,598 person-years, the overall incidence rate of diabetes was 6.6 cases per 1000 person-years, with 132 cases in 13,369 person-years among black women and 50 cases in 14,229 person-years among white women. The incidence of diabetes per 1000 person-years was significantly higher among black women compared with white women (9.9% vs 3.5; P < .001).
Women with GD also had a higher incidence of diabetes per 1000 person-years compared with women who did not have GD (18.0 vs 5.1; P < .001).
Most (93%) of the postbaseline births occurred within 15 years after baseline, and 86% of incident diabetes cases were diagnosed from 15 to 30 years postbaseline.
"We have known for a long time that breastfeeding has many benefits both for mothers and babies; however, previous evidence showed only weak effects on chronic disease in women," Tracy Flanagan, MD, director of women's health for Kaiser Permanente Northern California, said in the news release.
"Now we see much stronger protection from this new study showing that mothers who breastfeed for months after their delivery may be reducing their risk of developing type 2 diabetes by up to one half as they get older. This is yet another reason that doctors, nurses, and hospitals as well as policymakers should support women and their families to breastfeed as long as possible."
The researchers say several mechanisms could explain their findings, including the effects of lactation-associated hormones on pancreatic cells, which could result in decreased circulating glucose and reduced insulin secretion, even though glucose production increases during lactation.
"These processes for milk production have been associated with lower basal and glucose-stimulated β-cell secretory activity for a standardized glucose load and beneficial effects that unload the pancreatic β-cells. Higher basal and sporadic increased prolactin levels in lactating women may preserve pancreatic β-cell mass and function" they explain.
Limitations of the study include the variable time of health assessments relative to pregnancies and the fact that women self-reported pregnancy complications. The researchers note that accurate reporting of GD and other perinatal outcomes is a strength of the study.
"Unlike previous studies of breastfeeding, which relied on self-reporting of diabetes onset and began to follow older women later in life, we were able to follow women specifically during the childbearing period and screen them regularly for diabetes before and after pregnancies," Dr Gunderson said in the news release.
"Lactation is a natural biological process with the enormous potential to provide long-term benefits to maternal health, but has been underappreciated as a potential key strategy for early primary prevention of metabolic diseases in women across the childbearing years and beyond," the researchers conclude.
Dr Gunderson reports receiving funding from Janssen Pharmaceuticals and Dr Quesenberry reports receiving research funding from Takeda, Merck & Company Inc, Sanofi-Aventis, Eli Lilly, Genentech, Valeant, and Pfizer. The remaining study authors have disclosed no relevant financial relationships.
JAMA Int Med. Published online January 16, 2018. Full text
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