Among women with a history of gestational diabetes, a longer period of breastfeeding was associated with a lower probability of going on to develop type 2 diabetes, as well as a more favorable glucose metabolic biomarker profile. Women who breastfed for 2 years or longer had a 27% lower risk than that of those who did not breastfeed at all, even after adjustment for age, ethnicity, family history of diabetes, parity, age at first birth, smoking, diet quality, physical activity, and prepregnancy body mass index, according to findings published in Diabetes Care.
It remains to be seen if the association is causal, and if so, what mechanisms might connect breastfeeding duration to risk for type 2 diabetes, wrote study leaders Sylvia Ley, PhD, of Tulane University School of Public Health and Tropical Medicine, New Orleans, and the Harvard T.H. Chan School of Public Health, Boston, and Cuilin Zhang, MD, PhD, of the Division of Intramural Population Health Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Md., and colleagues.
“It’s really nice to see this consistency and the long-term association being borne out in a very large sample of women with gestational diabetes,” Erica P. Gunderson, PhD, said in an interview about the study. Dr. Gunderson has conducted similar studies of her own, including one published in 2018 that showed an independent association between lactation and reduced diabetes risk in women (JAMA Intern Med. 2018;178:328-37). That analysis showed no sign that the presence of gestational diabetes affected the reduction of diabetes risk associated with lactation.
Dr. Gunderson noted that pregnancy is a hyperlipidemic state, with triglyceride levels sometimes doubling, likely in response to the need to support the placenta and the growing fetus. Lactation may help restore lipid levels to the prepregnancy state by redirecting lipids to breast milk. She and others are working to produce more direct evidence of metabolic changes in the postpartum period associated with lactation. “That’s where we don’t have much mechanistic evidence right now,” said Dr. Gunderson, a senior research scientist and epidemiologist at Kaiser Permanente Northern California, Oakland.
Gestational diabetes occurs in an estimated 5%-9% of pregnancies in the United States, and women who experience this complication are at greater risk of developing type 2 diabetes in the future. Findings from other studies have shown that longer lactation periods are associated with lowered risk of future type 2 disease (JAMA. 2005;294:2601-10).
In the latest study, the researchers included 4,372 women with a history of gestational diabetes, identified through the Nurses’ Health Study II. Participants were excluded if they had a history of cancer, cardiovascular disease, or multiple-birth pregnancy before the pregnancy during which they were diagnosed with gestational diabetes. In all, 873 women developed type 2 diabetes over 87,411 person-years of follow-up. The median age at gestational diabetes diagnosis was 31.8 years, and 49.8 years for diagnoses of type 2 diabetes.
After adjustment, the researchers found a steadying decline of risk for type 2 diabetes with increasing length of lactation: for up to 6 months of lactation, the hazard ratio was 1.05 (95% confidence interval, 0.82-1.34); for 6-12 months, the HR was 0.91 (95% CI, 0.71-1.15); 12-24 months, 0.84 (95% CI, 0.67-1.06); more than 24 months, 0.73 (95% CI, 0.57-0.93; P for trend = .004). Age, parity, primipara, prepregnancy body mass index, and age had no statistically significant effect modification on the association.
At a follow-up blood collection taken at median age of 58.2 years and 26.3 years after the gestational-diabetes index pregnancy, the researchers found associations between longer breastfeeding (greater than 24 months vs. 0 months) and lower hemoglobin A1c percentage (5.58 vs. 5.68; P for trend = .04), lower insulin levels (53.1 vs. 64.7 pmol/L; P for trend = .02), and lower C-peptide levels (3.42 vs. 3.88 ng/mL; P for trend = .02).
The study was supported by the National Institutes of Health. Dr. Ley was supported by a National Institute of General Medical Sciences grant from the NIH. None of the study authors reported any conflicts of interest, and neither did Dr. Gunderson.
SOURCE: Ley S et al. Diabetes Care. 2020 Feb 10. doi: 10.2337/dc19-2237.