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Higher intensity and duration of lactation may reduce the incidence of type 2 diabetes in women after gestational diabetes mellitus (GDM), a new study has suggested.

"We saw 36% to 57% relative reductions in the 2-year [diabetes mellitus] incidence with higher intensity of lactation at 6 to 9 weeks postpartum as well as for longer periods (>2 months through >10 months), independent of obesity, gestational glucose tolerance, and perinatal outcomes that can delay lactogenesis and thereby shorten lactation duration," Erica P. Gunderson, PhD, MPH, RD, an epidemiologist and senior research assistant at Kaiser Permanente, Oakland, California, and colleagues write.

The results of the prospective cohort study were published online November 23 in the Annals of Internal Medicine.

Growing evidence indicates that mothers who breast-feed have improved glucose and lipid metabolism, and that the favorable metabolic effects persist postweaning. However, evidence is limited and conflicting about whether lactation reduces future risk for type 2 diabetes among women with GDM. In addition, previous studies did not account for perinatal outcomes, subsequent pregnancies, or postpartum lifestyle behaviors.

Dr Gunderson and colleagues therefore conducted the Study of Women, Infant Feeding and Type 2 Diabetes After GDM Pregnancy (SWIFT) to evaluate the effect of exposure to prolonged, intensive lactation compared with that of formula feeding on the 2-year incidence of type 2 diabetes among women with a recent GDM pregnancy.

The study enrolled 1010 women with GDM who delivered singletons at 35 weeks' gestation or later. It excluded women who reported mixed feeding (formula supplementation of 7 - 13 oz per 24 hours) or inconsistent feeding (transition to ≥14 oz of formula per 24 hours after 3 weeks of lactation).

Enrollees underwent three in-person research examinations: at baseline (6 - 9 weeks after delivery) and then annually for 2 years of follow-up. During examinations, the researchers classified participants' infant feeding behaviors (including lactation intensity and duration) and performed oral glucose tolerance tests on the women. The primary study outcome was the development of incident type 2 diabetes during follow-up.

Overall, 959 (95%) of the women without diabetes at baseline were evaluated at least once during the 2-year follow-up, and 113 (11.8%) developed new-onset type 2 diabetes. The overall incidence rate of type 2 diabetes was 5.64 cases per 1000 person-months (95% confidence interval [CI], 4.60 - 6.68) and ranged from 3.95 cases per 1000 person-months (95% CI, 2.07 - 5.83) for exclusive lactation to 8.79 cases per 1000 person-months (95% CI, 5.47 - 12.11) for exclusive formula use at 6 to 9 weeks postpartum, in a graded manner (P trend = .004).

In multivariable regression models, after adjusting for age and covariates (including maternal and perinatal risk factors, newborn outcomes, and lifestyle behaviors), higher lactation intensity was associated with lower rates of incident type 2 diabetes (all P trends < .025). And compared with a short lactation duration of 0 to 2 months, longer lactation duration (>2 to 5 months, >5 to 10 months, and >10 months) was also independently associated with lower rates of incident type 2 diabetes (all P trends < .01).

Although the authors acknowledge the inability to evaluate the associations beyond 2 years of follow-up as one of the major limitations of this study, they emphasize that, by design, SWIFT minimized the reverse causality and residual confounding that reduce the validity of earlier studies. "[T]o date, SWIFT was the largest and most ethnically diverse prospective cohort of women with GDM to conduct glucose tolerance testing annually from the early postpartum period, and is the only study that prospectively quantified lactation intensity and duration and controlled for several perinatal and newborn potential confounders," they write.

On the basis of the results of this study, the researchers emphasize the need to refocus strategies to reduce the risk for incident type 2 diabetes in postpartum women.

"Modification of lactation behaviors to increase intensity and duration should be considered a high priority for pregnant and postpartum women with GDM because of their lasting metabolic benefits. Greater allocation of health care resources to promote and support exclusive and extended breastfeeding may benefit high-risk women by reducing their risk for midlife progression to [diabetes mellitus]," the authors conclude.

http://www.medscape.com/viewarticle/854928