Regularly breast-feeding your baby and getting regular exercise are two excellent ways for new mothers to lose the extra pounds packed on during pregnancy. In the past, however, clinical studies have shown contraindications involving the compatibility of these two activities, suggesting that running at a maximum effort affects the composition of breast milk.
Basis For The Belief
A now-outdated, but still widely circulated, study published in the journal "Pediatrics" in 1992 is the source for much consternation among breast-feeding mothers who like to run. In this study, researchers asked nursing mothers to exercise at 100 percent of their maximum heart rate. Then, the mothers were asked to supply their babies with breast milk. The research team found the babies were more likely to refuse their mother's milk after the period of intense exercise, suggesting the composition of the milk had changed during the run. However, the babies in this study were fed milk from a bottle by a stranger, not the child's mother, suggesting that the babies were rejecting the method in which the milk was presented, not the milk itself.
Lactic Acid
The physiological foundation for the change in composition to a mother's milk is real. When a person works out at an anaerobic level -- 80 percent or greater of her maximum heart rate -- her muscles begin to break down. A byproduct of this breakdown process is lactic acid, which makes its way into the bloodstream. Because blood vessels in the mammary glands are the source of breast milk, lactic acid in the bloodstream leaches into breast milk. Running at a lower intensity -- under 75 percent of a person's heart rate threshold -- does not increase the lactic acid levels in the bloodstream or breast milk. There is no evidence any lactic acid in the milk supply negatively impacts the health of a nursing baby.
New Research
A 2002 study published in the journal "Pediatrics" attempted to reevaluate the claims made by the 1992 study. In this study, breast-feeding mothers were asked to exercise at a variety of intensity levels, then feed their babies using a bottle with freshly expressed milk while a lactation consultant observed. Despite a small increase in lactic acid among mothers exercising at the highest intensities, neither the mothers nor the lactation consultants observed a change in the rate of infant acceptance of the breast milk.
Secondary Impacts on Breastfeeding
While running itself does not impact a mother's breast milk, other elements involved in this form of exercise might. For example, nursing mothers who lose too much weight because of their running regiment -- more than 4.5 pounds a month -- may suffer a drop in their milk production, compared to postpartum women who lost weight more gradually. Additionally, restrictive clothing often worn while running -- such as a tight sports bra -- physically prevents breast tissue from expanding, allowing for room to store milk once it is produced by the bloodstream. Physically binding the breasts also leads to plugged ducts and mastitis; in this condition, sodium and chloride levels in breast tissue rise, giving breast milk a salty taste. Finally, it is crucial to stay hydrated while running, as water is the main raw material used by your bloodstream in milk production.
References
- "Pediatrics"; Infant Acceptance of Post-Exercise Breast Milk; JP Wallace, et al; June 1992
- "Pediatrics"; Infant Acceptance of Breast Milk After Maternal Exercise; KS Wright, et al; April 2002
- Breastfeeding Basics: Breast Infections and Plugged Ducts
- "The Journal of Nutrition"; Effects of Maternal Calorie Restriction and Exercise During Lactation; KG Dewey; February 1998
- Kelly Mom; Exercise & Breastfeeding; Kelly Bonyata
