Friday, June 24, 2011

The Active Mommy Conundrum: Can Exercise Harm Breastfeeding? Part 2.

By Taryn Barrette, RD

In this short series, we’re sharing the research about the effect of exercise on breastfeeding. Last time, I explained that exercise does not reduce milk supply but what about lactic acid? Does lactic acid get into breast milk and affect the baby in any way? In today’s post, we will discuss the effect of exercise on the lactic acid content of breast milk.

What is Lactic Acid?

Lactic acid is found naturally in the blood and muscles and is a byproduct of normal metabolism. During high intensity (anaerobic) exercise, oxygen intake is decreased and CO2 is increased resulting in an excess of lactic acid. The high concentration of lactic acid eventually dissipates to normal levels after intense exercise is stopped and the lactic acid is recycled back during the normal metabolic process.

Lactic Acid in Breast milk

It is known that lactic acid produced from exercise is present in breast milk up to 90 minutes post-exercise; however, research shows that the concentration of lactic acid is only significantly higher after bouts of high intensity exercise. In one study, “High intensity” was determined by each participant’s “Rate of Perceived Exertion” (RPE) – meaning that the degree of difficulty was determined by the exerciser’s perception.

Thus, when an individual exerts themselves for 30 minutes of what is perceived to be highly intense exercise, there is a significant increase in lactic acid in their breast milk compared to breast milk expressed prior to exercise or during moderate exertion.

There was only an increase in lactic acid after an ‘intense’ (anaerobic) workout. When mothers engaged in moderate (aerobic) exercise that was 50% or 75% of maximum exertion (which is a more typical level of exercise for most people) there was not a significant or noticeable increase in lactic acid concentration in their breast milk.

Infant Acceptance of Breast Milk

Though all of the studies analyzing the contents of post-exercise breast milk showed that intense exercise was related to increased lactic acid concentration, we don’t have consistent evidence that lactic acid affects infant acceptance of the milk. Most of the studies of infant acceptance of breast milk required the mothers to offer expressed milk to their babies rather than feeding them directly from the breast.

One study with 12 mothers involved collecting breast milk samples before exercise and then 1 hour after intense exercise. The mothers then fed their infants the expressed breast milk (blinded as to whether they were feeding the pre- or post-exercise milk). They, along with a lactation consultant, then judged whether they felt their child accepted the milk well or not. This study found that there was no difference in infant acceptance of the breast milk despite there being significantly higher concentrations of lactic acid in the post-exercise milk.

In another study from 1992, breastfeeding mothers were asked to collect breast milk samples at 10 and 30 minutes post-high intensity exercise. The mothers then fed their infants the expressed breast milk with a dropper and rated the infant’s acceptance. These mothers reported that their infants rejected the post-exercise milk more often than the pre-exercise milk.

What’s a Mom to Do?

So can you still be the active mommy that you want to be? The answer is Yes! Moderate exercise does not affect the flavor, taste or nutritional quality of your breast milk at all. Intense exercise (completely based on your own perception) does alter the lactic acid concentration of your milk, but this change is not likely to be an issue with your infant. After all, the best evidence comes from longer term studies that included measurements of infant growth and findings from those studies did not show that exercise interfered with breastfeeding. Of course, talk to your doctor before you start any exercise program.

If you find a link between bouts of strenuous exercise and breast milk refusal, you can decrease the intensity or wait 30 minutes for the lactic acid to disappear but it may be more likely that your infant is distracted or there is another reason for the refusal.


1. Wright KS, Quinn TJ, Carey GB. “Infant acceptance of breast milk after maternal exercise.” Pediatrics. 2002;109(4):585-9.

2. Wallace JP, Inbar G, Ernsthausen K. “Infant acceptance of postexercise breast milk.” Pediatrics. 1992; 89(6 Pt 2):1245-7.

3. Dewey KG, Lovelady CA, Nommsen-Rivers LA et al. A randomized study of the effects of aerobic exercise by lactating women on breast milk volume and composition. N Engl J Med. 1994; 330: 449-453.


  1. Great information! Thanks for writing about this!

  2. This should reassure new Mom's that taking care of their own health and well-being won't harm their baby. Great information!

  3. I have a question/comment that's not exactly related to this post. My baby just turned 6 months old today and we have been successfully and exclusively breastfeeding, but I need some encouragement and tips to keep going. I love breastfeeding when I'm home with the baby. I work full-time and just really don't like to pump at work. The pump isn't particularly comfortable and it takes about 40 minutes out of my work day and I could really use that 40 minutes right now. For the past few days I've really been thinking about not pumping anymore. I've read that infants do better breastfeeding for the whole first year, but how much better? I guess I want to know is it really worth it to breastfeed after the first 6 months?

  4. Thank you all for your comments!

    To the anonymous reader who has a question about breastfeeding - I want to let you know that I know exactly how you feel! We have a lot in common - my baby just turned 6 months old and I work full time, and I have pumped exclusively for both my children. For our Friday post, we'll answer your questions and provide some tips that might help you stick with your original plan to breastfeed for a year.