Reader question: Marathon Running and Breast Milk Production

By Jennifer Goldbronn, MAS, RD

We recently received the following question from a reader: Could you post info on marathon training and how it impacts milk production? A mom says she has seen her pumping output cut in half since beginning her training.

To be honest, there is not a lot of evidence out there looking specifically at intense long-term exercise and its effects on breast milk production. However, a past post of ourslooked at a few related studies on the topic concluding that exercise doesnot affect breast milk production or infant growth. Below, we’ve provided a few more details of the studies reviewed in our past post and added a couple of other related references.

Intense Exercise

In a 1990 study, intense exercise (≥45 minutes per day, 5 days per week for at least the last 6 months prior to the study) did not affect milk volume or composition. Many women in this intervention were involved in competitive sports, one even training for a triathlon. Researchers found that study participants compensated for increased energy expenditure through exercise with increased food intake. Researchers also noted that while there was no increase in nursing frequency in this study, there could be in other situations where there was a large calorie deficit from exercise and/or dieting. The limitation of this study was the very small sample size (8 women in the exercise group and 8 in the no exercise group). This makes it difficult to draw conclusions based on this study alone. (Lovelady 1990)

Moderate Exercise

In a follow-up study, Lovelady (2000) found that moderate exercise (45 minutes per day for 4 days per week) combined with energy restriction (500 kcal per day) in healthy, previously inactive, breastfeeding mothers did not harm the growth of their infants.

One additional study (Dewey 1994) found no difference in both the volume and composition of breast milk or in the infants’ weight gain with moderate exercise (4-5 times per week for 45 min., beginning at 6-8 weeks postpartum and lasting for 12 weeks.) This study was also conducted with previously inactive women and had a small sample size with only 18 women in the exercise group and 14 in the no exercise group.

Short-term Diet or Exercise

Two other studies (Strode 1986, McCrory 1999) showed no change in milk volume with short-term energy restriction (by diet or exercise), but the studies were no longer than 7-11 days in length. Further research needs to be conducted to see if there are changes in milk volume after the first week.

Conclusions:

More research needs to be completed to truly show how long term intense exercise effects breast milk production. We can only infer from the above studies that moderate exercise does not negatively affect breast milk production or infant growth. As mentioned in the previous post, separation of a mother from a young breastfeeding baby may effect milk production if feeding on-demand is interrupted. Also, if the mother was feeding on-demand and begins to pump to allow for longer workout sessions, breast milk production may be negatively affected because the breast pump is not as effective as the baby at removing milk. If less milk is removed, then less milk is produced by the mother’s body.

References

Strode MA, Dewey KG, Lonnerdal B. Effects of short-term caloric restriction on lactational performance of well-nourished women. Acta Paediatr Scand 1986;75:222-9.

McCrory MA, Nommsen-Rivers LA, Mole PA, Lonnerdal B, Dewey KG. Randomized trial of the short-term effects of dieting compared with dieting plus aerobic exercise on lactation performance. Am J Clin Nutr 1999;69:959-67.

Lovelady CA, Lonnerdal B, Dewey KG. Lactation performance of exercising women.Am J Clin Nutr. 1990 Jul;52(1):103-9.

Lovelady CA, Garner KE, Moreno KL, Williams JP. The effect of weight loss in overweight, lactating women on the growth of their infants. N Engl J Med. 2000 Feb 17;342(7):449-53.

Dewey KG, Lovelady CA, Nommsen-Rivers LA, McCrory MA, Lönnerdal B. A randomized study of the effects of aerobic exercise by lactating women on breast-milk volume and composition. N Engl J Med. 1994 Feb 17;330(7):449-53.