REF-570 · Dewey, Lovelady, Nommsen-Rivers, McCrory & Lönnerdal, 1994 · New England Journal of Medicine, 1994 Feb;330 · Paper · PubMed
Aerobic exercise by lactating women does not impair breast-milk volume or composition (RCT)
Participants
n=33 sedentary women whose infants were exclusively breast-fed, randomized at 6–8 weeks postpartum to exercise (n=18) or control (n=15).
Protocol
Exercise group: supervised aerobic exercise at 60–70% of heart-rate reserve, 45 min/day, 5 days/week, 12 weeks; control: no exercise programme.
Energy expenditure, dietary intake, body composition and milk volume/composition assessed at 6–8, 12–14 and 18–20 weeks postpartum; VO₂max and prolactin response to nursing at 6–8 and 18–20 weeks.
Key Findings
Exercisers spent about 400 kcal/day in sessions but compensated with higher intake (2497±436 vs 2168±328 kcal/day at 18–20 weeks; P<0.05).
VO₂max rose 25% in exercisers vs 5% in controls (P<0.001).
No significant differences between groups in maternal body weight or fat loss, milk volume or composition, infant weight gain, or maternal prolactin during the 12-week study.
Practical Application
RCT, n=33 sedentary breastfeeding women: 12 weeks of aerobic exercise (60–70% HRR, 45 min, 5 days/week) raised VO₂max 25% vs 5% in controls with no adverse effect on milk volume or composition, infant weight gain or prolactin — the exercisers ate about 330 kcal/day more (2497 vs 2168 kcal/day) and covered the ~400 kcal/day session cost.
Reassure lactating athletes: moderate aerobic training is compatible with exclusive breastfeeding — supply and infant growth were unaffected in this trial.
The protective mechanism was eating more to cover the ~400 kcal/day cost — match intake to lactation + training demand; do NOT layer a deficit on lactation (RED-S risk) — inference, not tested: the trial did not test exercise under an energy deficit.
Tested moderate volume in sedentary women — does not license aggressive dieting or high-volume return while lactating (untested; medical/dietetic conversation).
Limitations
Small (n=33), single study, previously sedentary subjects, moderate loads only; findings apply with adequate energy intake, not concurrent weight-loss dieting.
How this entry was checked and changed
Why this level
regraded 2026-09-27 from B: parallel RCT with n=33 (rubric: parallel RCT with 20-39 = B+)
Content correction 2026-09-27
Level B -> B+ per rubric (parallel RCT, n=33). Lead rewritten to the trial's result with its numbers; the "eat more" advice labelled as inference (the trial did not test a deficit). Cross-references given refIds; Sport line set to "general".