The Lactating Athlete: Bone, REDs Risk, and Training Smart While Breastfeeding
What the emerging research on bone loss, energy availability, and lactation physiology means for postpartum athletic rehabilitation
The Variable Most Frameworks Ignore
Ask most return-to-sport frameworks whether a postpartum athlete is breastfeeding, and you'll find silence. Lactation is treated as a personal detail — relevant to nutrition counseling, perhaps, but not to rehabilitation. The emerging evidence suggests otherwise. Breastfeeding has measurable effects on bone density, hormonal milieu, energy availability, and injury risk that every PT working with postpartum athletes needs to understand.
This article covers three interconnected topics: bone mineral density changes during lactation, Relative Energy Deficiency in Sport (REDs) risk in the lactating athlete, and practical guidance for training and rehabilitation in this population.
What Lactation Does to Bone
Bone mineral density can decline 3–10% during pregnancy and lactation, with losses occurring most significantly in the lumbar spine and femoral neck — the very sites that matter for impact sport loading. This is driven by a hormonal mechanism: elevated prolactin suppresses estrogen and increases parathyroid hormone-related protein, increasing bone resorption to mobilize calcium for breast milk.
The good news is that bone density largely recovers following weaning — typically within 6–12 months. The clinical concern is the window during active lactation: an athlete with already-reduced BMD who begins high-impact training faces an elevated risk of stress fracture, particularly in the femur, tibia, and lumbar spine. This risk is compounded if energy availability is inadequate.
“Breastfeeding adds ~400–500 kcal/day to energy demands. In an athlete who is also training and managing postpartum body image pressures, REDs risk is real,
underscreened, and consequential.”
REDs Risk in the Lactating Athlete
Relative Energy Deficiency in Sport describes a condition in which energy intake is insufficient to meet the combined demands of exercise and physiological function. In lactating athletes, the energetic calculus is significantly altered: breastfeeding adds approximately 400–500 kcal/day to baseline energy expenditure, and postpartum social pressures — the desire to 'get back to pre-pregnancy weight' — may lead athletes to restrict intake precisely when they need more.
The IOC's 2023 consensus statement on REDs identifies lactation as an independent risk factor for energy deficiency in athletic populations. Symptoms include impaired recovery, fatigue, mood disturbance, hormonal disruption, and increased stress fracture risk. A PT who includes a brief energy availability screen in postpartum intake is filling a gap that most providers leave open.
BONE & LACTATION: KEY FACTS
BMD can decline 3–10% during pregnancy and lactation, particularly at lumbar spine and femoral neck
Calcium losses during 9 months of lactation exceed those of pregnancy itself
Estrogen suppression during lactation reduces bone protection typically present in reproductive-age women
BMD largely recovers 6–12 months post-weaning — but the window of low BMD coincides with typical return-to-sport period
Stress fracture risk is elevated during active lactation with high-impact training (Woodroffe et al., Sports Health 2025)
What Exercise Does (and Doesn't Do) to Breast Milk
A common concern among lactating athletes is that exercise will negatively affect milk quality or supply. The evidence does not support this. Moderate-to-vigorous exercise does not impair milk volume, macronutrient content, or infant feeding behavior. Lactic acid concentrations may transiently increase after high-intensity exercise — some infants temporarily refuse milk post-maximal exercise — but this is resolved by feeding before training or waiting 30–60 minutes post-session.
The clinical takeaway: exercise is safe during lactation. The variable that matters is energy intake, not exercise itself. Athletes should be counseled to fuel adequately — not to limit training based on unfounded concerns about milk quality.
A Multidisciplinary Approach
The lactating athlete represents a case where PT alone is insufficient. The ideal care team includes a sports dietitian to assess and support energy availability, a lactation consultant if supply or feeding issues arise, and a PT to guide progressive rehabilitation and monitor for musculoskeletal warning signs — including bone stress symptoms.
Screening questions for energy availability and lactation status should be standard in the postpartum PT intake. Lactation is not a passive backdrop to athletic rehabilitation — it is an active physiological state that affects bone, hormones, energy, and injury risk. The PT who understands lactation physiology is better equipped to protect, guide, and advocate for this population.
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Woodroffe L, et al. Return to running for postpartum elite and subelite athletes. Sports Health. 2025;17(3):614–620.
Mountjoy M, et al. 2023 IOC consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med.
2023;57(17):1073–1097.
Gatorade Sports Science Institute. Postpartum athlete nutritional considerations and REDs risk. GSSI. 2025.
Davenport MH, et al. Exercise and lactation: evidence-based guidance for health professionals. Can J Appl Physiol. 2004.
Pearce EN. Thyroid disease in pregnancy and postpartum. Best Pract Res Clin Obstet Gynaecol. 2015.