Researchers wanted to understand whether proteins from cow’s milk and soy drinks pass into breast milk when mothers consume these beverages. They studied 38 breastfeeding women who drank increasing amounts of cow’s milk or soy drink over several days while researchers tested their breast milk. They found that cow’s milk proteins do appear in breast milk after mothers drink it, though in small amounts. However, they couldn’t confidently detect soy proteins in the breast milk. This research helps doctors understand how food allergens might reach babies through breast milk and could help guide advice for mothers with allergic infants.

The Quick Take

  • What they studied: Whether proteins from cow’s milk and soy drinks that mothers consume show up in their breast milk, and how much appears after drinking these beverages.
  • Who participated: 38 healthy breastfeeding women participated in the full study, with 24 of them providing breast milk samples for detailed analysis. All were able to produce breast milk and had no major health conditions.
  • Key finding: Cow’s milk proteins were found in breast milk samples, with some variation in the amount detected 2 and 4 hours after mothers drank cow’s milk. Soy proteins were not confidently identified in breast milk after mothers drank soy beverages.
  • What it means for you: If you’re breastfeeding and your baby has a milk allergy, be aware that small amounts of milk proteins may pass into your breast milk even after you stop drinking milk. However, this research suggests soy beverages may be a safer alternative, as soy proteins don’t appear to transfer into breast milk in detectable amounts.

The Research Details

This was a carefully controlled experiment where breastfeeding mothers followed two separate test periods. In each period, they first avoided cow’s milk or soy drinks for 5 days, then drank increasing amounts (starting small and building up to about 415 milliliters, or roughly 14 ounces) over 3 days, followed by 2 days without these drinks. Researchers collected breast milk samples at different times and used advanced laboratory techniques to identify and measure any proteins from these drinks in the milk.

The study used a ‘cross-over’ design, meaning each mother participated in both the cow’s milk phase and the soy drink phase at different times. This approach helps researchers see how each mother’s body responds to both beverages, making the results more reliable.

To find the proteins, scientists used a technique called peptidomics, which breaks down proteins into smaller pieces called peptides and identifies them using specialized machines that can detect tiny amounts of these pieces in breast milk samples.

Understanding whether and how much food proteins pass into breast milk is crucial for mothers with allergic babies. If a baby has a milk allergy, knowing whether cow’s milk proteins reach breast milk helps doctors give better advice about what mothers should eat. This research provides actual evidence rather than just guesses about protein transfer.

The study was well-designed with careful controls, including elimination periods and specific testing times. However, the sample size was relatively small (24 mothers for the detailed analysis), which means the results may not apply equally to all breastfeeding women. The study was rigorous in how it identified proteins, but some soy proteins couldn’t be identified with complete confidence, suggesting the detection method may have limitations for soy.

What the Results Show

Researchers identified 121 different cow’s milk-derived peptides (protein pieces) in breast milk samples collected during the cow’s milk phase. These peptides came from six different milk proteins, with beta-lactoglobulin being the most abundant, followed by several casein proteins and other milk components.

When comparing breast milk samples taken before mothers drank cow’s milk versus samples taken 2 and 4 hours after drinking, the researchers found that 14 of these peptides showed different amounts in the milk at different times. This suggests that drinking cow’s milk does increase the amount of milk proteins in breast milk, though the overall levels remain quite low.

Interestingly, even after mothers avoided cow’s milk for 5 days, some cow’s milk peptides were still detectable in their breast milk. This could mean either that 5 days isn’t long enough to completely clear these proteins, or that tiny amounts of milk proteins come from other dietary sources.

For soy beverages, the results were quite different. Researchers found only 8 peptides that might have come from soy, and they couldn’t confidently match these to soy proteins with the certainty they achieved with cow’s milk. This suggests that soy proteins either don’t pass into breast milk in significant amounts, or they pass in forms that are harder to detect.

The study found that the specific cow’s milk proteins that appeared in breast milk were consistent across most mothers, suggesting this is a common pattern rather than something unique to a few individuals. The timing of protein appearance (showing up within 2-4 hours of consumption) suggests these proteins are absorbed relatively quickly from the mother’s digestive system into her milk. The persistence of some proteins even after elimination suggests that complete avoidance might require longer than 5 days.

Previous research has suggested that food proteins can pass into breast milk, but this study provides more detailed information about exactly which proteins appear and in what amounts. The findings align with earlier observations that cow’s milk proteins are more likely to transfer into breast milk than plant-based proteins, though this study provides stronger evidence for soy specifically.

The study included only 24 mothers for the detailed protein analysis, which is a relatively small group. Results might differ in mothers with different diets, health conditions, or genetic backgrounds. The 5-day elimination period may not have been long enough to completely clear all proteins, making it unclear whether longer avoidance would reduce protein transfer further. The study couldn’t confidently identify soy proteins, so we can’t be completely certain that soy proteins don’t transfer—they might just be harder to detect with current methods. Additionally, the study measured proteins at only two time points (2 and 4 hours), so we don’t know the full timeline of when proteins appear and disappear from breast milk.

The Bottom Line

If your baby has a confirmed cow’s milk allergy, be aware that cow’s milk proteins likely will appear in your breast milk after you drink milk, though in small amounts. Avoiding cow’s milk may reduce but not completely eliminate these proteins from your breast milk. Soy beverages appear to be a safer alternative, as soy proteins don’t seem to transfer into breast milk in detectable amounts. However, always consult with your pediatrician or allergist before making dietary changes, as individual babies may react differently. (Confidence level: Moderate—based on a small study with some limitations)

This research is most relevant for mothers whose babies have confirmed cow’s milk allergies or suspected milk sensitivities. It’s less relevant for mothers whose babies don’t have allergies. Healthcare providers caring for allergic infants should consider this information when advising mothers about diet. Mothers considering soy as a milk alternative may find reassurance in these findings.

Changes in breast milk composition happen relatively quickly—within 2-4 hours of the mother consuming cow’s milk. If a mother eliminates cow’s milk, some proteins may clear quickly, but the study suggests complete clearance might take longer than 5 days. Babies with milk allergies might show improvement within days to weeks of the mother eliminating cow’s milk, but individual responses vary.

Want to Apply This Research?

  • If tracking for a milk-allergic infant, log maternal milk consumption (type and amount) and infant symptoms (fussiness, rash, digestive issues) at the same times each day. Note the timing between maternal consumption and symptom appearance to identify patterns specific to that baby.
  • Users with allergic infants can use the app to track which dairy products they consume and monitor their baby’s symptoms in real-time. Set reminders to avoid cow’s milk products and track alternative beverages like soy, oat, or almond milk to find which options work best for their family.
  • Maintain a 2-week food and symptom diary, recording all dairy intake and infant reactions. Use the app to identify patterns—for example, whether symptoms consistently appear 2-4 hours after maternal dairy consumption. Share this data with your pediatrician to confirm milk allergy and track improvement after dietary changes.

This research provides scientific information about protein transfer in breast milk but should not replace medical advice from your pediatrician or allergist. If you suspect your baby has a milk allergy, consult a healthcare provider for proper diagnosis and personalized recommendations. Do not eliminate foods from your diet without professional guidance, as this could affect your nutrition and milk supply. Individual babies may respond differently to maternal diet changes, and what works for one baby may not work for another.