What Component of Breastmilk Causes Jaundice
The specific component in breast milk that causes breast milk jaundice remains unidentified, though research confirms that an unknown factor in the milk of a small percentage of mothers increases enterohepatic circulation of bilirubin, leading to prolonged unconjugated hyperbilirubinemia. 1, 2
The Unidentified Culprit
Despite decades of research, the exact substance responsible for breast milk jaundice has not been definitively identified:
- An as-yet unidentified factor exists in the human milk of a small minority of women (1-2% of breastfeeding mothers) that inhibits bilirubin clearance and increases enterohepatic circulation 1, 3
- This unknown component triggers jaundice particularly in infants who have underlying genetic mutations in the bilirubin uridine diphosphate-glucuronosyltransferase gene (UGT1A1), the same mutations found in Gilbert's syndrome 2
- The factor works by enhancing the recycling of bilirubin through the enterohepatic circulation rather than through direct hepatotoxicity 1, 4
Critical Distinction: Two Different Mechanisms
It's essential to differentiate between two types of jaundice in breastfed infants, as they have different causes:
Breastfeeding Jaundice (Early-Onset)
- Caused by inadequate caloric intake and dehydration, not by a milk component 5, 4
- Occurs in the first week of life when feeding frequency is insufficient (fewer than 8-12 times per day) 6, 5
- This represents "starvation jaundice" due to poor feeding technique, not a breast milk factor 4
Breast Milk Jaundice (Late-Onset)
- Caused by the unidentified factor in breast milk itself 1, 2
- Appears later (typically after the first week), peaks around week 2-3, and can persist until weeks 7-13 3
- Occurs even with adequate feeding and hydration 1
Genetic Susceptibility
The severity of breast milk jaundice appears related to infant genetics rather than variation in the milk component:
- 16 of 17 infants with prolonged breast milk jaundice had mutations in the UGT1A1 gene, with the G71R mutation being most common in Japanese populations 2
- These mutations reduce bilirubin conjugation enzyme activity to approximately one-third of normal 2
- The breast milk factor acts as a trigger in genetically susceptible infants rather than causing jaundice in all exposed infants 2
Clinical Implications
Breastfeeding should continue without interruption in most cases of breast milk jaundice, as the condition is benign and self-limited 5, 7
- Temporary interruption is only considered if bilirubin exceeds 270 μmol/L (approximately 16 mg/dL) and continues rising, or if phototherapy becomes necessary 5
- The benefits of continued breastfeeding—including reduced infections, enhanced organ maturity, and potential tissue protection—outweigh the risks of mild to moderate hyperbilirubinemia 8
- Colostrum and frequent feeding (8-12 times per 24 hours) increase stooling, which actually helps prevent pathologic hyperbilirubinemia by reducing enterohepatic circulation 9, 7
Common Pitfall to Avoid
Do not assume all jaundice in breastfed infants is benign breast milk jaundice. Always measure bilirubin levels, exclude pathologic causes (hemolysis, G6PD deficiency, hypothyroidism), and optimize breastfeeding technique before attributing jaundice solely to breast milk 5, 7