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What is the difference between foremilk and hindmilk?
Is my baby's fussiness caused by the lactose in my milk?

You may have heard that mothers produce two kinds of milk: foremilk, the thinner milk the baby gets first, which has a lower fat content; and hindmilk, the high-fat, creamier milk that follows. These terms can make it seem as if the breasts produce two distinct kinds of milk, which is not the case. The milk-making cells in the breasts actually produce only one type of milk, but the fat content of the milk that is removed varies according to how long the milk has been collecting in the ducts and how much of the breast is drained at the moment.

As milk is made, the fat sticks to the sides of the milk-making cells while the watery portion of the milk moves down the ducts toward the nipple, where it mixes with any milk left there from the last feeding. The longer the amount of time between feedings, the more diluted that leftover milk becomes. This "watery" milk has a higher lactose content and less fat than the milk stored in the milk-making cells higher up in the breast.

As baby begins nursing, the first thing he receives is this lower-fat foremilk, which quenches his thirst. Baby's nursing triggers the mother's milk ejection reflex, which squeezes milk and the sticking fat cells from the milk-making cells into the ducts. This higher-fat hindmilk mixes with the high-lactose foremilk and baby receives the perfect food, with fat calories for growth and lactose for energy and brain development. (1), (2) However, when milk production is too high, baby may fill up on the foremilk and then have difficulty digesting all the lactose that is not balanced by fat. This is known as foremilk/hindmilk imbalance or oversupply.

You may know adults who don't drink milk because they are "lactose intolerant," a medical condition that occurs when the body no longer makes enough of the enzyme lactase, which is needed to digest lactose, the main carbohydrate in milk. Lactose intolerance is not a problem for babies. They are born with the ability to produce lots of lactase because they depend on their mother's milk for nutrition in the first year of life and the lactose in mother's milk is needed for brain development. Lactase production decreases as children get older, because in the world of mammals, milk is a food for babies, not adults. This is why some adults (especially the elderly), become gassy and uncomfortable when they eat dairy foods high in lactose, which their bodies can no longer digest. True lactose intolerance in infants is called galactosemia, an extremely rare genetic condition (approximately 1 in 30,000 US births) that is present from birth and fatal if not treated; a baby with the disorder would not gain weight well and would have clear symptoms of malabsorption and dehydration.(3)

Although infants are not lactose intolerant by nature, a high volume of lactose can overwhelm a baby's digestive system. When there is not enough lactase to break down all the lactose, the excess lactose causes gassiness and discomfort, and frequently green, watery or foamy stools. Over time, large amounts of undigested lactose can irritate the lining of the intestines so that even a little bit passing through can cause irritation. Occasionally, this can result in small amounts of bleeding into stools that can be misdiagnosed as a food allergy. Some pediatricians will mistakenly diagnose lactose intolerance if there is undigested sugar in the baby's stool.

Occasionally, mothers whose babies are receiving a high level of lactose are advised to reduce the amount of dairy products in their diets so that there will be less lactose in their milk. This is neither necessary nor helpful, because the amount of lactose in a mother's milk has nothing to do with her diet; her body manufactures it especially for baby. If, however, limiting dairy products in mother's diet improves baby's condition, the baby was probably reacting to the proteins found in cow's milk that can appear in a mother's milk. (See our collection of LLL resources on Allergies for more information.)

For most mothers, allowing baby to nurse long enough on one side so that he gets more of the creamy, higher fat milk helps balance lactose and fat to ease digestion and usually corrects the problem. For more information about foremilk/hindmilk imbalance and oversupply, see our Oversupply FAQ, and the article Finish the First Breast First from LEAVEN, our publication for Leaders.

by Diana West, BA, IBCLC, co-author with Lisa Marasco of The Breastfeeding Mother's Guide to Making More Milk (McGraw-Hill, December 2008) and Elliot Hirsch of Breastfeeding After Breast and Nipple Procedures (Hale Publishing, July 2008), and author of The Clinician's Breastfeeding Triage Tool (International Lactation Consultants Association, 2006) and DEFINING YOUR OWN SUCCESS: BREASTFEEDING AFTER BREAST REDUCTION SURGERY (LLLI, 2001).

References

1) Lawlor-Smith, C. and Lawlor-Smith, L. Lactose intolerance. Breastfeeding Rev 1998; 6(1): 29-30.

2) Rings, E. et al. Lactose intolerance and lactase deficiency in children. Curr Op Ped 1994; 6: 562-67.

3) Leeson, R. Lactose intolerance: What does it mean? ALCA News 1995; 6(1) 24-25, 27.

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