Paired versus alternate breastfeeding: Does hindmilk become foremilk?

Breast milk fat content increases four-fold from foremilk to hindmilk. Within each nursing episode, women may partially feed on both breasts (paired feeding) or nurse from one breast at a feed and the other breast at the next feed (alternate breastfeeding). Despite varied breastfeeding recommendations, there are no well-designed studies that examine milk volume or composition differences with paired versus alternate feeding. We sought to determine the potential nutrient impact of paired versus alternate breastfeeding, and assess if during paired feeding, does "hindmilk become foremilk" at the next feed.

Women (n = 9) breastfeeding between 7 and 10^6/7 weeks postpartum were studied. For the First Pumping session, breast milk samples were obtained continuously (10 mL aliquots) until Breast A was emptied. Thereafter, the second breast (Breast B) was pumped to 50% of Breast A volume. After a 2-h hiatus, Second Pumping was undertaken with both breasts (A and B) pumped until empty. Maternal dietary intake was assessed and maternal blood and breast milk analyzed.

In First Pumping, Breasts A and B showed parallel increases in milk fat and calorie content. With the Second Pumping, Breast B milk fat demonstrated further increases consistent with the hindmilk of Breast A. Unexpectedly, with the Second Pumping, Breast A foremilk fat content began at levels consistent with Breast A hindmilk, and further increased to values not seen during the First Pumping. Milk calorie content demonstrated identical patterns as milk fat. Milk protein and carbohydrate content did not change from foremilk to hindmilk, nor was there a significant change between the two milk sampling periods.

The fat concentration in the Second Pumping of Breast B was essentially equivalent to the hindmilk content of First Pumping of Breast A, suggesting that "hindmilk becomes foremilk" with a paired feeding process. With the refilling of Breast A, it appears that high fat "hindmilk" is produced first with the fat content further increasing from the initial to the final value in Breast A. These findings raise important questions regarding the diurnal, dietary, and hormonal regulation of breast milk fat content and its potential impact on infant growth and nutrition.