Following on their study on milk volume and method of expression, which showed that a combination of pumping and hand expression (a technique termed "hands on pumping") yields more milk for preterm infants, a team at Stanford University School of Medicine has shown that pumping method also affects milk composition.
The study, led by Dr. Jane Morton, found:
The researchers’ findings confirmed that moms [of very preterm infants] who used hands-on pumping had higher fat content in their milk than women relying on electric pumps alone...
“People have suspected that mothers would be able to get more fat-rich milk with hands-on pumping but it’s never been demonstrated before,” said Jane Morton, MD, a community pediatrician who was the new paper’s first author. The suspicion arose because milk composition changes during a feeding, shifting from more-dilute milk at first to richer, higher-fat “hindmilk” at the end. Because of its high fat content, the hindmilk is more viscous, which may explain why it’s difficult to remove this milk with an electric pump alone. But extracting more high-fat hindmilk could give preemies an important calorie boost.
Dr. Morton says that her next research question is whether this higher-fat milk is sufficient to meet preemies' needs without the use of human milk fortifier. This fortifier, with the exception of the human-milk product made by Prolacta Bioscience, is made from cow's milk. Cow's milk products have been shown to increase the risk of the life-threatening condition necrotizing enterocolitis.
Another question many moms may have is whether the same applies for full term infants. Here's a post I wrote on two different methods of hand expression, in case you'd like to conduct an experiment of your own!