Dec. 22, 1997 WEST LAFAYETTE, Ind. -- Leptin, a hormone that appears to play an important role in body metabolism and obesity, has been found for the first time in human breast milk.
In a study of 23 lactating women, researchers at Purdue University, the University of Idaho and Washington State University found that the hormone is present in human breast milk in levels that are lower than, but correlate with, levels in the mother's bloodstream.
The research also found that the amount of leptin in the breast milk correlates with the amount of body fat of the mother; obese mothers produce large amounts of leptin, thin mothers produce almost no leptin in their breast milk. The study was published in the current issue of the scientific journal Biochemical and Biophysical Research Communications.
Leptin is produced by fat cells and the placenta in the body. Since its discovery in the early 1990s, researchers have been racing to learn more about the protein. The reason is simple: Obese mice that are injected with leptin soon lose their excess weight. Scientists are hoping that by learning more about leptin, they can control the problems of obesity and its related maladies, such as diabetes and heart disease.
Karen Houseknecht, assistant professor of animal science at Purdue and adjunct assistant professor of endocrinology and metabolism at the Indiana University School of Medicine, is interested in the role of leptin during transitional periods of the body. She decided to investigate whether this newly discovered hormone was in breast milk, because lactation is an obviously important physiological transition for many women.
Although the researchers found leptin in the milk, at this point they aren't sure what role the hormone plays for mothers and infants. "Like many hormones in breast milk, it is difficult to determine what it is doing," Houseknecht says. "It may be that leptin is doing nothing; it may be that the leptin is just there."
On the other hand, there are reasons to believe that leptin could be important to neonatal development. "We know that breast milk contains many bioactive hormones and peptides. Research by many groups has shown that breast-fed infants have many health advantages over formula-fed infants," Houseknecht says. "Experiments in suckling rats have shown that rat newborns respond to leptin injections by increasing their metabolic rates.
"Leptin in breast milk could mean lots of things to the infant's development. There are receptors for leptin throughout the gastrointestinal tract of adults. If leptin is important to the neonate, we know there is a mechanism for it to get from the intestine to the bloodstream."
No one knows yet if leptin is present in cow's milk, or if the pasteurization process damages the hormone, Houseknecht adds.
Researchers don't know whether leptin offers any advantages or disadvantages for infant development, but they do know that infants whose nursing mothers have significant fat tissue will be exposed to more leptin in the milk. "Leptin levels reflect the mom's leptin levels," Houseknecht says. "Very thin mothers don't produce very much leptin. This adds another interesting twist to this story, because if leptin is important for infant development, these varying levels may mean that some infants are at a disadvantage."
Another theory about how leptin could be important is that leptin may play a role in passing obesity from one generation to the next. An obese mother who produces high levels of leptin and passes the hormone on to her infant may influence the child's metabolism into adulthood. "This is a possibility, but we have no data on this," Houseknecht says. "We know from studies of identical twins that have been separately adopted that there is a huge genetic component to obesity. The animal studies cause us to wonder if milk-borne leptin may play a role, too."
Another possibility is that leptin is important for the lactating mother and not important for the infant. Research has found that leptin levels in mothers are elevated during the pregnancy, and that it may be involved in milk production. "We also know that obesity plays a role in lactation, because obese mothers don't breast-feed as well as other women. Studies have shown that obese women start breastfeeding later and don't stay with it as long," Houseknecht says. "So it may be that the leptin is in the milk because in some way it is involved in lactation."
Houseknecht has discovered previously that leptin circulates in the bloodstream bound to specific proteins, and this binding process is saturated -- or at its upper limit -- in obese people. The next step for the researchers is to see whether breast milk has leptin-binding proteins that may influence the amount of active leptin available for the infant.
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