I am thrilled to post my interview, with human milk researcher, Shannon L Kelleher, PhD. Dr. Kelleher just published a paper about her research on zinc levels in human milk. She found that zinc in the diet is not correlated to the amount of zinc in breast milk! This suggests that certain breastfed infants, are at risk for zinc deficiency and might benefit from zinc supplements, like is already being done for vitamin D and iron. Adequate zinc levels are critical to develop the immune system, cognitive development and general infant growth. Please read on for the rest of my interview. I promise, there are exciting results!
Shannon L Kelleher, PhD
Associate Professor in Nutritional Sciences, Surgery and Cell and Molecular Physiology; Penn State Hershey Cancer Institute
University of San Diego – BA in Biology/Chemistry
University of California Davis – PhD in Nutritional Biochemistry/Cell Biology
Jody: You just submitted a paper and I would like to know if a certain population of lactating moms could be at risk for a Zn deficiency and if a supplement could prevent a Zn deficiency?
Dr. Kelleher: It estimated that ~80% of people do not consume enough zinc! Women of reproductive age and children are at greatest risk due the high demand for zinc during pregnancy, the even greater demand for zinc during lactation, and the large amount that is needed for post-natal growth and development. You actually need more zinc when you are breastfeeding than you do when you are pregnant due to the large amount (1-5 milligrams/day) that is secreted into milk to meet the needs of the infant. So yes, most women are at risk for zinc deficiency and a zinc-rich diet or zinc supplement could help ensure women get adequate zinc. However, zinc in the diet is not correlated to the amount of zinc in breast milk; several studies have shown that zinc supplementation during breastfeeding does not affect milk zinc concentration. So this is what we have addressed in the study we just completed…..if it is not zinc in the diet, what governs the amount of zinc in breast milk?
We think is a major contributor is mom’s genetics. What we found, much to our surprise, is that there is a lot of variation in a gene that we have shown is very important to secreting zinc into milk. This gene is called ZnT2. We previously found that some women who have very low zinc levels in their milk, so low that their infants become very zinc deficient and sick, have mutations in ZnT2. In this study, we recruited healthy breastfeeding women from the Harrisburg/Hershey area, measured the amount of zinc in their milk and analyzed their DNA for mutations in ZnT2 to see if there was an association. We were shocked to find that ~30% of otherwise “healthy” women had defects in ZnT2. Moreover, we found that if a woman had a defect in ZnT2, she had milk zinc levels that were lower than women who had “normal” ZnT2. Because 75% of women with low milk zinc levels had some variation in this gene we think that variation in ZnT2 is an important factor that governs milk zinc levels, but surely not the only thing. Importantly, this suggests that these infants are at risk for zinc deficiency and might benefit from zinc supplementation, like is already being done for iron and vitamin D. Interestingly, ~30% of women who had milk zinc levels higher than “normal” also had variation in ZnT2. However, there is no evidence that milk zinc levels that are slightly higher than “normal” have any health consequences in the infant, and in fact may be beneficial.
Jody: What is Zn responsible for, during the development of an infant?
Dr. Kelleher: Breast milk has a lot of zinc in it to meet the needs of the infant. Infants have enhanced demand for zinc because of the role that zinc plays in so many biological processes. In fact, 10% of the proteins in our body require zinc, that’s over 3000 different proteins, which means that zinc regulates over 300 different biological processes. Adequate zinc is particularly critical for the infant as it plays an important role in the development of the immune system, cognitive development and general infant growth.
What symptoms would indicate there could be a Zn deficiency or too much Zn, in a breastfed baby? Is Zn toxic in high amounts? Zinc deficiency goes undiagnosed for a long time, often until the infant falls off the growth chart and fails to thrive, because the symptoms are so non-descript. It first presents as skin issues, irritably, and increased illness….think about how many infants suffer with bad diaper rashes, colic and ear infections. The main problem is, there is no sensitive marker for zinc deficiency. The amount of zinc in blood plasma is often measured, but it is not affected until one is very zinc deficient so there is no simple way to assess someone’s zinc status. Yes, zinc is also toxic and can cause nausea, vomiting, cramps, diarrhea and headaches, in infants and adults. Too much zinc also depress your immune system and interfere with other minerals such as copper and iron. Nutrition is always about finding balance.
Jody: Could there be a Zn deficiency in a formula fed baby?
Dr. Kelleher: Most likely not. Infant formula contains about 5 times more zinc than breast milk, so it is unlikely an infant born healthy and at term would become zinc deficient if fed formula.
Jody: What foods are high in Zn?
Dr. Kelleher: Zinc is high in seafood, particularly oysters, meats, nuts and some legumes.
Jody: As a researcher, you already know human milk can be deficient in Vitamin D and iron. What other nutrients do you know of, if any, could be deficient?
Dr. Kelleher: So, milk is normally low in iron and vitamin D for a reason, it’s not that it is “deficient”. If you’re healthy and you eat a well-balanced diet, the amount of iron and vitamin D in breast milk is just low. I think this is Mother Nature at her best! Iron is very toxic, and because infants draw upon iron stores in their liver for their needs during early infancy, the amount of iron in milk is low. The same can be said for copper. With respect to vitamin D, because as humans we produce vitamin D in our skin from sunlight and too much vitamin D causes calcification of soft tissues, my bias is that Mother Nature didn’t think to add it to milk! There is also naturally little vitamin K in milk, likely because the bacteria in our gut produce it and it is recycled time and time again. This is not to say that some infants don’t need extra iron, vitamin D or vitamin K to meet their needs, but that it is not because milk is “deficient”.
Now, you can contrast that with other micronutrients that may be deficient in breast milk because of mom’s diet, genetics, medications or environmental exposures. For example, many of the B-vitamins (thiamin, riboflavin, niacin, B6, B12) and iodine are very sensitive to maternal intake. In addition to our work on zinc, a study found that genetic variation in the vitamin D receptor was associated with lower levels of calcium in milk and genetic variation in genes that produce omega-3 fatty acids (DHA and EPA) alter milk lipid profiles and affect infant health. I am convinced that diet and genetics plays a larger role in milk volume and composition than we think. With the help of breastfeeding moms, we now have the tools to answer these important questions.
Jody: What advice can you give lactating moms about the nutrient composition of their milk, related to their diet?
Dr. Kelleher: I guess the best advice I could give at this point would be to eat a well-balanced and colorful diet to ensure you get as many antioxidants and bioactive factors as possible. Some of our other work shows that inflammation in the breast during lactation prematurely activates involution so the more antioxidants you eat the better your body is at combatting inflammation, whether you’re breastfeeding or not. A simple rule I’ve heard is try aim for 3 colors every meal, because different colored foods play different roles in the body…..and get some sleep!
Jody: How did Zn become the focus of your research?
Back when I was a graduate student I had the great fortune to train with one of the world’s leading researchers in maternal/infant nutrition, Dr. Bo Lonnerdal at the University of California Davis. He had long been interested in understanding how the mammary gland secreted so much zinc into milk and why the milk of some women was too low to support the needs of their infants. Although I worked on many different aspects of maternal and infant nutrition, understanding how the mammary gland regulates zinc transport was part of my PhD dissertation and working together we were the first to determine that ZnT2 was critical to zinc secretion into milk and that some women had mutations in this gene. I have continued to work to understand this very complex process ever since, isolated cells, animal models and more recently clinical studies in breastfeeding women. Zinc is used for so many processes it is fascinating to study how cells and tissues regulate zinc biology. The overall goal of my research is to understand how maternal diet and genetics affects breast development, lactation and infant health and also breast cancer, from a zinc-centric perspective. In fact you can follow me on twitter at @zinciscool where I post the latest buzz about zinc or lactation.