The Basics: Protein – Cow? Soy? Casein? Whey?
Protein Source! This is one of the biggest first decisions you make when deciding what formula to feed your baby. This is where you can find a lot of the variation between formula brands and types. The standard (and default) source of protein in infant formula comes from cow’s milk protein. I am going to talk about the different formulations of cow’s milk protein that are available before I explain about the alternative protein source in formula, which is soy. There are 2 main types of cow’s milk protein that can go into formula. These are whey proteins and casein proteins. Whey and casein are each actually a group or category of different types of individual proteins (or chains of amino acids linked together). Whey proteins are those that remain in liquid when milk coagulates (curdles) This is the portion that is discarded when making cheese (remember Little Miss Muffet, famous for eating her curds and whey?). These proteins empty the stomach into the intestines faster than the casein proteins, and are rarely allergenic. Casein proteins take longer to digest and include the culprit proteins that most people with a true milk allergy react to. Casein proteins are considered more “reactive” meaning, they are more likely to trigger allergies or general indigestion than whey proteins. Now I want to explain 2 major differences between cow’s milk and human milk proteins: 1) The individual proteins (chains of amino acids) that compose human milk whey are not the same as the individual proteins that compose cow’s milk whey. This holds true for human vs cow’s milk casein. 2) Cow’s milk protein is ~20% whey and 80% casein while human milk is ~60% whey and 40% casein. It is important to understand the above differences together! Many formula companies brag that their casein:whey ratio is more similar to human milk. Others brag that they use 100% whey because it empties the stomach faster, which is also similar to human milk. This point will come up again when we talk about acid reflux (see article 8). As a parent, you must choose which you think is best. I recommend that you look for a formula with a protein ratio of at least more whey than casein. This is more similar to human milk. 100% whey may be better for your baby if your baby is prone to reflux, and/or is at risk for developing a milk allergy. Very few (if any) labels will break down the percentage of whey vs. casein for you. You have to infer it based on the order of ingredients. All ingredients are listed in order of amount. If the label just reads “cow’s milk protein”, you need to know that cow’s milk protein has a lot more casein than whey. In this case, I like to see additional whey further down in the ingredients list which will even out the ratio a bit. The alternative source from cow’s milk altogether is a soy-based formula. The protein in these formulas appears on the ingredients list as “soy protein isolate”. I am not a huge fan of soy formula. There are many reasons for this, but the first is the protein. Soy is, by definition, a “complete” protein. This means that it contains all 9 essential amino acids (the building blocks of protein) that humans need. However, soy protein does not contain an adequate amount of the amino acid methionine. Additional methionine must be added to soy formula to meet babies’ needs. You’ll find “L-methionine” on the list of ingredients only in soy formulas. Cow’s milk protein is an excellent source of all 9 amino acids and does not require supplementation. I am also not a huge fan of the carbohydrates added to soy formulas, but I’ll talk about that in article 6. Another major concern that has been raised about soy formula is the phytoestrogen content of soy. Phytoestrogens are compounds that can act like estrogen in the human body by binding to estrogen receptors. You can imagine that this would be an undesirable thing to occur in growing infants of either sex. A large study was conducted in 248 adults who were fed soy formula as infants (comparing them to 563 adults who were fed cow’s milk formula) to determine if the soy formula caused any problems or differences in sexual and pubertal development. Basically, the researchers found no differences between the groups (6), and soy formula has been deemed safe for infants. However, as a researcher myself, I am not 100% convinced. First this sample was “self-selected”. That means parents chose for themselves which formula to feed their infant which can introduce error into the outcomes. A gold-standard for this study would have randomly assigned parents to either feed a soy or cow’s milk based formula once they had decided to not breastfeed. Secondly more research is needed to study additional health outcomes. Pubertal characteristics are the natural first place to look for differences when we’re talking about phytoestrogen exposure, and I would have done the same thing. However, does this early phytoestrogen exposure affect later risk of cancer (especially breast, uterine and ovarian), cardiovascular disease, metabolic syndrome? I would like to see more research. Lastly (and not nearly as scientific), soy formula seems much less natural as a food source for human infants. Humans are mammals. This means they must consume milk to survive after birth. Cows’ milk is hardly breast milk, but at least it serves the same purpose for another species. Soy is initially a plant that cannot solely sustain life and needs to be extensively processed in order to yield the protein that we put in formula. This is one of the main reasons I prefer and recommend cows’ milk-based over soy-based formula. I will end this article here. Next, I’ll walk you through what the heck hydrolyzed protein is, and why I am so enthusiastic about it for little tummies. Take-Home Messages
- Milk protein is made of two groups of protein: Casein and Whey. Whey empties the stomach faster, and casein is more “reactive”.
- The individual proteins that compose human casein and milk whey are not the same as the individual proteins that compose cow’s milk casein and whey.
- Cow’s milk protein is ~20% whey and 80% casein while human milk is ~60% whey and 40% casein.
- I recommend a formula that has additional whey added or is 100% whey to more closely resemble breast milk.
- Soy protein has been deemed safe, but I do not recommend it as a protein source in formula due to the extensive modification and supplementation necessary to make this protein able to sustain life.
6. Strom, B. L., Schinnar, R., Ziegler, E. E., Barnhart, K. T., Sammel, M. D., Macones, G. A., Stallings, V. A., Drulis, J. M., Nelson, S. E., and Hanson, S. A. (2001) Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood. JAMA 286, 807-814