Know Your Macronutrients: Proteins
- 5 December 2024


Introduction
Dietary protein makes up one of three macronutrients (components of foods that are broken down and absorbed by the body to support the maintenance of life) in the diet, crucial for every cell in the body, and important as a fuel source.1 Infants require dietary protein for growth and development of all organs and organ systems.2 Amino acids are the building blocks of dietary proteins, and dietary protein quality influences functional health outcomes including, but not limited to, digestion, sleep, and dietary tolerance. To learn more about protein structure and classifications, see the ‘Fast Facts: Proteins’ box for more detailed information.
Essential amino acids cannot be made intrinsically and must be consumed in the diet, while nonessential amino acids can be manufactured by the body. A third classification, conditionally essential amino acids, can be made by the body except under times of illness, stress or infancy. This foundational knowledge is crucial to understanding how proteins are classified – complete versus incomplete. Complete proteins contain all 9 essential amino acids required from food, whereas incomplete proteins lack one or more essential amino acids.
Complete proteins are found in animal products like meat, chicken, eggs, and dairy products, (and some non-animal sources such as soy) while incomplete proteins include foods like pulses (beans, lentils, and peas), whole grains, nuts, and seeds.3 Human milk is composed of whey proteins, such as alpha-lactalbumin, lactoferrin and beta-lactoglobulin), casein proteins (such as alpha-caseins and beta-caseins) and free amino acids.4 Infant formula also contains whey proteins, casein proteins and free amino acids, but often in different ratios than those found in human milk. Early human milk is whey-dominant with an 80:20 whey:casein ratio and mature human milk has a 60:40 whey:casein ratio.4 Both whey and casein proteins that come from cow’s milk are whole (intact) and complete proteins (provide all 9 EAAs).
FDA Requirements
The United States Food and Drug Administration (FDA) requires protein content of 1.8 g/100 kcal, but not exceeding 4.5 g/100 kcal, in infant formula, and the source of protein must be at least nutritionally equivalent to casein, one of the two major protein components of human milk.5 While the FDA outlines an acceptable range for protein quantity in infant formula, manufacturers can use discretion to determine whether their products should err on the lower or higher end of the acceptable range. This is especially relevant considering some research that now suggests a link between higher protein consumption in early life and longer-term health outcomes including metabolic syndrome and obesity.6 Infant formula companies also have significant flexibility in terms of protein source. Given that proteins found in infant formula differ in terms of type and structure, formulas may contain any combination of the following: whey (including whey fractions such as alpha lactalbumin and lactoferrin), casein, and soy protein. Selected proteins may include any combination of intact proteins, partially hydrolyzed proteins (polypeptide chains and peptides), or extensively hydrolyzed proteins (peptides + free amino acids). Formulas with solely free amino acids are also available and recommended for infants unable to tolerate extensively hydrolyzed or partially hydrolyzed proteins. Cow’s milk formulas often differ in terms of their whey:casein ratios, as well.
Dietary Protein for Infant Health: A Shift in Paradigm
It has been historically understood that dietary protein served as building blocks for growth. While this sentiment still holds true today, more recent discoveries in human milk science now tell a more nuanced story around the functional roles of dietary proteins. Dietary protein is known to affect digestion and tolerance, growth, sleep, the immune system, and more. Whey and casein are the two predominant protein types in human milk. Whey proteins remain liquid in the stomach and therefore digest more quickly than casein protein, which can form curds that take longer to digest.7 Additionally, selective proteolysis occurs in the mammary gland, meaning that human milk-fed infants receive a combination of peptides and intact proteins which provide nutritional, immunomodulatory, and antimicrobial benefits.8 Pre-hydrolysis also increases gastric emptying speed which is associated with softer stools,9 and reduced spit up.10
Regarding specific proteins, lactoferrin is the most abundant whey protein in colostrum, and known for its involvement in iron transport and immune function.11 Alpha-lactalbumin is the most abundant whey protein in mature milk, making up 20-25% of the total protein content and an important contributor of essential amino acids12 (more on this star ingredient below). Other proteins, such as osteopontin and lysozyme, also present in human milk help support elements of infant development including immune and intestinal development.4,13
Nutrient Spotlight: Alpha-lactalbumin
Alpha-lactalbumin is the most abundant protein in mature human milk, making up ~22% of the total whey proteins.12 Among its many roles, alpha lactalbumin is important for lactose development within the mammary gland14 (lactose is the predominant carbohydrate source in human milk), and it provides functional benefits to babies including increased infant formula tolerance, and reduced spit up.15,16 Furthermore, alpha lactalbumin is a rich source of amino acids (the building blocks of protein), including tryptophan, which supports infant sleep.14 Since the addition of alpha-lactalbumin to infant formula helps manufacturers more easily achieve required amounts of essential amino acids in the final formulation, products with added alpha-lactalbumin can be lower in total protein content.
Relevance For Your Clinical Practice
In order to give evidence-based infant formula advice and recommendations to families in your practice, it is important to understand the potential health implications of different nutrients found in commercially available products. Here are health outcomes associated with some of the ingredients discussed:
Infant formula with a whey-predominant composition has been shown to support ease of digestion.17
Alpha-lactalbumin is a whey protein and a rich source of essential amino acids (in other words, it is a high-quality protein). Since the addition of alpha-lactalbumin to infant formula helps manufacturers more easily achieve required amounts of essential amino acids in the final formulation, products with added alpha-lactalbumin can be lower in total protein content. Studies have shown that infant formula with reduced protein and added alpha-lactalbumin supports increased energetic efficiency (i.e., a ratio of weight gin to the average energy or protein intake per day) from improved protein composition.18
Studies have demonstrated beneficial outcomes on gut health,19 cognition,20 and immune function21 in infants fed infant formula containing lactoferrin, a whey protein.
Products incorporating intact proteins are usually well-tolerated by healthy, term infants. Outcomes that have been evaluated in the literature include spit up, crying/fussiness, gas, stool frequency and consistency.22
Formulas that include a percentage of partially hydrolyzed proteins or are all partially hydrolyzed proteins may result in increased tolerance, less spit up and softer stools.23
Indicated for infants with cow’s milk protein allergy or infants with maldigestion or malabsorption.24
These formulas are indicated for severe cases of cow’s milk protein allergy or infants with severe maldigestion or malabsorption.25
Complete proteins: Meat, chicken, eggs, and dairy products, (and some non-animal sources such as soy)
Incomplete proteins: Pulses (beans, lentils, and peas), whole grains, nuts, seeds
Human milk is composed of whey proteins, such as alpha-lactalbumin, lactoferrin and beta-lactoglobulin), casein proteins (such as alpha-caseins and beta-caseins) and free amino acids.
Infant formula also contains whey proteins, casein proteins and free amino acids, but often in different ratios than those found in human milk.
Ingredients described here represent a subset of many other factors, including other macronutrients and bioactive factors, that may impact infant health. All ingredients in commercially available infant formula are approved for use by the FDA. Beyond acknowledged health outcomes, infant formula purchasing decisions should be made holistically and with respect to financial means and accessibility, the infant’s tolerance of the product, personal values, and more.
Wu G. Dietary protein intake and human health. Food Funct. 2016;7(3):1251-1265. doi:10.1039/C5FO01530H
Garlick PJ. Protein requirements of infants and children. Nestle Nutr Workshop Ser Pediatr Program. 2006;58(58). doi:10.1159/000095009
Hoffman JR, Falvo MJ. Protein – Which is Best? J Sports Sci Med. 2004;3(3):118. Accessed April 10, 2024. /pmc/articles/PMC3905294
Donovan SM. Human Milk Proteins: Composition and Physiological Significance. Nestle Nutr Inst Workshop Ser. 2019;90:93-101. doi:10.1159/000490298
21 USC 350a: Infant formulas. Accessed April 10, 2024. https://uscode.house.gov/view.xhtml?req=(title:21%20section:350a%20edition:prelim)
Koletzko B, Demmelmair H, Grote V, Totzauer M. Optimized protein intakes in term infants support physiological growth and promote long-term health. Semin Perinatol. 2019;43(7). doi:10.1053/J.SEMPERI.2019.06.001
Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufrère B. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proceedings of the National Academy of Sciences. 1997;94(26):14930-14935. doi:10.1073/PNAS.94.26.14930
Dallas DC, Guerrero A, Khaldi N, et al. Extensive in vivo human milk peptidomics reveals specific proteolysis yielding protective antimicrobial peptides. J Proteome Res. 2013;12(5):2295. doi:10.1021/PR400212Z
Fabrizio V, Harris CL, Walsh KR, Wampler JL, Zhuang W, Wu SS. Softer More Frequent Stools in Infants With Difficult Stooling Fed Hydrolyzed Protein Formula With Added Prebiotics: Randomized Controlled Trial. Front Pediatr. 2022;10. doi:10.3389/FPED.2022.894626
Vandenplas Y, Leluyer B, Cazaubiel M, Housez B, Bocquet A. Double-blind comparative trial with 2 antiregurgitation formulae. J Pediatr Gastroenterol Nutr. 2013;57(3):389-393. doi:10.1097/MPG.0B013E318299993E
Lönnerdal B, Du X, Jiang R. Biological activities of commercial bovine lactoferrin sources. Biochem Cell Biol. 2021;99(1):35-46. doi:10.1139/BCB-2020-0182
Lönnerdal B, Lien EL. Nutritional and physiologic significance of alpha-lactalbumin in infants. Nutr Rev. 2003;61(9):295-305. doi:10.1301/NR.2003.SEPT.295-305
Jiang R, Lönnerdal B. Biological roles of milk osteopontin. Curr Opin Clin Nutr Metab Care. 2016;19(3):214-219. doi:10.1097/MCO.0000000000000275
Layman DK, Lönnerdal B, Fernstrom JD. Applications for α-lactalbumin in human nutrition. Nutr Rev. 2018;76(6):444. doi:10.1093/NUTRIT/NUY004
Davis AM, Harris BJ, Lien EL, Pramuk K, Trabulsi J. Alpha-lactalbumin-rich infant formula fed to healthy term infants in a multicenter study: plasma essential amino acids and gastrointestinal tolerance. Eur J Clin Nutr. 2008;62(11):1294-1301. doi:10.1038/SJ.EJCN.1602848
Lien EL, Davis AM, Euler AR. Growth and safety in term infants fed reduced-protein formula with added bovine alpha-lactalbumin. J Pediatr Gastroenterol Nutr. 2004;38(2):170-176. doi:10.1097/00005176-200402000-00013
Martin CR, Ling PR, Blackburn GL. Review of Infant Feeding: Key Features of Breast Milk and Infant Formula. Nutrients. 2016;8(5). doi:10.3390/NU8050279
Fleddermann M, Demmelmair H, Grote V, Nikolic T, Trisic B, Koletzko B. Infant formula composition affects energetic efficiency for growth: The BeMIM study, a randomized controlled trial. Clinical Nutrition. 2014;33(4):588-595. doi:10.1016/j.clnu.2013.12.007
Johnston WH, Ashley C, Yeiser M, et al. Growth and tolerance of formula with lactoferrin in infants through one year of age: double-blind, randomized, controlled trial. BMC Pediatr. 2015;15(1). doi:10.1186/S12887-015-0488-3
Colombo J, Harris CL, Wampler JL, et al. Improved Neurodevelopmental Outcomes at 5.5 Years of Age in Children Who Received Bovine Milk Fat Globule Membrane and Lactoferrin in Infant Formula Through 12 Months: A Randomized Controlled Trial. J Pediatr. 2023;261. doi:10.1016/J.JPEDS.2023.113483
Björmsjö M, Hernell O, Lönnerdal B, Berglund SK. Immunological Effects of Adding Bovine Lactoferrin and Reducing Iron in Infant Formula: A Randomized Controlled Trial. J Pediatr Gastroenterol Nutr. 2022;74(3):e65. doi:10.1097/MPG.0000000000003367
Berseth CL, Mitmesser SH, Ziegler EE, Marunycz JD, Vanderhoof J. Tolerance of a standard intact protein formula versus a partially hydrolyzed formula in healthy, term infants. Nutr J. 2009;8(1):27. doi:10.1186/1475-2891-8-27
Vandenplas Y, Salvatore S. Infant Formula with Partially Hydrolyzed Proteins in Functional Gastrointestinal Disorders. Nestle Nutr Inst Workshop Ser. 2016;86:29-37. doi:10.1159/000442723
Lemale J, Decline JL, Dive-Pouletty C, Touboul C, Pichon N, Dupont C. Managing Cow’s Milk Protein Allergy with an Extensively Hydrolyzed Formula: Results from a Prospective, Non-Interventional Study in France (EVA Study). Nutrients. 2022;14(6). doi:10.3390/NU14061203
Meyer R, Groetch M, Venter C. When Should Infants with Cow’s Milk Protein Allergy Use an Amino Acid Formula? A Practical Guide. J Allergy Clin Immunol Pract. 2018;6(2):383-399. doi:10.1016/J.JAIP.2017.09.003
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