When navigating the world of infant formulas, the ingredient list can be a bit overwhelming. All formulas are mainly composed of carbohydrates, proteins and fats, plus vitamins, minerals and other additives.  Today we’re looking at whey and casein, which make up the protein portion of infant formulas. It’s so important to understand this because the protein source is closely linked to the tolerability of formula, meaning most dairy sensitivity is due to the protein type. And we know all parents want a baby that tolerates his formula well!

So, let’s take a closer look at protein in formula. We will discuss their importance in baby’s growth and development, the different ratios among different milks and why they matter.

What are Whey and Casein? How do They Behave in the Body?

Both breast milk and cow milk-based formula contain whey and casein as the main forms of proteins. The main difference between the two is the way they behave when they encounter an acid (think stomach juices). Whey proteins stay in the liquid form, and because they stay in the liquid form, they are easier to digest and empty from the stomach faster than casein proteins. They are also less likely to cause allergic reactions. Casein proteins will curdle when they hit the stomach and will take longer to empty.  Casein proteins are considered more “reactive,” and will likely be responsible for some of the more common digestion problems in babies. 

The Benefits of Whey

Why is whey important? We know it makes up part of the protein profile in breast milk and cow’s milk. Here are some of the benefits of whey: 

  • It is a high-quality source of protein
  • Promotes muscle growth and development
  • Supports circulatory function
  • Supports the body’s antioxidant defenses
  • It is filling and helps reduce hunger
  • Promotes healthy weight and metabolism

What About the Benefits of Casein? 

  • Takes longer to digest, keeping your baby full
  • Important for muscle growth and development
  • Supports the immune system
  • Helps maintain healthy triglyceride levels
  • Promotes antioxidant function
  • Promotes healthy weight and metabolism

Let’s Talk Ratios

Whey and casein have a similar profile when it comes to benefits, but because they are digested differently, their ratios are important in understanding infant formula tolerability. In other words, what ratio might be part of a formula that would make it easiest to digest?

  1. Human Milk

Most articles quote breast milk as having a whey: casein ratio of 60:40 percent. In reality, as with many other nutrients in breast milk, the ratio fluctuates, anywhere between 70:30 and 80:20 in early lactation and decreases to 50:50 in late lactation. 

  1. Cow’s Milk

Cow’s milk, which is the basis of most infant formulas, has a whey: casein ratio of close to 20:80 percent. Part of the reason we don’t give infants straight up cow’s milk is because it would be tough for them to digest this amount of casein.

What Ratio Should I look for in My Baby’s Formula?

We discussed the difference between whey and casein, how they are digested, and the different ratios in human vs. cow’s milk. Since whey is easiest to digest and less likely to be the culprit of an allergic reaction, it makes sense to look for a formula that has a higher concentration of whey, which would be more similar to breast milk. But the tricky part is that infant formulas will likely say “cow’s milk protein” on the label, but the ratio is not disclosed! So how do you figure this out?

Remember that ingredients are listed in the order of amount. The first ingredient takes the most volume in the recipe. Then others will follow in order of concentration. If the formula is only labeled as having “cow’s milk protein,” look for additional whey in the list of ingredients. This will mean the proportion of whey to casein will be higher, therefore more easily digestible.

Keep in mind there are formulas on the market that are made up of 100% whey, which makes them easy to digest and empty the stomach faster. These formulas might be appropriate for babies at risk for milk protein allergy. 

Our Final Thoughts

  1. Milk protein (breast or cow’s) is made up of whey and casein. Whey stays in the liquid form and empties faster, and casein curdles and seems to be the culprit in most formula intolerance.  
  2. The ratios vary in breastmilk, but they average 60:40 whey: casein, while cow’s milk has a ratio of close to 20:80 whey: casein. 
  3. Look for cow milk-based infant formulas with a higher ratio of whey for easier digestion. Look at the list of ingredients and for additional whey added to the formula to increase its proportion. 
  4. All babies are different, and your little one will tell you if he’s happy with what he’s drinking!      


Review of Infant Feeding: Key Features of Breast Milk and Infant Formula. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4882692/

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