Partially hydrolyzed formula is infant formula in which the milk proteins have been broken into smaller pieces, making them easier to digest. These formulas are often marketed as “gentle” and sit between standard formula (with intact proteins) and extensively hydrolyzed formula (where proteins are broken down much further). About 27% of cow’s milk-based infant formulas on the market use partially hydrolyzed protein.
How the Proteins Are Broken Down
In standard infant formula, cow’s milk proteins remain whole. In partially hydrolyzed formula, enzymes break those proteins into smaller fragments called peptides. The size of these fragments is what separates partial from extensive hydrolysis: partially hydrolyzed formulas contain most peptides in the 3,000 to 10,000 dalton range, while extensively hydrolyzed formulas break them down to under 3,000 daltons. For context, intact cow’s milk protein is far larger than either.
About 30% of the total protein in a partially hydrolyzed formula is hydrolyzed. In extensively hydrolyzed formulas, that figure is 90%, and amino acid-based formulas are 100% broken down. So partially hydrolyzed formula still contains a significant amount of intact protein alongside the smaller fragments. This matters for both digestion and allergy considerations.
The resulting protein structure may actually resemble the protein in human breast milk more closely than intact whey protein formulas do, which is part of the rationale behind these products.
What It Does (and Doesn’t Do) for Digestion
The “gentle” label suggests easier digestion, but the reality is more nuanced. In a study of newborns, gastric emptying time for partially hydrolyzed formula averaged about 53 minutes, compared to 55 minutes for standard intact protein formula. That difference was not statistically significant. Extensively hydrolyzed formula, by contrast, emptied in about 46 minutes, which was meaningfully faster than both.
Where partially hydrolyzed formula does show a clearer effect is on stool patterns. Infants fed these formulas tend to have softer, more frequent stools compared to those on standard or soy formulas. In one clinical trial of infants with difficult stooling, those on partially hydrolyzed formula averaged about 1.5 stools per day versus 1.0 in the control group, with noticeably softer consistency. If your baby seems uncomfortable with hard or infrequent stools on standard formula, this is one area where a switch could make a practical difference.
Lactose Content Varies by Brand
There’s no single rule for how much lactose partially hydrolyzed formulas contain. Some brands keep full lactose levels, while others reduce lactose significantly. Specialty versions designed for babies with mild gastrointestinal discomfort may contain less than 3.1 grams of lactose per 100 mL, well below what standard formula provides. If lactose content matters for your baby, you’ll need to check the specific product label rather than assuming all partially hydrolyzed formulas are the same.
Allergy Prevention vs. Allergy Treatment
This is the most important distinction to understand about partially hydrolyzed formula: it may help prevent allergies in some cases, but it is not safe for treating an existing cow’s milk allergy.
A meta-analysis of 18 studies involving about 1,000 infants found that high-risk babies (those with a family history of allergies) who were fed partially hydrolyzed whey formula had a 45% lower risk of developing eczema compared to those fed standard cow’s milk formula. This is a meaningful reduction, and it’s the main evidence behind recommendations to consider these formulas for babies with strong family allergy histories.
However, if your baby has already been diagnosed with a cow’s milk protein allergy, partially hydrolyzed formula is not appropriate. Because 70% of the protein remains intact, residual milk proteins, particularly caseins, can still trigger allergic reactions. In one study of 10 children with confirmed cow’s milk allergy, none tolerated partially hydrolyzed whey formula. Zero percent. These children need extensively hydrolyzed or amino acid-based formulas instead.
Fussiness and Colic
Many parents consider partially hydrolyzed formula because their baby is fussy or colicky. The evidence here is mixed and worth parsing carefully. One double-blind trial of 43 colicky infants (crying more than 3 hours a day, at least 3 days a week) found that switching to a whey hydrolysate formula reduced crying by about 63 minutes per day compared to standard formula. That’s roughly an hour less crying daily, which for exhausted parents can feel significant. It’s worth noting, though, that this study used an extensively hydrolyzed whey formula, not a partially hydrolyzed one. The distinction matters because the deeper protein breakdown in extensive hydrolysates likely drives much of the benefit.
Partially hydrolyzed formulas may still help some fussy babies, particularly if the fussiness is related to general digestive comfort rather than an underlying protein sensitivity. But if your baby has severe colic that doesn’t improve after a trial period, a more thoroughly broken-down formula might be the next step to discuss with your pediatrician.
Who These Formulas Are Designed For
Partially hydrolyzed formulas occupy a specific middle ground. They’re generally recommended for infants who show signs of mild formula intolerance, such as gas, fussiness, or hard stools, but who don’t have a diagnosed milk allergy. They may also be considered for babies at high genetic risk for allergic conditions like eczema, where the goal is prevention rather than treatment.
They are not designed for babies with confirmed cow’s milk protein allergy, severe gastrointestinal conditions, or symptoms like blood in the stool, significant vomiting, or failure to gain weight. Those situations call for extensively hydrolyzed or amino acid-based formulas, which break proteins down far enough to avoid triggering immune reactions.
If you’re switching from standard formula, expect to see changes in your baby’s stool within a few days. Softer and slightly more frequent bowel movements are normal and generally a sign the formula is being digested as expected, not a reason for concern.