Formulas that skip palm oil, include prebiotic fiber, and use partially hydrolyzed whey protein are the most likely to help a constipated infant pass softer stools. No single “constipation formula” works for every baby, but understanding which ingredients soften stool and which harden it can help you choose wisely.
Constipation in formula-fed babies is common and usually not dangerous. It’s also one of the top reasons parents switch formulas. Before making a change, it helps to know what’s actually happening in your baby’s gut and which ingredient swaps have real evidence behind them.
Why Palm Oil Matters More Than You Think
The single biggest ingredient difference between formulas that cause hard stools and those that don’t is the fat source. Many formulas use palm olein oil because its fatty acid profile resembles breast milk on paper. In practice, though, it behaves very differently during digestion.
The key fatty acid in palm oil, palmitic acid, sits in a position on the fat molecule that causes it to break free during digestion and bind with calcium in the gut. This creates calcium soaps, which are essentially tiny chunks of calcium-fat that can’t be absorbed. They make stools harder, drier, and more difficult to pass. A meta-analysis of randomized trials found that infants fed palm oil formulas had measurably firmer stools compared to those fed formulas without palm oil, with a statistically significant difference on standardized stool consistency scales.
The fix is straightforward: look for a formula that does not list palm olein oil (sometimes labeled “palm oil” or “palm olein”) as a primary fat source. Formulas that use blends of other vegetable oils like coconut, soy, and high-oleic sunflower oil tend to produce softer stools. This is one of the most evidence-backed changes you can make.
Prebiotics That Soften Stools
Breast milk naturally contains a complex mix of sugars that feed beneficial gut bacteria. Formula makers try to replicate this effect with prebiotic fibers, most commonly galacto-oligosaccharides (GOS) and fructo-oligosaccharides (FOS). These aren’t digested in the upper gut. Instead, they travel to the large intestine where they feed bacteria that produce short-chain fatty acids, drawing water into the stool and making it softer.
The ratio used in most studied formulas is 90% GOS to 10% FOS. In a randomized trial of infants with constipation, those given FOS had a mouth-to-anus transit time of about 22 hours compared to nearly 25 hours in the control group. That difference of a few hours may sound small, but in a constipated baby it can mean the difference between straining and discomfort versus a normal bowel movement. When shopping, check the ingredient list for these prebiotic fibers. Not all formulas include them, and the ones marketed for digestive comfort are more likely to.
Partially Hydrolyzed Whey Protein
Standard formulas use intact cow’s milk proteins, either whey or casein or a mix. Partially hydrolyzed formulas break those proteins into smaller pieces before they reach your baby. This appears to help constipation through a couple of mechanisms. Smaller protein fragments increase the osmotic load in the intestines, which draws in more water. They also reduce the activity of certain compounds derived from intact milk protein that can slow gut motility.
There’s another possible benefit: some babies have a low-grade sensitivity to intact cow’s milk protein that hasn’t risen to the level of a diagnosed allergy. In these infants, the immune response can slow digestion and alter gut motility. Partially hydrolyzed protein may ease this without requiring a full switch to an extensively hydrolyzed or amino acid formula.
A clinical trial tested a formula combining partially hydrolyzed whey protein with prebiotics in infants who had difficult stooling. Babies on this formula produced softer, more frequent stools compared to those on a standard formula. The combination of hydrolyzed protein and prebiotic fiber appears to work better together than either change alone.
What About Lactose-Free or Low-Lactose Formulas?
Parents sometimes reach for a low-lactose formula hoping it will help constipation, but the evidence doesn’t support this. In a clinical trial of a partially hydrolyzed formula with reduced lactose and probiotics, stool characteristics did not meaningfully change over two weeks. The formula did help with vomiting, gas, and crying, often within the first few days, but constipation stayed about the same.
Lactose actually has a mild stool-softening effect. It’s a sugar that promotes water retention in the intestines and supports the growth of beneficial bacteria. Removing it can theoretically make constipation worse, not better. Unless your baby has a diagnosed lactose intolerance (which is rare in infancy), switching to a lactose-free formula for constipation is unlikely to help.
Probiotics as an Add-On
Some formulas now include probiotics, and certain strains have shown real benefits for constipation. The most studied strain for infant bowel regularity is Lactobacillus reuteri DSM 17938. In a double-blind, placebo-controlled trial, infants receiving this probiotic had significantly more bowel movements than those on placebo by week two, and the difference held through week eight.
The catch: probiotics in that study improved bowel movement frequency but not stool consistency. Your baby may go more often without the stools themselves becoming softer. This makes probiotics a useful complement to other formula changes (like removing palm oil or adding prebiotics) rather than a standalone solution. Probiotic drops are also available separately if you’d rather not switch formulas entirely.
Formulas That May Make Constipation Worse
If your baby is on an anti-reflux formula thickened with rice starch or carob bean gum, that could be contributing to the problem. Thickened formulas are designed to stay down, but the same properties that reduce spit-up can slow gut transit and firm up stools. If your baby needs a reflux formula and is also constipated, talk to your pediatrician about whether the trade-off is worth it or whether a different approach to reflux might work.
Casein-dominant formulas (where the protein is mostly casein rather than whey) also tend to produce firmer stools. Whey-dominant or whey-only formulas are generally a better choice for babies prone to constipation.
How to Switch and What to Expect
You can typically switch formulas without a gradual transition, though some parents prefer to mix the old and new formulas over three to four days. Either approach is fine. What matters more is giving the new formula enough time to work. Monitor your baby’s digestion for at least 10 days before deciding whether the switch helped. Some gas, mild spit-up, and temporary stool changes are normal during the adjustment period and usually settle within the first week.
If you’re choosing a new formula specifically for constipation, prioritize these features in order of evidence strength: no palm olein oil, added prebiotic fiber (GOS/FOS), and partially hydrolyzed whey protein. A formula that combines all three addresses the problem from multiple angles. Several “comfort” or “gentle” formulas on the market include this combination, though you’ll need to check ingredient lists carefully since marketing terms aren’t standardized.
When Formula Alone Isn’t Enough
The European Society for Paediatric Gastroenterology, Hepatology and Nutrition notes that functional digestive issues like constipation are normal in infants and typically don’t require medical treatment. For breastfed babies experiencing constipation, switching to formula is not recommended, as breast milk generally produces softer stools than any commercial formula.
If your formula-fed baby is still constipated after a well-chosen formula switch and a couple of weeks of adjustment, the issue may go beyond what dietary changes can address. Persistent constipation with hard, pellet-like stools, visible straining with every bowel movement, or blood in the stool warrants a conversation with your pediatrician, who may recommend additional interventions beyond formula selection.