Yes, soy formula can cause diarrhea in some infants. The most common reasons are a sensitivity or allergy to soy protein, difficulty digesting the carbohydrate source in the formula, or simply the normal adjustment period when switching from one formula to another. In most cases, mild loose stools after starting soy formula resolve within a few days, but persistent watery or bloody diarrhea signals something more serious.
Why Soy Formula Can Trigger Loose Stools
Soy protein isolate makes up about 14 to 16% of soy formula by weight. It contains compounds like phytic acid and protease inhibitors that can interfere with digestion, particularly in very young guts. Phytic acid blocks the absorption of certain minerals, which is why soy formulas are heavily fortified with iron, zinc, calcium, and a long list of vitamins. These added nutrients are necessary but can themselves contribute to digestive upset in some babies.
The carbohydrate source matters too. Soy formulas use either sucrose or glucose polymers instead of lactose (the sugar found in cow’s milk formula). Research comparing soy formulas with different sugars found that infants fed a sucrose-based soy formula had significantly lower stool output and shorter episodes of diarrhea than those fed a lactose-containing soy formula. Infants on the lactose version were also 3.5 times more likely to become dehydrated again after feeding resumed. If your baby already has some degree of lactose intolerance, picking the right soy formula matters.
The Adjustment Period After Switching
Any time you change formulas, your baby’s digestive system needs a few days to adapt. Increased gas, fussiness, and looser stools are common during this window. Most babies adjust within about five days. To ease the transition, you can mix equal parts of the old and new formula for two to three days before switching entirely. For larger bottles, replace one scoop at a time every two days. If loose stools persist well beyond that first week, the issue is likely the formula itself rather than the adjustment process.
Soy Protein Allergy and Cross-Reactivity
Many parents switch to soy formula because their baby reacted poorly to cow’s milk formula. But this swap doesn’t always solve the problem. Among infants with a confirmed cow’s milk allergy driven by the immune system’s typical allergic pathway (IgE-mediated), roughly 10 to 14% will also react to soy protein. The situation is worse for babies whose cow’s milk allergy works through a different immune mechanism (non-IgE-mediated): about half of those infants will also be allergic to soy.
The American Academy of Pediatrics actually recommends against using soy formula as a first-line substitute for babies with confirmed cow’s milk protein allergy. Instead, extensively hydrolyzed or amino acid-based formulas are preferred because the proteins in those products are broken down small enough that the immune system doesn’t recognize them.
FPIES: A More Serious Reaction to Watch For
Food Protein-Induced Enterocolitis Syndrome, or FPIES, is an uncommon but important condition where an infant’s gut reacts severely to specific food proteins, including soy. It was originally identified in babies under three months old who were fed cow’s milk or soy-based formula and developed poor growth, persistent diarrhea, and signs of gut inflammation.
FPIES can take two forms. In the acute form, vomiting starts one to four hours after a feeding, followed by diarrhea within 5 to 10 hours. There are no hives or breathing problems, which distinguishes it from a typical allergic reaction. In the chronic form, which develops when a baby drinks the trigger formula daily over weeks, symptoms build gradually: frequent watery diarrhea (sometimes bloody or mucousy), worsening vomiting, poor weight gain, and eventually anemia from ongoing blood loss in the stool. Chronic FPIES resolves within days once the trigger food is removed, and symptoms flare again if it’s reintroduced.
Normal Soy Formula Stools vs. Diarrhea
Formula-fed babies typically produce yellow or brown stools with a thick, pasty consistency, similar to peanut butter. They tend to pass fewer but larger and smellier stools than breastfed babies. This is the baseline you’re looking for, regardless of whether the formula is cow’s milk or soy-based.
Diarrhea looks different: the stool is runny and watery, green, yellow, or brown, and comes more frequently than your baby’s normal pattern. One loose stool after a formula change isn’t cause for alarm. Multiple watery stools over the course of a day, especially if they continue beyond the first week on the new formula, suggest your baby isn’t tolerating it well.
Signs the Diarrhea Needs Attention
Persistent diarrhea in an infant can lead to dehydration quickly because babies have less fluid reserve than older children. Watch for fewer than four wet diapers in 24 hours, no tears when crying, a dry mouth or tongue, sunken eyes, or a sunken soft spot on top of the head. Grayish skin or a noticeably faster heartbeat are later signs that dehydration is becoming serious. If your baby shows any of these alongside ongoing diarrhea, they need medical evaluation promptly.
Blood or mucus in the stool, weight loss, or vomiting that accompanies the diarrhea also point toward a protein allergy or FPIES rather than a simple adjustment issue. In these cases, stopping the soy formula and switching to a hypoallergenic alternative is the typical next step.