What causes diarrhea in newborns who drink formula milk?

Diarrhea in newborns who consume formula milk may be caused by improper feeding methods, cow's milk protein allergy, lactose intolerance, and other reasons.

1. Improper Feeding Methods

The gastrointestinal system of newborns is not yet fully developed, and their digestive capacity is generally weak. If a newborn consumes a small amount of milk but the caregiver gives too much formula at once, feeds too frequently, or provides formula that is too cold, it may cause gastrointestinal discomfort, which often leads to diarrhea after consuming formula.

2. Cow's Milk Protein Allergy

Newborns' immune systems are not fully mature, and their intestinal barrier function is also typically underdeveloped. After consuming formula, they may develop an allergy to cow's milk protein—especially those with sensitive constitutions, who have a higher likelihood of allergies. This can result in symptoms such as diarrhea, vomiting, and abdominal pain.

3. Lactose Intolerance

If a newborn has insufficient lactase enzyme activity or lacks lactase altogether, lactose intolerance may occur. In such cases, the body cannot properly break down lactose after consuming formula, commonly leading to diarrhea in newborns following formula feeding.