Can Standard Cow's Milk Formula Worsen Diarrhea in Infants?
In most formula-fed infants with acute diarrhea, standard cow's milk formula can be safely continued and does NOT need to be changed, as children previously receiving lactose-containing formula can tolerate the same product in most instances. 1
When Cow's Milk Formula Is Well-Tolerated
Continue regular cow's milk formula immediately after rehydration in the majority of infants with acute diarrhea, as this approach is clinically well-tolerated and nutritionally advantageous. 1
Despite concerns about acquired lactase deficiency (which occurs in 88% of infants hospitalized with rotavirus diarrhea), lactase deficiency must be distinguished from actual lactose malabsorption—many infants with lactase deficiency will NOT have clinical symptoms. 1
The 2017 IDSA guidelines explicitly state that after rehydration, children previously receiving lactose-containing formula can tolerate the same product in most instances, and diluted formula does not confer any benefit. 1
When to Switch to Lactose-Free Formula
Switch to full-strength lactose-free formula only if the infant develops clear clinical worsening (increased stool output, more watery stools, or prolonged diarrhea) after reintroducing standard cow's milk formula. 2
For bottle-fed infants with confirmed lactose intolerance (defined as exacerbation of diarrhea upon introduction of lactose-containing formula), switch to full-strength lactose-free or soy-based formula immediately—this reduces both stool output and duration of diarrhea by approximately 50%. 1, 2, 3
In cases of acute diarrhea with severe symptoms, lactose-free or lactose-reduced formulas may be considered for 2-4 weeks after rehydration. 4
Do NOT diagnose lactose intolerance based solely on stool pH or reducing substances without clinical symptoms—this leads to unnecessary dietary restrictions. 2
When to Suspect Cow's Milk Protein Allergy
Consider cow's milk protein allergy (CMA) if diarrhea persists beyond 7 days despite appropriate rehydration, or if the infant has additional concerning features beyond simple watery diarrhea. 5, 6
Red Flags for CMA Rather Than Simple Lactose Intolerance:
Blood-streaked stools in an otherwise healthy-appearing infant (allergic proctocolitis). 7
Chronic emesis, failure to thrive, or profuse repetitive vomiting 2-3 hours after milk ingestion (food protein-induced enterocolitis syndrome). 7
Protein-losing enteropathy with hypoalbuminemia, edema, or significant weight loss. 1, 7
Very young age (under 2 months) with persistent diarrhea after acute gastroenteritis—the jejunal mucosa in these infants is particularly vulnerable to cow's milk protein damage. 8
Management of Suspected CMA:
Switch to extensively hydrolyzed formula (eHF) or amino acid-based formula, NOT just lactose-free formula, as lactose-free formulas still contain intact cow's milk proteins. 1, 6
Amino acid-based formulas are more efficient in decreasing parenteral nutrition requirements than extensively hydrolyzed feeds in infants with severe intestinal disease. 1
If breast milk is available, it may be the enteral feed of first choice, as it is associated with shorter duration of parenteral nutrition and improved prognosis in infants with intestinal inflammation. 1
Common Pitfalls to Avoid
Do not routinely switch all infants with diarrhea to lactose-free formula—this is unnecessary in most cases and the older AAP recommendation for gradual reintroduction with diluted formulas is being reevaluated. 1
Do not confuse lactose intolerance with cow's milk protein allergy—they require completely different formulas (lactose-free vs. extensively hydrolyzed/amino acid-based). 6
Do not use soy formula for suspected CMA in very young infants—there may be an increased incidence of soy protein intolerance in newborns with intestinal damage. 1
Do not delay refeeding—full-strength formula (whether regular or lactose-free) should be introduced immediately after rehydration, as fasting increases intestinal permeability and delays mucosal recovery. 1