Normal stool pH is typically about 6.5–7.5, meaning slightly acidic to near-neutral. Values vary with diet (high‑carbohydrate intake can lower pH), gut microbiota composition, and infections. Markedly acidic or alkaline stools may signal malabsorption, bacterial overgrowth, or other gastrointestinal problems and should be evaluated by a clinician if persistent. Measurement generally requires lab testing of a fresh sample.

Low stool pH usually reflects excess acid from undigested carbohydrates being fermented by gut bacteria. Causes include lactose or fructose malabsorption, small intestinal bacterial overgrowth, recent antibiotic use, viral or bacterial gastroenteritis, and pancreatic exocrine insufficiency. Conditions like celiac disease can contribute. In infants, breast‑milk fermentation often produces acidic stools; testing helps identify the cause.

Acidic stool often means excess unabsorbed sugars or an infection and typically causes foul, watery diarrhea. Short-lived cases from diet or mild gastroenteritis aren’t usually serious. Seek medical care if diarrhea is severe, persistent (>48 hours), contains blood, causes dehydration, high fever, severe abdominal pain, or affects infants/elderly. Tests and treatment (rehydration, antibiotics, or managing malabsorption) may be needed.

Stool pH testing helps identify causes of diarrhea and malabsorption. Acidic stools suggest carbohydrate malabsorption (for example, lactose intolerance) or excess bacterial fermentation, while neutral/alkaline stools argue against those causes. It’s a simple, noninvasive, inexpensive screening tool used in infants and adults to support diagnosis, guide further testing, and monitor response to dietary or medical treatment.

Alkaline stool (pH above 7) is not usually considered "good" — it often signals carbohydrate malabsorption (e.g., lactose intolerance), bacterial overgrowth, or infections that cause diarrhea. Occasional mild shifts aren’t alarming, but persistent alkaline stools, especially with loose stools, weight loss, or blood, warrant medical evaluation and stool testing to identify causes and guide treatment.

To lower stool pH, increase fermentation in the colon by eating more soluble and fermentable fiber (fruits, vegetables, legumes, whole grains, resistant starch like cooled potatoes and underripe bananas) and prebiotics (onion, garlic, leeks) to boost short‑chain fatty acid production. Add probiotics/fermented foods (yogurt, kefir), limit high‑fat/high‑protein processed foods, avoid unnecessary antibiotics, stay hydrated and introduce changes gradually.