Stool pH High or Low: What Your Result Means (Acidic vs Alkaline)

Stool pH measures the acidity or alkalinity of your stool. A normal result falls between 6.1 and 7.9 — slightly acidic to neutral. A low (acidic) result is most commonly linked to carbohydrate malabsorption, infection, or fast gut transit. A high (alkaline) result may indicate reduced beneficial bacteria, low fibre intake, or other changes in gut fermentation. Your result should always be interpreted alongside other findings from your stool test.

At a Glance

• Stool pH measures the acidity of your stool — a reflection of gut fermentation, microbial activity, and digestion
• Normal range: 6.1–7.9 (slightly acidic to neutral); average in healthy adults is approximately pH 6.6
• Low (acidic) result — below 6.0 — most often indicates carbohydrate malabsorption, infection, or rapid gut transit
• High (alkaline) result — above 7.9 — may reflect reduced beneficial bacteria, low dietary fibre, dysbiosis, or antibiotic use
• On lab reports, stool pH may be labelled as "reaction: acidic" or "reaction: alkaline" rather than a numeric pH value
• Infants and young children have a different normal range — typically more acidic (pH 4.5–7.0) than adults
• Stool pH is most meaningful when interpreted alongside the full stool test, symptoms, and clinical history

What is stool pH?

pH is a scale from 0 to 14 that measures acidity or alkalinity. A pH below 7 is acidic, a pH above 7 is alkaline, and pH 7 is neutral. Stool pH measures where your stool falls on this scale and reflects the chemical environment in your colon.

Normal stool is slightly acidic. This acidity is primarily produced by the fermentation of dietary carbohydrates and fibre by beneficial bacteria in the colon. When bacteria ferment carbohydrates, they produce short-chain fatty acids (SCFAs) — including acetate, propionate, and butyrate — which lower the pH of the stool. This slightly acidic environment is considered healthy and helps suppress the growth of pathogenic bacteria and yeast.

When fermentation is reduced — due to low fibre intake, disrupted gut microbiota, or antibiotic use — less acid is produced and stool pH rises toward alkaline. Conversely, when undigested sugars reach the colon in large amounts (as in carbohydrate malabsorption), bacteria ferment the excess, producing more acid and driving pH down.

Stool pH is measured using pH paper or a pH meter applied directly to a stool sample. The test is most accurate on liquid or semi-liquid samples. Results may be reported as a numeric value or as a qualitative result — "acidic," "neutral," or "alkaline."

On your lab report, this test may appear as: Stool pH, Fecal pH, pH (Stool), Stool Reaction, Reaction: Acidic, Reaction: Alkaline, or Stool Acidity.

What does "reaction: acidic" or "reaction: alkaline" mean on a stool test?

Many standard stool tests — particularly in Asia, the Middle East, South America, and parts of Europe — report stool pH as a qualitative result rather than a number. Instead of a pH value, you may see:

• Reaction: Acidic — stool pH below the lab's lower reference limit (typically below 6.0 or 6.5)
• Reaction: Alkaline — stool pH above the lab's upper reference limit (typically above 7.5 or 8.0)
• Reaction: Neutral — stool pH within or close to the neutral range

These labels mean the same thing as a numeric pH result — they just present the finding in a simplified form. "Reaction: acidic" indicates low stool pH; "reaction: alkaline" indicates high stool pH. The clinical interpretation of each is covered in the sections below.

Stool pH normal range

Note: Reference ranges vary between laboratories and test methods. Always compare your result to the range on your own lab report. GI Effects panels (Genova Diagnostics) and similar functional stool tests may use slightly different reference intervals.

The average stool pH in healthy adults is approximately 6.6 — slightly acidic. This slight acidity reflects healthy bacterial fermentation producing short-chain fatty acids in the colon.

What does a low (acidic) stool pH mean?

A stool pH below 6.0 — reported as a low numeric value or as "reaction: acidic" — indicates the stool is more acidic than normal. The most common causes involve increased fermentation of undigested carbohydrates or rapid transit through the gut.

Carbohydrate and sugar malabsorption

• Lactose intolerance — undigested lactose reaching the colon is fermented by bacteria, producing excess acid; a stool pH below 6.0 is a common finding and a stool pH below 5.5 is considered suggestive evidence of sugar malabsorption
• Other disaccharide intolerances — sucrase-isomaltase deficiency and similar conditions
• Malabsorption syndromes — coeliac disease, short bowel syndrome, pancreatic insufficiency
• High carbohydrate intake — excessive dietary sugars can overwhelm normal absorption

Infections and gut inflammation

• Bacterial infections — including E. coli, Salmonella, and other enteric pathogens
• Viral gastroenteritis — particularly rotavirus in infants and children
• Parasitic infections

Small intestinal bacterial overgrowth (SIBO) Abnormally high numbers of bacteria in the small intestine ferment carbohydrates before they reach the colon, producing excess acid and driving stool pH down. SIBO is associated with high carbohydrate diets, low stomach acid, reduced gut motility, and conditions including chronic fatigue syndrome, fibromyalgia, and IBS.

Rapid gut transit / diarrhea Fast transit time reduces the time available for normal fermentation and absorption, often resulting in more acidic stool. This is why diarrhea — regardless of cause — is commonly associated with low stool pH.

Overgrowth of acid-producing bacteria An imbalance in the gut microbiome favouring acid-producing bacteria such as lactic acid bacteria can drive stool pH down even without obvious malabsorption.

What does a high (alkaline) stool pH mean?

A stool pH above 7.9 — reported as a high numeric value or as "reaction: alkaline" — indicates reduced acid production in the colon, most commonly due to disrupted microbial fermentation.

Reduced beneficial bacteria and dysbiosis The most common cause of alkaline stool is a deficiency of the beneficial bacteria that normally produce short-chain fatty acids. Without adequate SCFA production, the colon environment becomes less acidic. Dysbiosis — an imbalance between beneficial and pathogenic microorganisms — is frequently associated with alkaline stool pH.

Low dietary fibre Dietary fibre is the primary substrate for colonic fermentation. A low-fibre diet reduces fermentation and SCFA production, reducing colonic acidity. Fibre types particularly associated with lowering stool pH include oat bran, wheat bran, and prebiotics such as fructooligosaccharides (FOS).

Antibiotic use Antibiotics disrupt the gut microbiome, reducing populations of fermenting bacteria and impairing colonic fermentation. This commonly raises stool pH, particularly during and shortly after antibiotic courses.

Secretory diarrhoea In some forms of secretory diarrhoea — particularly without food intake — impaired fermentation leads to alkaline stool despite loose consistency.

Colitis and gut inflammation Inflammatory bowel conditions including colitis can alter the colonic environment and raise stool pH.

Villous adenoma This type of colonic polyp can cause secretory diarrhoea and is associated with alkaline stool pH.

Colorectal cancer risk association High fecal pH has been identified in research as a potential risk factor for colorectal cancer, though the relationship is considered secondary rather than causative — high pH may reflect the same microbiome and dietary changes that underlie cancer risk rather than being a direct cause. Elevated stool pH alone is not diagnostic of colorectal cancer.

What do specific stool pH values mean?

pH 5 or below Significantly acidic. Strong indicator of carbohydrate malabsorption — particularly lactose intolerance, sucrase deficiency, or other sugar absorption disorders. Also seen in acute infectious gastroenteritis, particularly rotavirus in young children. Perianal skin irritation and burning are common at this pH level.

pH 6 (e.g. "stool pH 6") Mildly acidic. May reflect borderline carbohydrate malabsorption, early SIBO, mild diarrhoea, or dietary factors. A pH of 6 in isolation is not always clinically significant, but warrants interpretation alongside symptoms and other stool findings.

pH 6.5 Within or at the lower end of the normal range for most labs. Generally considered acceptable in adults. May reflect a slightly higher carbohydrate load or mild dysbiosis without definitive pathology.

pH 7 ("stool pH 7 means") Neutral — within normal range for adults. No specific clinical concern.

pH 7.5 At the upper limit of normal for many labs. May reflect early reduction in beneficial bacteria, mildly low fibre intake, or post-antibiotic changes. Often not clinically significant in isolation.

pH 8 ("stool pH 8 means") Alkaline — above normal for most adult reference ranges. Consistent with reduced SCFA production, low fibre diet, dysbiosis, antibiotic use, or colitis. Warrants interpretation alongside other stool markers including calprotectin, secretory IgA, and microbiome profile if available.

Stool pH in infants and children

Infants and young children normally have more acidic stool than adults, and reference ranges differ significantly.

Breastfed infants typically have stool pH between 4.5 and 5.5 — strongly acidic. This is normal and reflects the fermentation of lactose and other breast milk carbohydrates by bifidobacteria. Acidic stool in breastfed infants is actually a marker of a healthy microbiome.

Formula-fed infants tend to have less acidic stool (pH 5.5–7.0) than breastfed infants, reflecting differences in gut microbiome composition.

Older children (beyond infancy) gradually move toward the adult range of 6.0–7.5.

A stool pH above 7.5 in infants may indicate formula intolerance, infection, or other gut disturbance. A very low pH (below 5.0) in infants with diarrhoea is consistent with sugar malabsorption or viral gastroenteritis and should be investigated.

Why does acidic stool cause burning?

Acidic stool — particularly at a pH of 6.0 or below — can cause irritation, burning, or excoriation of the perianal skin during and after a bowel movement. This is sometimes described as:

• Burning sensation when pooping
• Butt burns after a bowel movement
• Perianal skin soreness or redness

The mechanism is direct chemical irritation: highly acidic stool in contact with perianal skin breaks down the skin barrier, particularly with repeated or profuse diarrhoea. This is especially common in infants with acidic stools from viral gastroenteritis or lactose malabsorption — and is the reason for the diaper rash often seen with diarrhoeal illness.

Managing the underlying cause of acidic stool (addressing malabsorption, infection, or rapid transit) is the most effective way to resolve the burning. Protective barrier creams applied to the perianal area can provide symptomatic relief while the cause is treated.

What to do next based on your result