Stool pH (faecal pH)

The pH value indicates how acidic or alkaline a sample is. In stool it reflects the environment in the colon, which is determined mainly by the metabolic products of the gut bacteria. It is measured in a stool sample, usually together with the digestive residues, the microbiome analysis and inflammation markers. The value is not stable: it varies with the diet of the past few days, with stool consistency and with the duration of gut transit. In the first part of the colon the environment is usually more acidic than in the last section, because more carbohydrates are fermented there.

What determines the pH in the gut

When bacteria ferment fibre, resistant starch and undigested sugars, short-chain fatty acids such as acetate, propionate and butyrate as well as lactic acid are produced; they acidify the gut contents. When bacteria break down protein instead, ammonia and amines are formed, which have an alkaline effect. A slightly acidic environment favours bifidobacteria and lactobacilli and makes it harder for many acid-sensitive microbes to grow. It keeps calcium and magnesium dissolved so that they can also be absorbed in the colon. In addition, at acidic pH, ammonia is present mainly as ammonium, which passes less easily through the gut wall into the blood. The gut lining takes up the fatty acids quickly; how acidic the stool is therefore depends on production and uptake at the same time.

Stool pH too acidic (low) – what happens in the body?

Clearly acidic stool develops when large amounts of carbohydrates are fermented in the colon. This is the case, for example, when lactose, fructose or sorbitol are incompletely absorbed in the small intestine, with very fast gut transit or with diarrhoea, when the fatty acids are no longer taken up by the lining in time. The many acids and the gases produced alongside them lead to bloating, rumbling and soft, often sour-smelling stool. The skin around the anus can become irritated and sore, which is particularly noticeable in babies and toddlers. Lower enzyme output from the pancreas can also contribute to more acidic stool via undigested carbohydrates.

Stool pH alkaline (high) – what happens in the body?

Alkaline stool shows that little acid is being produced in the colon or that protein putrefaction predominates. This occurs with a low-fibre, high-protein diet, when carbohydrate-fermenting bacteria such as bifidobacteria are scarce, for example after antibiotics, and with slow gut transit and constipation, because the stool is then broken down for longer. Protein putrefaction produces not only ammonia but also phenols, indoles and hydrogen sulphide, which make stool and wind smell foul. In an alkaline environment, acid-sensitive microbes can multiply more easily. Together with indican, skatole, iso fatty acids and the microbiome analysis, a picture emerges of which fermentation processes predominate in the gut.

The interpretation is given in your report.