A burnt, smoky, or chemical-like odor during a bowel movement can be unsettling. Stool naturally smells unpleasant due to bacterial fermentation byproducts, but a change in aroma signals a shift in the chemical compounds produced in the digestive tract. This phenomenon is typically a benign, temporary consequence of diet or medication, though it may occasionally indicate an underlying issue. The odor usually relates to sulfur-containing gases, helping determine if the change is due to recent food intake or a more complex digestive process.
Common Dietary and Lifestyle Causes
The most frequent reason for a burnt or acrid smell in feces is the consumption of foods rich in sulfur compounds. Gut bacteria metabolize these compounds, releasing volatile organic compounds like hydrogen sulfide gas. This gas is often perceived as a rotten egg smell but can be concentrated enough to smell chemical or burnt. Cruciferous vegetables, such as broccoli, cauliflower, and cabbage, are common offenders due to their high sulfur content, as are allium vegetables like garlic and onions.
High-protein diets can also contribute to this odor because proteins contain sulfur-bearing amino acids. When the small intestine fails to fully digest a large protein load, the undigested portions reach the colon where bacteria ferment them into odorous waste products. Similarly, certain medications and supplements can alter the chemical composition of stool or disrupt the natural balance of the gut microbiome. Broad-spectrum antibiotics, for instance, can eliminate beneficial bacteria, allowing odor-producing microbes to flourish and changing the overall gaseous profile.
Specific supplements high in sulfates, such as glucosamine, chondroitin, or iron supplements containing ferrous sulfate, can produce a sulfur-like or metallic odor. Iron, in particular, reacts in the gut as the body processes and excretes excess minerals, creating distinct smells. These diet and supplement-related changes are usually short-lived, resolving quickly once the specific food or compound is reduced or removed from the diet.
Digestive Processing and Malabsorption Issues
When the digestive system cannot properly break down nutrients, it creates a mechanism for the production of strongly odorous compounds regardless of the diet. This failure in processing, known as malabsorption, means that fats, carbohydrates, or proteins pass into the large intestine undigested. The colonic bacteria then ferment these excess nutrients, leading to a significant increase in gas production with an unusual profile.
Malabsorption of fats, specifically, often results in a condition called steatorrhea, where the stools are greasy, pale, bulky, and particularly foul-smelling. The unabsorbed fat content can cause the stool to float or stick to the toilet bowl, and the putrid smell stems from the bacterial breakdown of the fatty acids. Carbohydrate malabsorption, such as in lactose intolerance, causes undigested sugars to ferment, leading to gas, bloating, and explosive, odorous diarrhea.
Conditions that affect the organs responsible for digestion, like chronic pancreatitis, can lead to exocrine pancreatic insufficiency, meaning insufficient digestive enzymes are released to break down food. Small Intestinal Bacterial Overgrowth (SIBO) is another cause, where an excess of bacteria in the small intestine prematurely ferments food. This produces high levels of gases that can be perceived as burnt or chemical.
When This Odor Signals a Medical Concern
While a temporary change in fecal odor is often benign, a persistent, unexplained burnt or chemical smell, especially when accompanied by other symptoms, warrants medical evaluation. One serious cause of a strong, metallic, or burnt odor is gastrointestinal bleeding, where oxidized blood passing through the digestive tract creates a distinct smell. This may result in stools that appear black and tarry, known as melena, which requires immediate medical attention.
Other red flag symptoms associated with a concerning change in stool odor include:
- Sudden and unexplained weight loss
- Chronic diarrhea or constipation
- Persistent abdominal pain
- Fever
Chronic inflammatory conditions like Crohn’s disease or ulcerative colitis, as well as infections from pathogens like Clostridium difficile (C. diff) or Giardia, can significantly alter the gut environment and produce unusually foul odors. If the change in odor persists for more than a few days, or if it is part of a cluster of symptoms, consult a healthcare provider.