The characteristic foul smell of human feces is a well-known biological reality, but an odor described as sweet, fruity, or sickly sweet is a significant deviation from the norm. This change suggests an alteration in the chemical composition of the waste, often resulting from changes in diet, gut bacteria, or the body’s digestive processes. Understanding the origins of normal and abnormal fecal odors can help determine whether the sweet smell is a harmless anomaly or a sign of an underlying medical issue.
The Chemistry of Typical Fecal Odors
The typical, unpleasant odor of human stool results from bacteria in the large intestine breaking down unabsorbed food components. This process, known as putrefaction, primarily targets proteins and amino acids that escape digestion in the small intestine. The breakdown produces a collection of volatile organic compounds (VOCs) that are highly odorous.
Key among these foul-smelling compounds are nitrogen-containing molecules like skatole and indole, derived from the amino acid tryptophan. Sulfur-containing compounds, such as hydrogen sulfide and various methyl sulfides, also contribute distinct rotten-egg or cabbage-like notes. When gut bacteria focus on protein breakdown, these pungent compounds dominate the final scent of the stool.
Dietary and Metabolic Causes of Sweetness
When the balance shifts away from protein putrefaction, the odor profile changes toward a sweeter scent, similar to the feces of breastfed infants. Breast milk is high in lactose, which is often not fully digested by the infant’s immature system. This excess sugar is fermented by gut bacteria, producing lactic acid and short-chain fatty acids that result in a milder, often sweet smell.
In adults, a high intake of rapidly fermentable carbohydrates and sugars can lead to a similar fermentation process in the colon. If a person consumes large amounts of sugars or artificial sweeteners that are not fully absorbed, these compounds feed the gut bacteria. The bacteria then produce sweet-smelling fermentation byproducts. This temporary dietary shift can alter microbial activity, leading to a noticeable change in the stool’s odor.
Ketosis
A metabolic cause of a sweet odor is the body entering a state of ketosis, often induced by a low-carbohydrate diet or fasting. When the body burns fat instead of glucose for energy, it produces ketones, including acetone. While ketosis is more commonly known to cause a fruity smell on the breath and in the urine, this metabolic shift can sometimes influence the composition of bodily wastes, potentially contributing to a sweet-smelling stool.
Gastrointestinal Conditions and Sweet-Smelling Stool
For an adult, a persistently sweet or fruity stool odor often points toward a problem with nutrient absorption in the gastrointestinal tract. Conditions causing malabsorption, such as celiac disease or chronic pancreatitis, allow sugars and starches to pass undigested into the large intestine. The resulting excessive fermentation of these carbohydrates generates gases and volatile compounds that can smell sweet or sour.
Specific carbohydrate intolerances, such as lactose or fructose intolerance, operate through this same mechanism. The unabsorbed sugar draws water into the intestine, causing diarrhea, and is then fermented by bacteria. This results in a watery, sweet-smelling stool and indicates that the necessary digestive enzymes are insufficient or absent.
A serious infectious cause of a sickly sweet or strong odor is an infection with Clostridioides difficile (C. diff) bacteria. This infection typically occurs following a course of antibiotics, which disrupt the gut’s normal bacterial population. The toxins produced by C. diff can cause severe, watery diarrhea with an odor some people describe as sickeningly sweet.
When to Seek Medical Guidance
While a single instance of a sweet odor might be linked to a recent dietary change, any persistent or recurring sweet smell in an adult’s stool warrants a conversation with a healthcare provider. The most urgent concern is when the sweet smell is accompanied by other symptoms suggesting a serious metabolic or infectious issue.
Immediate medical attention is necessary if the sweet odor is accompanied by symptoms like persistent fever, severe abdominal pain, unexplained weight loss, or severe dehydration. For individuals with diabetes, a fruity odor combined with excessive thirst, frequent urination, or confusion could signal Diabetic Ketoacidosis (DKA). If the sweet-smelling stool is loose and frequent, especially after recent antibiotic use, an evaluation for a C. diff infection is appropriate.