The odor of the faeces is mainly linked to putrefactive processes that affect the amino acids escaped from intestinal absorption. These nutrients are in fact intercepted by the colon bacteria, undergoing a decarboxylation process that produces the following toxic amines:
arginine → agmatine
cystine and cysteine → mercaptan
histidine → histamine
lysine → cadaverine
ornithine → putrescine
tyrosine → tiratina
tryptophan → indole and skatole
Many of these substances give the stool a particularly unpleasant odor. The stools become rather malodorous even when they remain in the intestine for a long time, as well as in the aforementioned cases of malabsorption (celiac disease, pancreatic insufficiency, intestinal infections, Crohn's disease, ulcerative colitis, short bowel syndrome, cystic fibrosis, liver disease and biliary tract and gastric diseases accompanied by hypochlorhydria). When the malabsorption is mainly protein, the smell of the faeces is nauseating, while it becomes acrid and penetrating when large quantities of unabsorbed or badly digested sugars and starches arrive in the colon, then fermented by the local flora with the production of volatile fatty acids.
The vegetarian's diarrhea and stools are poorly smelling.
Feces with a smell of fish or sperm are characteristic of bacillary dysentery, cholera and more generally of mucorrhea.
In summary, the smell of feces is therefore influenced by eating habits (amount of protein, aromatic foods such as garlic or onions, etc.), by intestinal bacterial flora, by the state of health and by particular habits (such as smoking or chewing tobacco) . In general, the consumption of small and well balanced meals, without associating large quantities of "incompatible" foods (see dissociated diet), helps to optimize digestive processes, reducing the incidence of disorders such as meteorism and flatulence, and giving the stool a sui generis smell.
Any case in which the bad odor of the stool is accompanied by symptoms such as black, tarry or pale stools, presence of blood in the same, fever, chills, severe abdominal cramps, abundant presence of mucus in the stool and weight loss.
