Why do some bowel movements smell like sulphur or rotten eggs?

The smell associated with some bowel movements you mentioned is commonly caused by the presence of sulfur-containing compounds, notably hydrogen sulfide gas (H2S) and certain volatile organic compounds (VOCs) such as methanethiol and dimethyl sulfide. These compounds are produced as a result of the breakdown and bacterial fermentation of dietary fiber and proteins in the large intestine (colon).

Here are some factors that can influence the production of these sulfur-containing compounds and contribute to foul-smelling stools:

1. Diet: Certain foods, such as red meat, garlic, onions, cruciferous vegetables (e.g., broccoli, cabbage), legumes, and high-protein diets, can increase the sulfur content of stools and contribute to their strong odor.

2. Digestion and Absorption: Incomplete digestion or malabsorption of certain nutrients, particularly carbohydrates and proteins, can lead to excessive fermentation in the colon, resulting in the production of foul-smelling compounds.

3. Gut Microbiota: The composition of gut bacteria plays a crucial role in the breakdown of dietary compounds. An imbalance in gut flora or the overgrowth of certain bacteria, such as sulfate-reducing bacteria, can enhance the production of sulfur-containing gases and VOCs.

4. Medical Conditions: Certain medical conditions can also cause foul-smelling stools. For example, irritable bowel syndrome (IBS), inflammatory bowel diseases (e.g., Crohn's disease, ulcerative colitis), celiac disease, and gastrointestinal infections (such as bacterial overgrowth) can alter bowel movements and produce foul odors.

5. Medications: Some medications can affect digestion and alter the composition of gut bacteria, which may contribute to changes in stool odor.

It's important to note that occasional changes in stool odor are usually not a cause for concern and can be related to dietary choices or temporary digestive disturbances. However, persistent or severe foul-smelling stools, especially accompanied by other symptoms like abdominal pain, changes in bowel habits, or unexplained weight loss, warrant a visit to a healthcare professional for further evaluation to rule out any underlying medical conditions.