What’s That Smell?
STEVEN SANDBERG-LEWIS,
N.D., DHANP
The odor of stool and intestinal gas are generally unpleasant and often the butt (pun intended) of jokes. Below we will discuss where these odors are created and how this knowledge can help inform hunches as to what is in or out of balance with one’s gastrointestinal system.
Prior to the availability of tests to measure sugar in the urine of diabetics, doctors would taste the patient’s urine to check for a sweet taste, and aid in the diagnosis of diabetes. The breath odor of acetone is still part of the information that can be used in the diagnosis of ketoacidosis in poorly controlled diabetes. It turns out that in similar fashion stool and gas odors can also be informative about digestive health.
Several types of gas are a normal part of a healthy gut. These gases are made by bacteria, archaebacteria and yeast. The gases produced in the gut include hydrogen, methane, hydrogen sulfide and carbon dioxide. Of these, hydrogen sulfide (H2S) is the most likely one to have an odor. In a healthy gut, a small amount of hydrogen sulfide gas is produced by the cells lining the intestine. At high levels, H2S smells like “rotten eggs” while the other gases tend to be rather neutral.
When foods with higher levels of fermentable carbohydrates are consumed, the bacteria, archaebacteria and yeast organisms have more fuel to ferment, creating more gas. Much gas is produced in the small intestine, but in the case of indigestible carbohydrates (fiber), this happens in the colon. Since fiber is not broken down by the human digestive tract, it continues on from the small bowel into the colon, where an inordinate number of residing organisms ferment the fiber into gas. Little odor is produced by any of these gasses, apart from the odor created by excess H2S.
Another source of odor can be produced in the large intestine. Bacteria in the large intestine convert soluble fiber, which is a very particular type of fiber, into substances called short chain fatty acids. The most studied of these short chain fatty acids is called butyric acid (or butyrate). The name butyrate comes from both Latin and Greek words meaning “butter”; maybe not so coincidentally butter contains butyrate. Butter or ghee derived from goat and/or sheep milk have higher butyrate content, and therefore may have a stronger taste and odor.
Short chain fatty acids have important health promoting effects for the cells lining the gut, for example, production of mucus and reduction of inflammation. Short chain fatty acids also promote brain health. A healthy stool derives a large part of its odor from these important chemicals.
So why does some gas and stool smell so much stronger, and in some cases more unpleasant?
I previously mentioned the hydrogen sulfide odor. When higher amounts of this gas are present, as in hydrogen sulfide small intestine bacterial overgrowth, the rotten egg smell may be strong. In addition, if the ability to digest protein is deficient – new odors may be added to stool and gas. Proper protein digestion requires gastric hydrochloric acid (“stomach acid”) as well as pancreatic and brush border enzymes in the small intestine. Proteins are long chains of amino acids. The protein needs to be cleaved into small chains and eventually single amino acids in order to be properly absorbed into the blood from the small intestine.
In cases of hypochlorhydria (low levels of stomach acid) or exocrine pancreatic insufficiency (deficient production of pancreatic digestive enzymes), protein remains more intact, keeping its initial structure. Rather than being absorbed in the small bowel, this undigested protein gets into the colon where huge numbers of bacteria putrefy the protein, rather than ferment it, into extremely malodorous chemicals. Their names, cadaverine and putrescine, are derived from the odor of these chemicals.
Recognizing the types of odors being given off by one’s gas and stool can help one determine where potential issues with gut health originate, and subsequently, inform treatment options for optimal digestive and absorptive health.
Steven Sandberg-Lewis, ND, has been in clinical practice for 43 years, with a focus on functional gastroenterology. He has been a professor since 1985, teaching a variety of courses but primarily focusing on gastroenterology and GI physical medicine. In 2019, Dr. Sandberg-Lewis co-founded Hive Mind Medicine, continuing his specialization in gastroenterology with a focus on reflux, SIBO, inflammatory bowel disease and functional GI disorders. He is currently working on his new book, Getting Real About Reflux, written to help non-medical and medical professionals alike develop a better understanding of diseases affecting the gastrointestinal tract.
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