The Large Intestine

In healthy individuals after eating a mixed meal, it usually takes about 4-5 hours for that meal to completely empty the stomach and 5-6 hours for that meal to empty the small intestine. This all can vary depending on what you eat and how well your gut muscles are working. 
 
Eventually, the remaining unabsorbed food matter (like fiber) and water, move through the ileocecal valve, the doorway from the small intestine to the large intestine. As the unabsorbed matter passes through the ileocecal valve, the large intestine  monitors how much undigested material there is. If there are increasing amounts of undigested food, especially fats, it signals to the small intestine to S L O W  D O W N.  This is called the ileal break and is necessary to help maximize our absorption of nutrients. This mechanism also reduces our appetite. This is why when individuals struggle with diarrhea, they often do not have much appetite. 
 
Once the unabsorbed liquid food matter passes through into the large intestine a few things happen:

• Water, sodium, and chloride are absorbed to create a dehydrated mass including sloughed off dead gut cells, fiber, waste (hormones, chemicals, toxins), bile salts, bacteria, and small amounts of unabsorbed nutrients and food. Normal stool is about 75% water and 25% solids.
• Most of our gut bacteria reside in the large intestine and ferment undigested food especially fiber. I call this our FERMENTATION STATION. However, undigested protein and fats also act as fuel for gut microbes (this generally leads to a more dysbiotic profile). 
• When bacteria ferment fiber, they produce short chain fatty acids and vitamins such as biotin and vitamin K2. 
• The dehydrated mass of waste is then compacted in the rectum, along with bilirubin from broken down red blood cells (which makes poop brown), and is released through the anus, as POOP!

​On average in healthy folks it takes about 30-40 hours for the mass to travel the entire 1.5 meters of the large intestine, going up along your right side (called the ascending colon), across your upper abdomen (called the transverse colon) and back down your left side (called the descending colon). In individuals struggling with idiopathic constipation, their colonic transit time can be greater than 100 hours! When colonic transit time is slowed and stool is stagnant in the colon, it allows chemicals, toxins, and hormones originally bound for elimination, to be reabsorbed into circulation. This can increase your risk of hormonal imbalance and impaired detoxification due to increased stress on the liver and kidneys. Also, slowed transit time can contribute to diverticulosis and colon cancer, as well as the overgrowth of bacteria and fungus in both the small and large intestines. Furthermore, slowed motility usually presents along with hard to pass stools and straining, leading to uncomfortable hemorrhoids. 
 
The total amount of time for a meal to be digested and absorbed and the remainder excreted as a bowel movement is called your gastrointestinal transit time (GTT). If your transit time is <12 hours you are likely struggling with nutrient malabsorption, if your transit time is >48 hours then you are likely struggling with constipation. If it takes more than 72 hours for food to travel from mouth to toilet, there is significant backup. I find that around 24 hours is usually the sweet spot for most—what you ate yesterday, leaves you today!
 
Are you curious what your stool transit time is? Generally, if you eat a higher fiber diet, stool transit time should be faster than if you eat a low fiber diet. However, if you eat a high fiber diet and still struggle with constipation something else is going on. This is the perfect time to work with a gut health dietitian for guidance. 

TESTING YOUR STOOL TRANSIT TIME
Although not the gold standard, testing stool transit time at home can give you a rough estimate on your personal window. Sesame seeds remain undigested and pass through the gut intact. White hulled sesame seeds are more easily seen than dark sesame seeds. Alternatively, eat a steamed red beet.
 
THE SESAME SEED (or red beet) CHALLENGE

1. Mix  ~2 tablespoons of white sesame seeds into a glass of room temperature water or eat a steamed medium red beet.
2. Drink it up and write down the date and time in the chart below. Then wait and monitor every stool until you see the white sesame seeds or colored red stool. 
3. Write down the date and time and calculate the difference in hours. This is your personal stool transit time. 
4. Continue to monitor your stools until all the sesame seeds or red stool has been evacuated. Make note of last bowel movement with sesame seeds or red color. 

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References:
Gastrointestinal Tract: How Long Does it Take?
http://www.vivo.colostate.edu/hbooks/pathphys/digestion/basics/transit.html
Ileal Brake: neuropeptidergic control of intestinal transit.
https://pubmed.ncbi.nlm.nih.gov/16968603/
Physiology, Large Intestine. 
https://www.ncbi.nlm.nih.gov/books/NBK507857/
Measuring colonic transit time in chronic idiopathic constipation.
https://pubmed.ncbi.nlm.nih.gov

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