When you're tucking into one of your favorite foods — perhaps a steaming bowl of ramen or spectacularly cheesy slice of pizza — you're probably not thinking about the journey each morsel is about to take through your digestive system.
Of course, what started out as a delectable meal will eventually end up at the bottom of a toilet bowl. But how long does it take to digest food, exactly?
The answer to this question is more complicated than you might think. Different types of foods are broken down and absorbed by the body at different rates, meaning some parts of the meal may be entering the large intestine when other parts are still in the stomach. It's also common for healthy people to digest food at slightly different rates, according to Colorado State University.
Scientists have conducted studies assessing "gut transit time," or how long it takes a substance to move through the whole digestive tract, using ingestible capsules that can be tracked throughout their journey. These studies suggest that it can take anywhere from 0.4 to 15.3 hours for food to leave the stomach, and from 3.3 to 7 hours for it to pass through the entire small intestine. The leftover, indigestible parts of food then enter the large intestine, where they may remain for approximately 15.9 to 28.9 hours, according to a review published in the Journal of Clinical Medicine in 2023.
Related: Why can't humans digest corn?
Foods rich in dietary fiber, protein, complex carbohydrates and fats tend to take longer to digest than foods low in these nutrients, Dr. Nina Nandy, a gastroenterologist based in Texas and American Gastroenterological Association spokesperson, told Live Science by email.
"Fiber adds bulk to the diet, which slows down the movement of food through the digestive tract," she said. "Heavily processed foods tend to be digested more quickly as they lack this nutrient." Meanwhile, while they can process less-nutrient-dense foods fairly quickly, the stomach and small intestine take more time to break down foods rich in proteins and fats into nutrients that can be used by the body.
Similarly, complex carbohydrates — like those found in whole grains, legumes and starchy vegetables — take longer to digest than simple sugars. That's because complex carbs are made of long, complex chains of three or more types of sugar molecules, while simple sugars contain only one or two sugars.
"The body has to break [complex carbs] down into simple sugars before absorption can happen," Nandy said. (Fiber is a complex carb that can't be broken down at all.)
Lifestyle factors also affect gut transit time. Chewing thoroughly and staying hydrated can help speed up the digestive process by increasing the food's surface area for digestive enzymes and helping to soften the food particles, respectively, Nandy said. In addition, "exercise helps increase gut motility and promotes peristalsis, which is the rhythmic contraction of digestive muscles," Nandy said. Conversely, peristalsis can slow down during periods of inactivity.
A person's age and stress levels can also affect digestion. With age, adults tend to produce less stomach acid and digestive enzymes, while their guts become less motile, she said. "Stress and anxiety can also increase gut transit time by altering gut motility and reducing gastrointestinal blood flow," she added. Having a "nervous stomach" reflects an activation of the fight-or-flight system, which generally suppresses digestion in the stomach and small intestine while stimulating the large intestine.
And finally, certain medical conditions and medications can either speed or slow digestion. For example, diabetes is the most common cause of gastroparesis, or "delayed stomach emptying," which makes food linger in the stomach for a long time. Certain medications, including opiates and anticholinergic drugs, which suppress nerve signals responsible for involuntary muscle movements, can slow gut transit and cause constipation, Nandy said.
This article is for informational purposes only and is not meant to offer medical advice.
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Anna Gora is a health writer at Live Science, having previously worked across Coach, Fit&Well, T3, TechRadar and Tom's Guide. She is a certified personal trainer, nutritionist and health coach with nearly 10 years of professional experience. Anna holds a Bachelor's degree in Nutrition from the Warsaw University of Life Sciences, a Master’s degree in Nutrition, Physical Activity & Public Health from the University of Bristol, as well as various health coaching certificates. She is passionate about empowering people to live a healthy lifestyle and promoting the benefits of a plant-based diet.