July 26, 2023

How Exactly Does Digestion…Work?

This piece originally appeared on katiecouric.com.

What happens to your food after you eat it? In our series “The Exact Science Of,” we aim to find out.

You’re eating a sandwich: You chew, you swallow…and then what happens? Unless you’re a doctor, or have a great memory of high school biology class, what occurs inside your stomach after you eat something is probably a mystery. So we asked Paul Limburg, MD, MPH, board-certified gastroenterologist and chief medical officer for screening at Exact Sciences, to help us answer the all-important digestion question.

In the latest edition of our “Exact Science Of” series, we’re going to tackle this topic, and we’re leaving no stomach unturned. From what happens to your food after you eat it, to why some people experience that dreaded phenomenon we call flatulence, we’re going to get down and dirty about those mysterious bodily functions that happen inside all of us.

What is digestion?

Dr. Limburg says digestion is “a wonderfully complex system,” but let’s start with the basics. When you eat something solid, the first thing you’ll do (unless you’re hot-dog-eating champion Joey Chestnut) is thoroughly chew it. “This starts the process of breaking down food so the body can absorb it,” explains Dr. Limburg. This process is also aided by enzymes in your saliva.

Once you swallow, what you’ve eaten goes down what Dr. Limburg affectionately calls your “food tube,” otherwise known as your esophagus. It then enters your stomach, where acid and enzymes from the pancreas and other sites break that food down further. Dr. Limburg explains, “The stomach then squeezes, to make what are known as peristaltic waves, which help break the food into smaller pieces and advance those pieces farther down the intestinal tract.”

Next up, your food hits the small intestine, followed by the large intestine. This is where most of the nutrient absorption happens, and anything your body can use is absorbed across the gut lining, then moves into your bloodstream. “As this process continues,” Dr. Limburg explains, ”the leftover broken-down product travels through the colon, where water is reabsorbed into the body, and bacteria can further break down the residual food particles. Then eventually, what’s left is eliminated.” And there you have it.

How long does it take to digest food?

The answer to this one shocked us: “Typically, the food has to be churned in the stomach for about six to eight hours for it to pass into the small intestine,” says Dr. Limburg. And that’s just the beginning. The rest of the digestion process can take up to 36 hours, so that’s about 42 hours in total. That means when you have a bowel movement, it probably contains something you ate about two days ago.

Why do we get diarrhea?

If the whole digestive process typically takes days, you might be wondering why a stomach ache can lead to the process…speeding up. Unfortunately, there’s no single cause for diarrhea, but Dr. Limburg shared a few common examples: “Diarrhea can be caused by too much of something in the intestinal tract that the body can’t effectively break down, or from excess fluid within the intestine. Microbes present in various locations throughout the intestinal tract can also contribute to loose, frequent bowel movements. This could be from a virus, bacteria, or even a parasite.” (Just one more reason to steer clear of the ham and mayo sandwich that’s been sitting out in the conference room since yesterday.)

Any changes in your bowel movements, including diarrhea, can also be a sign of a more serious issue, like colon cancer. If you’re experiencing symptoms like frequent diarrhea, blood in your stool, fevers, or weight loss, contact your primary care provider immediately.

Which foods help digestion?

The answer to this one is a tale as old as time — think of anything deemed “healthy” on those posters in your grade school cafeteria. “Whole grains, fruits, and vegetables, or anything with high amounts of vitamins and iron will provide more good stuff for your body to absorb,” Dr. Limburg says.

Fiber is also key when it comes to effective digestion. “Fiber helps to bulk up what’s in the intestinal tract,” says Dr. Limburg, “so water can stay in the intestines for longer. This allows gravity to move the stool more easily through the body, which then makes bowel movements easier and more regular.” So go ahead and sprinkle another spoonful of bran flakes on your yogurt.

And although certain dairy products can also have health benefits, if you’re lactose-intolerant, you’ll do more harm than good by loading up on yogurt or cheese.

Speaking of cheese — specifically, “cutting it” — why do some people experience more flatulence than others, and what makes that gas smell?

It’s time to talk about tooting. “Passing gas is partially related to what we eat and how quickly we eat it,” says Dr. Limburg. So if you’re a fast eater, you’re going to swallow more air, and that air may come out either end of your body.

It’s also got to do with one of our favorite topics, the microbiome. “The profile of the bacteria that live in our intestinal tracts are pretty unique to each individual,” Dr. Limburg says. “So your body’s bacteria and the way it interacts with food in the large intestine may impact the frequency of flatulence, which will differ from person to person.” So the next time somebody clears out a room with gas, blame their gut biome.

Other foul smells could be a sign of trouble. “Odor can also be a signal of something abnormal in the digestive process, like trouble with the pancreas,” says Dr. Limburg. “When the pancreas doesn’t secrete enough enzymes to digest fatty foods, for example, a person can experience malabsorption, which can cause diarrhea, or foul-smelling flatulence or stool.”

Are there any supplements that can help with digestion?

The short answer is yes…but proceed with caution. “I’m not a disbeliever in supplements,” says Dr. Limburg, “but you should give them some careful thought, and ideally request a medical consultation to make sure you’ll benefit from any supplement.”

Two of the most common supplements for digestion are prebiotics and probiotics. Prebiotics act as “food” for the good bacteria you already have in your gut, to help them multiply and work more efficiently. Probiotics are living strains of these bacteria that will add to those you’ve already got.

Many prebiotics are essentially dietary fiber, so they’re found in natural sources like whole grains. Unless you’ve got an allergy or intolerance, it’s pretty likely they’re going to be safe for you to add to your daily routine. Probiotics are also pretty safe to use, as long as your body can tolerate them. Dr. Limburg explains: “You can find probiotics naturally in things like yogurt, but some people have enzyme deficiencies that won’t allow them to break down dairy products. This might not impact the potential benefits of a probiotic on its own, but the delivery method — meaning, naturally occurring versus in supplement form — could give you some negative symptoms. Before you make any major changes in your diet that could alter your microbiome, speak with your healthcare provider.”

How do issues with digestion and the gut biome relate to cancer?

“There are some bacteria that could be harmful, and epidemiology studies suggest these could be associated with a higher risk of conditions like colon cancer,” explains Dr. Limburg. “We’ve found some causal links between these microbes and disease, but there isn’t enough evidence to point to a single cancer-causing organism, or to explain how select bacteria might increase disease risk for some but not all people. It’s likely that these differences develop when a person is young and their gut microbiome profile is set, and then can very slowly impact the body over a long period of time. If you’re sick or start a new medication that disrupts the microbiome, it could change your bacterial balance, along with some of the important digestive processes that help to maintain good health.” This possibility highlights the importance of getting regularly screened for colorectal cancer.

Digestion is a complicated process that isn’t entirely the same for each individual. Understanding how the digestive system works can help you to determine whether something is wrong, how to potentially fix any issues, and when you should seek medical care. This magical internal system, from “top to tail,” confirms the old adage: “You are what you eat.”

The information provided is not clinical, diagnostic, or treatment advice. You should consult with a qualified healthcare provider about any questions you may have regarding a medical condition or treatment. Never disregard professional medical advice or delay in seeking it because of something you have read in this article.