You might want to rethink that burger.
The body digests different macronutrients at different rates, and the combination of protein, carbohydrates and fats in a meal affects how quickly it moves through your system. Try an experiment to see this first-hand: Today, eat an apple by itself—chances are, you’ll feel hungry an hour later. Tomorrow, eat an apple with a serving of peanut butter, and notice how you feel satiated longer. The peanut butter adds fat and protein to your snack, helping tide you over until dinner. But what takes place during digestion to make this true? First, you have to understand how each macronutrient is processed.
“Carbohydrates are the body’s primary fuel source, which means that they need to be readily available to provide fuel to every part of the body (and, in particular, your brain and working muscles) at any given time,” says Susan Bowerman, MS, RD, CSSD, FAND, director of worldwide nutrition education and training at Herbalife Nutrition.
As such, carbs have the shortest digestion time—and refined ones, like crackers and cookies, are digested quicker than unprocessed carbs, like the apple, which tend to be rich in fiber—so they can provide quick energy. Carbs also give the body an advantage in stockpiling any excess, says Bowerman, so it can pull from storage as needed (say, when you’re hitting a SoulCycle class after work).
Protein, on the other hand, is digested more slowly than carbohydrates. The digestion process doesn’t begin until it hits the stomach, as the molecules’ large size requires more work from the body to break them down. The full breakdown of proteins into amino acids, or “building blocks” that make up muscles, occurs in the small intestine, where they’re absorbed through the intestinal lining into the bloodstream, says Bowerman.
Dietary proteins aren’t intended to be used for energy, she notes, but instead manufacture hundreds of proteins in the body, from hair, skin and muscle to hormones and enzymes critical to healthy body function. This process happens continually, so proteins aren’t in demand the way carbs are.
Fats take the longest to digest—not only are they the last of the macronutrients to leave the stomach, but they also don’t go through the majority of the digestive process until they hit the small intestine. “Since fat and water don’t mix, the processing of dietary fat takes longer, because the end products have to be water-soluble before they can be transported in the watery environment of the bloodstream,” says Bowerman.
During digestion, fats are broken down into fatty acids and glycerol, which are then absorbed by the intestine; then they must be reassembled with some proteins to be transported into the blood. Because this can take a long time—up to six hours, says Bowerman—the body doesn’t use fats to provide quick energy, and as such, they become a primary way we store calories, as body fat.
The problem with understanding how each macronutrient is digested is that we rarely consume macronutrients in isolation—so how long it actually takes to digest a meal can vary widely (for example, even a high-fat food like peanut butter also contains protein, and even a few carbs). On average, it takes 24-72 hours for a meal to move completely through your digestive tract, says Mary Creel, a registered dietitian with eMeals. Yet that can vary greatly from person to person; digestion is affected by your sleep, stress level, water intake, activity level, gut health, metabolic rate and age, says Creel. A study from the Mayo Clinic even found a huge difference in digestion time among genders: The average transit time through the large intestine was 33 hours for men and 47 hours for women.
Keeping those points in mind, Creel breaks down average digestion time for some common foods:
A bowl of oatmeal: 1-2 hours
A complex carb, oatmeal is a great source of soluble fiber and has a high satiety ranking, as it soaks up water and delays emptying into the stomach. It has a longer digestion time than a refined cereal, like Frosted Flakes.
An apple: 1 hour
This also has a high satiety ranking, but due to the high water content, it might only take an hour to digest. Have a source of protein along with this carb to stay fuller longer.
A slice of pizza: 6-8 hours
Pizza has carbs in the crust, sauce, and vegetable toppings, plus high fat and protein in the cheese, and any meat toppings. The higher fat means it takes longer to digest.
A salad: 1 hour
If you add an oil-based dressing or a protein like cheese or chicken, digestion will take longer. While a salad on its own will digest quickly, the high water and fiber content of lettuce and vegetables helps you feel full.
A hamburger: 24 hours to 3 days
It depends on the size and toppings of the burger, but a meal like this requires a lot of digestive energy to break down the big molecules in protein and fat. Almost hard to believe it can take days to digest, isn’t it?
A slice of cheesecake: 12 hours
You can count on a full 12 hours for this one to break down, due to the high fat content with eggs and cream cheese (aka, don’t plan on hitting the gym a few hours after dessert, or you’ll experience some serious stomach pains.)
How to Speed Up Digestion
Drink at least 8-10 cups of water a day to keep things moving, and regularly consume fruits and vegetables with high water content, like watermelon or salad. Creel also recommends taking a daily probiotic for gut health. One additional tip: Consuming all your calories within a 12-hour time frame—a concept the science community calls “time-restricted feeding”—could also be key to optimum digestive health, according to recent research.
“It’s all about staying in sync with natural rhythms of your body clock,” says Dr. Pamela Peeke, MD, MPH, FACP, FACSM, chair of the Jenny Craig Science Advisory Board. She advises her clients to follow the 12-12 rule, meaning a 12-hour window of eating followed by a 12-hour window of fasting. (For example, if you finish dinner at 7:00 p.m., you don’t eat again until 7:00 a.m. the next morning.) This method allows your body to optimally digest your meal and convert from glucose metabolism to fat metabolism, using fat as fuel. Ultimately, she says, you should look at eating and digestion as a way to nourish your body and provide it with the fuel it needs.