Digestion vs. Metabolism

The misunderstood relationship between digestion and metabolism

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You can't blame me. The Internet is rife with articles dispelling myths about eating, diets, and nutrition—or what we put into our bodies. But in my line of work, I find that misunderstandings about what happens to our food once we eat it are equally common.

One of the misperceptions I frequently encounter is a false assumption that the rate of one's digestive processes implies something about the rate of one's metabolism. In other words, we are inclined to assume that just because the corn we ate at lunch shows up in our poop before bed, we must have a "fast metabolism."

In fact, however, digestion and metabolism are wholly separate processes, governed by multiple different—albeit sometimes overlapping—influences.

To start, some definitions: Digestion refers to how the body processes food in the gastrointestinal (GI) tract and eliminates food waste via the intestines. Metabolism refers to how the cells utilize the energy we have absorbed from food during digestion.

The rate of one's digestive process is generally measured in terms of "whole gut" transit time: how long it takes for food matter (or its residue) to make it from the mouth all the way to the end of the colon. There are some general ranges of what's considered "normal" at each stage of the process. When food matter travels from the mouth to the colon faster than the norm—a phenomenon which tends to result in loose stools, diarrhea, or brightly-colored (green, yellow) poops—it can be said that one has "rapid transit."

[See What Your Poo Says About You]

There are several factors that influence transit time: diet composition, exercise, functional disorders like Irritable Bowel Syndrome (IBS), and metabolic disorders like thyroid dysfunction or diabetes.

Diets high in insoluble fiber—found, for example, in bran, leafy veggies, seeds, and fruit and veggie skins—can speed up digestive transit. Diets that contain a relatively higher amount of soluble fiber—found in oats, barley, chia seeds, root veggies, and the flesh of pectin-rich fruits like apples—can slow down digestive transit.

[See Is All Fiber Created Equal?]

The rate of one's metabolism is measured in terms of calories (energy) expended over a specified period of time—usually a day—to conduct all of the body's necessary functions while at rest. Your metabolic rate is similarly governed by a host of factors. Age, sex, height, weight, body composition (amount of fat mass vs. muscle mass), presence of fever, and levels of various hormone levels—including thyroid and stress hormones—are all factors in determining the resting metabolic rate.

Vigorous exercise can increase the resting metabolic rate for a window of time even after the physical activity has ceased. Starvation and malnutrition can dampen the metabolic rate. To date, very few dietary components have been shown to measurably speed up metabolism; among those that have been cited—like caffeine, green tea, and capsaicin—the effect has been miniscule and short-lived.

Fiber, a dietary component that speeds up transit time specifically in the colon, is often credited with speeding up the metabolism; in fact, it does no such thing. As described above, fiber only speeds up colonic transit time—it does not influence the rate at which our body's cells utilize food energy. Similarly, taking laxatives to help you go to the bathroom does not speed up the metabolism such that you'd burn more calories than normal.

Why so much confusion between these two distinct bodily processes? There are at least two reasons:

First is the misunderstood phenomenon of the post-meal poop.

In response to the stimulus of your stomach stretching during a meal, an involuntary wave of motion (called peristalsis) happens in the colon to propel food waste forward, essentially to make room for whatever's about to come down the pipeline. This normal phenomenon is called the "gastro-colic reflex," and it can be particularly strong in the morning or after a large meal (or richer restaurant meal). It often results in a person having to go to the bathroom soon after a meal—and in some cases during the meal; this is particularly the case in the morning due to additional triggers like morning hormone levels and coffee intake. For some people, the resulting bowel movement can be loose or urgent, and this often leads them to believe that what they just ate "ran right through" them. In other words, the timing of the poop leads people to falsely conclude that the food they just ate was processed and metabolized within minutes—and expelled immediately. In reality, however, what's coming out is not that same food that just went in! No one's digestive process is that fast!

Second is the coincidence of certain metabolic disorders and GI symptoms.

One of the many symptoms of a metabolic disorder can be a change in transit time. For example, people with hyperthyroidism—an overactive thyroid gland—will have both an increased metabolic rate and be prone to hyper-motility of the gut. This is a fancy way of saying that they will poop a lot more often than normal, and the stool may be loose or watery as the result of too-speedy transit.

People with hypothyroidism—an underactive thyroid gland—may likewise be prone to constipation. People with diabetes—a metabolic condition that adversely affects how dietary sugar is absorbed by cells into usable energy—are prone to developing a sluggish rate of digestive transit as the result of damage to the nerves that control stomach emptying. In these cases, digestive transit time is indeed an indication of one's metabolic state of affairs. But these cases are the exception, not the rule.

In the absence of such metabolic disorders, a healthy person with a so-called "fast metabolism"—who burns a lot of calories while at rest—could be constipated or have a slow digestive transit time. Conversely, someone with a so-called "slow metabolism"—who requires very few calories to maintain his or her body's basic functioning at rest—can poop multiple times per day or suffer from chronic diarrhea as the result of poor diet or IBS.

Whether you're grappling with your weight or struggling to normalize your bowel function (or both), it can be helpful to understand the different factors that influence each of these separate areas and avoid misinterpreting the clues your body offers.

If you're concerned that a possible metabolic disorder could be causing you trouble in both departments, your doctor can evaluate that risk with simple blood tests and refer you to an endocrinologist if needed.

[See 8 Common Digestive Problems and How to End Them]

Hungry for more? Write to eatandrun@usnews.com with your questions, concerns, and feedback.

Tamara Duker Freuman, MS, RD, CDN, is a NYC-based registered dietitian whose clinical practice specializes in digestive disorders, Celiac Disease, and food intolerances. Her personal blog, www.tamaraduker.com, focuses on healthy eating and gluten-free living.