BodyRockers, beware! There's no end of fad diets that conflate improving digestion with improving metabolism to make a quick buck. These tacts are so successful because they play on two common weaknesses: first, our insecurities about our bodies. Even the most shredded people often think they could lose one or two more pounds. The second weakness is our ignorance about our own bodies. While there's interplay between the various functions of both digestion and metabolism, they are not the same thing.
It's not hard to see how this misunderstanding might happen: the biggest similarity between these two systems is that they concern what happens to food once it's in your body. Other than that, however, the don't have much functionality in common. Let's take a look at the differences so you can better understand how they work. This will help you make more informed choices about how (and what) you eat — and this will help you lose unwanted body fat. No fad. Just fact.
Digestion refers to how your body processes food in your gastrointestinal tract (GI tract) and gets rid of food waste through the intestines.
Digestion is generally measured by how long it takes food to move from your mouth to your colon's end. Factors that inform digestion include the composition of your diet, your exercise, disorders like IBS and diabetes. It is worth mentioning here that diabetes is a metabolic disorder though...but we'll get back to that.
Metabolism involves the way in which your cells use the energy you have absorbed from food during the digestive process.
Metabolism is measured by how much energy (calories) your body expends over a given period of time to conduct your body's needs. This is called to your resting metabolic rate (RMR). Factors that influence metabolism includes sex, age, height, weight and how much fat and muscle you have on your body (i.e. your body composition). Genetics also plays a role, though just how much of a role is still up for debate.
Just like you can speed up your rate of digestion by eating more insoluble fibre, like leafy greens or bran, you can speed up your metabolism indefinitely by beefing up your lean muscle mass, and for hours following a hard core sweat session — especially if that sweat ‘sesh involves pumping heavy iron.
You see? There is interplay between the systems, but they are not governed by the same operations. Strictly speaking, that is.
Remember diabetes? That's a metabolic condition that impacts how cells absorb dietary sugar and convert it to energy. People with diabetes will often experience slow digestion due to damage to nerves controlling the emptying of the stomach. So, in this case, one's metabolic condition does have an effect on digestive health. Other similar exceptions to the rule? Thyroid disorders which can increase the rate of digestion (hyperthyroidism) or slow the rate of digestion (hypothyroidism).
But again, exceptions. Just because food 'runs through' you, doesn't mean you have a super fast metabolism. And being constipated doesn't mean your metabolism is slow. So, while having a healthy gut will contribute to a healthy body and your ability to have the energy and motivation to make the lifestyle adjustments to increase your metabolism (i.e. exercise), what happens in your gut is only part of digestion. Moreover, good digestion does not guarantee your metabolism is operating at peak.