Science 101

What a High or Low Metabolism Really Means

And what you can do about it.

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A high or low metabolism relates to the speed at which your body uses energy to operate. Low metabolism means the body burns fewer calories to function, high metabolism means more calories burned and, typically, less fat stored. But your metabolic rate and the calorie-to-energy-to-fat equation (that is, the calories you consume turn into energy that your body uses) is constantly varying. Your metabolism is not set in place.

How Your Metabolism Changes

The malleable parts include age, fat-to-lean-mass ratio, and weight. If you have two people of the same weight and age, but one has more lean mass and less body fat, that person will, most likely, have a faster metabolism. 

The more adjustable bits include things as variable as diet, temperature, sleep and exercise. “If you were out in the cold working, you burn through more calories,” Baur says. “If you stay in a warm place and don't move much, you're going to burn fewer calories.”

Dietary-induced thermogenesis (DIT), exercise activity-related thermogenesis (EAT) and non-exercise activity thermogenesis (NEAT) all play a role. DIT and EAT are as they sound: the increase in calories burned while processing food and during exercise. NEAT is calories burned during other activities, like wandering around or, say, washing the car.

DIT, EAT and NEAT can all vary depending on inputs. The type of food and the time that food is consumed can greatly change the DIT. EAT can vary based on type and time of exercise, as well as muscle mass and weight. NEAT can change based on numerous factors, both related to malleable and adjustable variables.

And to mix things up a bit more, another factor to consider is what changes one’s body weight is currently undergoing. The concept of “adaptive thermogenesis” plays a notable role in metabolism — especially in keeping the body at a stable weight. Baur puts it succinctly: “If you lose weight, your metabolism does slow down to resist that.”

With weight loss, the body slows metabolism and increases the appetite. But considering evolutionary and survival terms, it makes sense. The body slows to maintain energy stores; it tries to keep things at the current “default” weight. But this weight loss metabolism shift can also have to do with muscle changes. “You tend to get more slow twitch fibers when you're losing weight,” Baur says. “So you're able to basically accomplish the same movements for less energy than if you were using fast twitch fibers to do it.”

Baur also posits that this adaptation may have to do with a drop in body temperature when losing weight — which has been seen in mice. In humans, though, this is still being studied, although even a small drop in body temperature could lead to energy saved and fewer calories burned.

In sum, what a high or low metabolism really means is that the intricate and highly integrated metabolic processes your body follows are, at that moment, running quickly or slowly. Yet those are subject to change — within the hour, throughout the day, and over the years. Metabolism is not neat and tidy. It’s not one speed, and it’s not even one speed that changes simply due to age. Your metabolism is constantly changing, and there should be empowerment in knowing you are not forever stuck in the fast or slow lane.

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