When we think about weight regulation, eating and exercise are easy to observe. Metabolism seems more abstract. You might imagine it’s a hand-waving explanation for why some people don’t weigh what you’d expect based on their behavior. In casual conversation, the concept is sometimes used that way, but metabolism has a concrete meaning to physiologists.
When you digest food, your body can do three things with the energy: use it to do work on the outside world (like lifting heavy objects), store it as fat, or turn it into heat. By far the most energy turns into heat, but that’s not as wasteful as it sounds. That heat maintains your body temperature, and some of it is a byproduct of life-sustaining activities like pumping blood through your veins. The rest (about 60%) is released because your digestion is inefficient at turning food into useable energy.
Metabolism is simply how much heat your body produces. It’s measured in calories (the kind that are listed on food labels, which scientists call “kilocalories”) by placing a person in an insulated room with water-filled walls and then measuring the change in water temperature. Or more commonly (because it’s less expensive), metabolic rate is estimated from oxygen consumption, though that approach requires some assumptions about the person’s diet.
Height, weight, age, and sex all influence metabolism. Thyroid hormone is the main regulator of metabolism, but it’s also increased by adrenaline released during the fight-or-flight reaction and by muscle activity, including shivering with cold or tensing up from anxiety.
Metabolism also changes when you move outside the weight range determined by your individual set point. If your weight is 10% below your set point, your metabolism is typically about 15% lower than someone of the same weight who’s at their set point. A similar compensation happens in the opposite direction when you eat too much – your body burns more energy for a while. That’s why you may wake up feeling too hot a few hours after a big meal. The problem for dieters is that the compensation for overeating seems to fade over time, while the low metabolism in successful dieters lasts as long as six or seven years, as long as anyone has measured it.