Although people can have a fast or slow metabolism, extremes are pretty rare, often resulting from disease or prior obesity. Around 96% of people stay within 200–300 kcal of the average metabolism. Moreover, resting metabolic rates (RMRs) are not well correlated with weight gain.
Protein burns, by far, the most energy of any of the three macronutrients. For that reason, and because of its protective effect on muscle mass, protein plays a central role in many diet templates.
Outside your resting metabolic rate and the energy that’s burned by eating food, somewhere between 15 and 50% of your energy burn comes from activity. This activity can be exercise or any of the dozens of activities/movements associated with living.
Regular exercise can increase your resting metabolic rate, as can other methods, such as cold exposure (with an eye to safety). Moreover, although taking the stairs rather than the elevator or standing while working won’t noticeably boost your RMR, this kind of small effort can be an easy way to burn more calories each day.
Your RMR will decrease as you lose weight, since weighing less means having less body tissue to support (among other factors). Unless you diet too hard for too long, your RMR should normalize after the dieting period is over.
In popular lore, someone with “a fast metabolism” is someone whose high resting metabolic rate (RMR) allows him or her to eat a seemingly infinite amount of food without any fat gain. Past their growing years, such people don’t exist. While some adults do have a higher natural RMR than their peers, interindividual variations tend to stay within a range of 200–300 kcal/day. Moreover, people with higher caloric requirements tend to compensate by eating more, so that one’s natural RMR is a poor predictor of weight gain.
The good news is that if you want to rev up your metabolism, you can. Anaerobic activities can increase your RMR for more than a day; they can also increase your muscle mass, thus further heightening your caloric requirements. They’re a safer way to lose fat than cutting your caloric intake drastically, which can result in your body fighting back, your RMR taking a lasting hit, and endless yo-yo dieting. Yes, to shed fat, you do need to spend more energy than you ingest; but the more severe the deficit, the easier it’ll be to regain whatever fat you lost.