Metabolism is the process by which the body breaks down foods into usable fuel.
Sometimes the complexity of metabolism makes people believe they can fool it, or that metabolism is a malleable force. While the jury is out on whether metabolism is a hackable system or whether people are programed at a set point, there are irrefutable laws which all must heed.
Basic chemistry teaches us that energy cannot be created or disappear. If someone eats more than they are using it has to go somewhere. There are zealots out there operating under the proclamation that you can eat as much as you want and still lose weight. Another common misconception: that you can speed up metabolism to burn more fat.
Law No. 1: When a person works out and burns fuel, it is being released from the body as CO2 and heat. It just doesn't disappear. When a person eats a food it is digested and stored; if they're working out regularly, this is likely stored in muscles for usable energy while excess is stored as fat.
The body strives to maintain its current healthful status; it is always adjusting to keep the body in a constant. If someone starts dieting, they're body senses the change in fuel and begins to upregulate processes to conserve or to seek out more to balance things. It's the dieter's paradox. Subconsciously the dieter begins to move less, think less clearly, and crave high-calorie foods.
This system of homeostasis is not a system build on 21st-century software, where food is available in ample qualities all around us.
On the other side of the coin, someone eating too much actually will increase the resting metabolism, the body will lose its red flags for being full and satisfied.
Weight gain is an easier process than weight loss.
Law No. 2: A large deficit or surplus of calories does slow down or speed up metabolism - but in small increments. This also influences the system of hunger and satiety (how full one feels). This leads to the body striving to maintain its current homeostatic level.
Metabolic individuality is a largely misunderstood and misinterpreted concept. One person claims they eat one doughnut and it goes straight to their thighs, while their friend can eat whatever they want and not gain an ounce. This is both true and false. A person who has more muscularity, moves more, or who has never been overweight will be more likely to burn the food as energy. Someone who sits, doesn't exercise, or has very little muscle mass will not use the energy. While resting metabolic rate does differ between individuals, it is much smaller than people are lead to believe.
Law No. 3: Lean body mass, movement, and diet composition contribute to the majority of your metabolic rate.
Often we'll be advised that green tea, spicy foods, or eating only fat will rev metabolism and turn us into a fat-burning machine. Such notions are arguable. There are, however, many metabolic rules that aren't arguable. Follow these rules and one can clearly implement weight-management.