What is metabolism?
Metabolism is the chemical reactions in the body's cells that change food into energy. Our bodies need energy to perform on all levels; from moving to thinking to growing.
Thousands of metabolic reactions happen at the same time — all regulated by the body — to keep our cells healthy and working.
How does it work?
After we eat/drink, the digestive system uses enzymes to:
break proteins down into amino acids
turn fats into fatty acids
turn carbohydrates into simple sugars (glucose)
The body can use sugar, amino acids, and fatty acids as energy sources when needed. These compounds are absorbed into the blood, which carries them to the cells.
After they enter the cells, other enzymes act to speed up or regulate the chemical reactions involved with "metabolizing" these compounds. During these processes, the energy from these compounds can be released for use by the body or stored in body tissues, especially the liver, muscles, and body fat.
Metabolism is a balancing act involving two kinds of activities that go on at the same time:
building up body tissues and energy stores (called anabolism)
breaking down body tissues and energy stores to get more fuel for body functions (called catabolism)
Anabolism is all about building and storing. It supports the growth of new cells, the maintenance of body tissues, and the storage of energy for future use.
Catabolism is the process that produces the energy needed for all activity in the cells. Cells break down large molecules (mostly carbs and fats) to release energy. This provides fuel for anabolism, heats the body, and enables the muscles to contract and the body to move.
As complex chemical units break down into more simple substances, the body releases the waste products through the skin, kidneys, lungs, and intestines.
Calories. Exercise. BMR.
Metabolism is a complicated chemical process. So it's not surprising that many people think of it in its simplest sense: As something that influences how easily our bodies gain or lose weight. That's where calories come in to play. A calorie is a unit that measures how much energy a particular food provides to the body. The body requires calories to function and uses them to sustain three main processes:
Basal metabolic rate (BMR): This refers to the number of calories needed to cover basic functions in the body, specifically at rest, including the proper functioning of the brain, kidneys, heart, lungs and nervous system.
Digestion: The body uses a certain number of calories to digest and metabolize the foods we eat. This is also known as the thermic effect of food (TEF).
Physical activity: This refers to the number of calories needed to fuel the body's everyday tasks and exercise.
The number of calories someone burns in a day is affected by how much that person exercises, the amount of fat vs. muscle his or her body has (body composition), and the person's BMR. BMR can play a role in a person's tendency to gain weight. For example, someone with a low BMR (who therefore burns fewer calories while at rest) will tend to gain more body fat over time than a similar-sized person with an average BMR, who eats the same amount of food and gets the same amount of exercise.
BMR can be affected by a person's genes and by underlying health problems. But most importantly, it's influenced by body composition - people with more muscle and less fat, generally have higher BMRs. Beings body composition is something that can be changed - through a healthy diet and exercise - one CAN control their own metabolism.
A donut has more calories than an apple, so it provides the body with more energy — yet, sometimes that can be too much of a good thing. Generally speaking, eating more calories than your body needs will cause you to gain weight, mostly in the form of body fat. Eating fewer calories than your body requires leads to weight loss.
This calorie balance concept, which is supported by strong scientific research, is why people wanting to lose weight often try to restrict their calorie intake.
Calorie restriction and dieting. As you have learned thus far, it sounds pretty simple, right? While eating less calories, moving more, and gaining more muscle mass may sound doable, there still is more to learn! Calorie restriction may be beneficial, however, restricting calories too much may be harmful to ones health. Several studies have shown that low-calorie intake (or calorie restrictive dieting) can cause the metabolism to decrease. This is called metabolic adaptation. It's our body's way of protecting us from starvation. Several studies show that low-calorie diets can decrease the number of calories the body burns by as much as 23%! What’s more, this lower metabolism can persist long after the calorie-restricted diet is stopped. In fact, researchers believe that the adapted lower metabolism may partly explain why more than 80% of people regain weight once they go off any given diet.