Metabolism
Overview
What Is Metabolism?
Metabolism (meh-TAB-uh-liz-um) is the set of chemical reactions in the body's cells that change food into energy. Your body needs this energy to do everything from moving and thinking to growing.
Specific proteins in the body called enzymes control the chemical reactions of metabolism. Thousands of metabolic reactions happen at the same time — all handled by the body — to keep our cells healthy and working.
How Does Metabolism Work?
After you eat food, your digestive system uses enzymes to break nutrients down into simpler forms:
- proteins become amino acids
- fats turn into fatty acids
- carbohydrates (carbs) become simple sugars, like glucose
The body can then use amino acids, fatty acids, and simple sugars as energy sources when needed. The blood absorbs these and carries them to the cells.
After they enter the cells, other enzymes speed up or control the chemical reactions involved with breaking down, or "metabolizing," these substances. The body can either use the energy right away or store it in body tissues — especially the liver, muscles, and body fat — for later.
Types of Metabolism
There are two kinds of metabolism, and they happen at the same time.
Anabolism
Anabolism (uh-NAB-uh-liz-um), or constructive metabolism, is all about building and storing. It helps the body grow new cells, repair tissues, and store energy for later. In anabolism, small building blocks are used to create larger, more complex molecules of carbohydrates, protein, and fat.
Catabolism
Catabolism (kuh-TAB-uh-liz-um), or destructive metabolism, is how cells get the energy they need. Cells break down large molecules (mostly carbs and fats) from body tissues and energy stores to release energy. This provides fuel for anabolism, heats the body, and lets the muscles contract — or tighten — and the body move.
As the body breaks things down into more simple substances, it creates waste products that are released through the skin, kidneys, lungs, and intestines.
What Controls Metabolism?
Hormones
Several hormones (chemical messengers) of the endocrine system help control the rate of metabolism. Thyroxine, a hormone made and released by the thyroid gland, plays a key role in deciding how fast or slow the chemical reactions of metabolism go in a person's body.
Another gland, the pancreas, releases hormones that help decide whether the body's main metabolic activity at any one time is anabolic (an-uh-BOL-ik) or catabolic (kat-uh-BOL-ik). For example, more anabolic activity usually happens after you eat a meal. That's because eating increases the blood's level of glucose — the body's most important fuel. The pancreas senses this increased glucose level and releases the hormone insulin, which tells cells to boost anabolism.
Calories
Metabolism is a complex chemical process, but many people think of it in its simplest sense: as something that affects how easily our bodies gain or lose weight. That's where calories come in. A calorie is a unit that measures how much energy a certain food provides for the body.
A chocolate bar has more calories than an apple, so it gives the body more energy — and sometimes that can be too much of a good thing. Just as a car stores gas in the tank until it’s needed to fuel the engine, the body stores calories — mainly as fat. If you overfill a car's gas tank, it spills onto the pavement. Likewise, if a person eats too many calories, they "spill over" in the form of too much body fat.
Basal Metabolic Rate
The number of calories someone burns in a day is affected by how much that person exercises, the amount of fat and muscle in the body, and the person's basal metabolic rate (BMR). BMR is a measure of how quickly a person's body "burns" energy, in the form of calories, while at rest.
The BMR can play a role in a person's chances of gaining weight. For example, someone with a low BMR (who burns fewer calories while at rest or sleeping) will tend to gain more pounds of body fat over time than someone of the same size, activity level, and diet with an average BMR.
What Can Affect the Basal Metabolic Rate?
A person's genes and some health problems can affect BMR. Also, BMR is influenced by body composition (the amount of muscle versus fat). People with more muscle and less fat generally have higher BMRs. But people can change their BMR in certain ways. For example, a person who exercises more burns more calories and becomes more fit, which increases the BMR.
Age
Age also affects metabolism. As people get older, their metabolism usually slows down. This means the body burns fewer calories at rest than it did when it was younger.
One reason is that muscle mass (the amount of muscle in your body) tends to decrease with age. Muscles burn more calories than fat, so when you lose muscle, you burn fewer calories. Hormone levels can also change, which affects how the body uses energy. So, even if you eat the same and move the same as you did when you were younger, the body might not burn energy as quickly.
Other Common Questions
What Are Fast and Slow Metabolisms?
A fast metabolism means your body burns energy (calories) quickly, even when you're resting. People with fast metabolisms often feel hungry more often and may have an easier time staying slim. Their bodies are like high-speed engines, using up fuel quickly to keep everything running.
By contrast, a slow metabolism means your body uses energy more slowly. This can make it easier to gain weight, even if you don’t eat much. People with slower metabolisms might feel tired more often or have a harder time losing weight. It’s like a car engine that runs on low speed — it still works, just not as fast.
How Can I Improve My Metabolism?
While there are lots of myths about how to improve your metabolism, you can’t control your BMR. But you can control how many calories you use by staying active — especially by combining aerobic activity with exercises that build muscle, like lifting weights or doing strength training. Eating enough lean protein, having lots of fruits and vegetables, drinking water, and getting enough sleep are other ways to stay healthy. Even small things, like getting up and walking around after you’ve been sitting for a few hours, can give you a little boost in your daily step count.