All About Metabolism

Medically Reviewed
metabolism
People with a slow metabolism may have a more difficult time losing weight because they burn fewer calories overall than those with a faster metabolism. Building muscle can help.
Tanja Ivanova/Getty Images; Anton Vierietin/iStock

Metabolism can often be a scapegoat if your weight isn’t where you want it to be. And while metabolism is certainly related to how much you weigh, its main duty is to keep the body humming with energy. Read on to learn more about metabolism, including how it works, how to speed it up or slow it down, how it’s related to a class of diseases, and more.

RELATED: 11 Best and Worst Foods for Boosting Metabolism

What Is Metabolism?

Metabolism refers to the chemical changes that take place within a cell or organism, according to the National Cancer Institute. These changes create the energy and materials that cells and organisms need to grow, reproduce, and stay healthy, and also help get rid of toxic substances.

In simpler terms, metabolism refers to everything in the body that has to do with energy conversion: blood circulation, body temperature control, muscle contraction, food digestion, nerve and brain function, and waste removal, according to MedlinePlus.

You’ll often hear metabolism discussed in relation to weight and weight loss, but that’s not the only reason it’s important. “Metabolism is the energy that's needed for our body to function — from breathing, to running, to exercise, everything,” says Marcio Griebeler, MD, an endocrinologist and obesity specialist with the Cleveland Clinic's endocrinology and metabolism institute in Avon, Ohio.

And it’s always at work, according to the Cleveland Clinic. There’s no turning your metabolism on or off.

RELATED: Metabolism May Not Decline With Age

Common Questions & Answers

What are the different types of metabolism?
There aren’t different types of metabolism, though your metabolism can be fast or slow. That’s determined by a number of factors such as age, sex, muscle mass, genetics, movement, and tobacco use.
What is a fast metabolism?
A fast metabolism means you burn more calories than someone with a slow metabolism, even when at rest. Your resting metabolic rate is known as your basal metabolic rate, or BMR.
What causes metabolism to slow down?
Metabolism can slow as a result of things within and outside of your control: losing muscle mass, which occurs naturally during the aging process; losing weight and muscle; skipping meals; and quitting smoking.
How does metabolism affect weight loss or gain?
People with a slow metabolism burn fewer calories than those with a fast metabolism. Because more calories then get stored as fat in the body, people with a slow metabolism may have a harder time losing weight. Metabolism can also be affected by weight changes and generally slows as a result of weight loss.
What is metabolic disorder?
Metabolic disorders are conditions that alter the metabolism in some way. These are generally inherited and not a result of your diet or exercise.

How Metabolism Works

People often refer to metabolism as being fast or slow, which means how quickly the body works through the energy conversion process. “We usually see patients coming to us saying, ‘My metabolism is down. My metabolism is slow,’” Dr. Griebeler says. “That's related to the basal metabolic rate (BMR), which is the resting metabolic rate.”

BMR refers to the minimum number of calories your body needs to survive when at rest and accounts for roughly 60 to 70 percent of your body’s overall energy needs, according to the Cleveland Clinic. This varies from person to person and isn’t steady over one’s lifetime either — it can decrease if you lose weight quickly or eat too few calories, which results in weight loss stalling. The higher your BMR, the more calories you burn, even when at rest.

Griebeler says that the calories burned by daily living and doing things like sitting, eating, and breathing are referred to as nonexercise activity thermogenesis. “The average person with minimal physical activity needs about 10 calories per pound of body weight to maintain their daily functions and remain the same weight,” says Felix Spiegel, MD, a bariatric surgeon with Memorial Hermann in Houston. “If someone is more active, their metabolism is increased, and they would require more calories.”

According to the Cleveland Clinic, the speed of your metabolism can be affected by several factors:

  • Muscle Mass Higher muscle mass translates to a faster metabolism. There’s an opposite relationship between fat and metabolism. “The more fat you carry, the slower your metabolism is generally,” Dr. Spiegel says.
  • Age Metabolism naturally slows as you age.
  • Sex Men typically have faster metabolisms than women due to higher muscle mass.
  • Genetics Your genes determine how much muscle mass your body will have.
  • Movement Any type of movement leads to more calories burned than being sedentary.
  • Smoking Nicotine stimulates metabolism, leading to more calories burned. That said, the negative health consequences of smoking far outweigh any metabolic benefit.

Many of these factors are out of your control but not all of them. You cannot control your genes, for instance. “It is what it is — some people are fast burners, and some are slow burners,” he says. Physical activity and your muscle mass, however, are within your power. “We can control [metabolism] — it's important to say that,” Griebeler says.

The Process of Metabolism

According to Rush University Medical Center, there are two main aspects to the process of metabolism: anabolism and catabolism.

Anabolism promotes new cell growth and builds body tissue such as muscle, while catabolism is the process by which the body breaks down tissue such as fat, Spiegel says.

These aren’t types of metabolism, rather they are related because of the link between metabolism and the weight loss and muscle gain process. “When you lose weight, you are in a catabolic state,” Spiegel says. “When you gain muscle or fat or both, you are in an anabolic state.”

How Metabolism Affects Your Weight

Oftentimes, when you hear about metabolism, it’s in conjunction with weight loss. That’s because you’ll probably hear that someone with a slow metabolism needs fewer calories than someone with a fast metabolism. But having a fast metabolism isn’t a characteristic shared by thin people; people who are overweight or obese often have fast metabolisms, per the Cleveland Clinic. In fact, metabolism is rarely the cause of weight loss or gain, since your metabolism naturally adjusts to meet your body’s needs. Burning more calories than you take in results in weight loss, no matter the speed of your metabolism.

However, people with a slow metabolism may have a more difficult time losing weight because they burn fewer calories than those with a fast metabolism, which means that more of the calories they consume get stored as fat in the body, according to Harvard Health Publishing.

Can You Boost Your Metabolism?

You cannot overcome your genetic predisposition, but Griebeler says that you can absolutely speed up your metabolism. The best way to do it? By building muscle.

Muscle uses more energy than fat does, even when you’re at rest, according to Rush University Medical Center. “If you increase your muscle mass, you need more calories to feed those muscles so your basal metabolic rate will increase,” Griebeler says. “You're burning calories in different ways: You're burning calories when you're doing the exercise, you're burning calories when you're recovering from exercise, and you're burning calories because you increase your metabolic rate.”

He says that there’s no one-size-fits-all recommendation on the best way to build muscle, but there are three ways to do it: increase exercise frequency, increase time spent exercising, or increase intensity.

According to Harvard Health Publishing, you can also speed up your metabolism by eating protein and drinking green tea, although the impact of both of these interventions is small.

All in all, a healthy lifestyle is key to a healthy metabolism, Griebeler says. In extreme cases, you can also consider bariatric or weight loss surgery to alter your metabolic rate. “These have been shown to increase your metabolism,” Spiegel says. “For that reason, we call them metabolic surgery.”

What Causes Metabolism to Slow Down?

There are also some things that can slow down your metabolism. One is aging, which of course you can’t prevent, though you can maintain a speedy metabolism with a little work. “As you age, your metabolism may decrease, but there is a very strong relationship with the muscle mass, and this is in our control,” Griebeler says. “Our goal as we age is always preservation of muscle mass so you can maintain or increase your metabolic rate.”

Additionally, your metabolism can slow as a result of certain situations or behaviors:

  • Losing Weight When you lose weight, you’re losing fat as well as muscle, and that can slow down your metabolism, according to the Mayo Clinic.
  • Quitting Smoking The nicotine in cigarettes has a metabolism-quickening effect and can increase calorie burn by 7 to 15 percent, according to MedlinePlus. So quitting smoking takes away that extra burn, though the potential weight gain is far better than continuing to smoke and increasing your risk of various health issues.
  • Skipping Meals It makes the body go into starvation mode and hold onto calories to survive. Breakfast is a particularly important one not to skip. According to a study published January 2021 in Nutrition & Metabolism, which looked at data from more than 21,000 people, the majority of people with abnormal metabolisms regularly skipped breakfast.

Metabolic Disorders: When Metabolism Goes Wrong

Certain conditions can affect metabolism. According to the Cleveland Clinic, endocrine disorders such as Cushing’s syndrome and hypothyroidism can result in a slower metabolism.

There’s also a whole host of metabolic disorders, which are inherited and not impacted by your diet or exercise routine. Metabolic disorders happen when abnormal chemical reactions within the body alter the metabolism in some way, according to MedlinePlus.

According to the Cleveland Clinic, examples of metabolic disorders include:

Your metabolism can also slow if you’re in the hospital or have a prolonged illness that limits your physical activity and therefore reduces your muscle mass, Griebeler says.

Resources We Love: Metabolism

Favorite Org for Essential Metabolism Information

Cleveland Clinic

For further reading on metabolism, visit this webpage from the Cleveland Clinic. It’s presented in an easy-to-digest Q&A format and covers what metabolism is all about, how it affects weight and certain health conditions, and when it’s time to see a doctor.

Favorite Podcasts on Metabolism

TED Health: Can I Speed Up My Metabolism?

This 30-minute episode from TED Health features Jen Gunter diving into what metabolism is all about and if it is truly possible to change yours in a meaningful way. She also addresses whether all the hacks that are marketed to consumers under the guise of speeding up metabolism are worthwhile.

Metabolism and Hormone Support for Women With Courtney Whitten

Need more about the topic than just one episode? You’re in luck — this entire podcast series and its 35-plus episode library centers on metabolism, weight loss, and the importance of achieving hormonal balance. Each episode covers a topic in this subject area in about 20 to 40 minutes each.

Favorite Book on Metabolism

Burn: The Misunderstood Science of Metabolism by Herman Pontzer

This 2021 release from evolutionary anthropologist and Duke professor Herman Pontzer covers the insights he gained while studying metabolism over the past decade. His research has led him to follow hunters and gatherers in Tanzania. In the book, he challenges the idea that exercise and movement can alter metabolism and that the body is intelligently designed to adjust to the changes in lifestyle.

Summary

Your metabolism is related to your weight, but you don’t have to assume having a slow metabolism means you’re doomed to have a slow metabolism forever. By prioritizing strength training and building muscle mass, you can speed up your metabolism.

Metabolism isn’t solely related to weight, either. “We need energy; that's your metabolism,” Griebeler says. “It’s not only to maintain your weight but necessary for everything that you do.”

Editorial Sources and Fact-Checking

Show Less