How Does Your Metabolism Work?

Two older men and a child holding surf boards

You probably hear a lot about your metabolism. From late night TV commercials to internet ads, the world is awash in solutions to speed up your metabolism. But what exactly is our metabolism and what role does it play in our health?

What is Metabolism?

Metabolism is the chemical process where nutrients are converted into energy or calories. It’s one of the most essential processes in our body. 

It starts with digestion. When we eat or drink, our stomach and intestines break down the foods we eat into smaller parts – amino acids, fatty acids and simple sugars. From there, hormones take those constituents and convert them to energy so that our cells can function properly. 

This energy is used to propel us forward when we exercise, but also for things like breathing, thinking, cell growth and division, sleeping — in fact, everything we do requires some at least some energy, even if we’re just sitting around.

Here’s another way to think of your metabolism: It’s the number of calories that keep you functioning, whether you’re a world class athlete, a mall walker or a couch potato.

What Factors Influence Your Metabolism?

Three major things affect your metabolism: Your age, body composition and size.

Age

Let’s focus on age first because this is where most people’s ideas about metabolism are wrong.

For many decades, scientists thought our metabolic rate peaked in our 20s and declined every year after – one of the reasons it’s hard to lose weight as we age. But this conventional wisdom has been challenged by new research. 

In a study published last year in the journal Science, researchers found that our metabolism is highest as infants, declines up to our 20s and then stabilizes through age 60. After 60, it declines, though not as fast as once thought.

Between ages 20 and 60 our metabolism is consistent. Of course, metabolisms vary from person to person — in the study, rates varied by 25 percent faster or slower on an individual basis. But on average, most people have a similar metabolism.

Body Composition

The other factor that influences your metabolism is your body composition. It does so in two ways. First, size matters. You may think skinnier people have faster metabolisms, but that’s not true. Bigger people need more energy and burn more calories just moving around, regardless of the amount of fat they have. 

Why? It takes more energy for weight-bearing activities like walking for someone who is heavier. Even at rest, people with more body mass tend to have a faster metabolism. This may explain why people who lose weight find it harder to continue to lose weight; their bodies are lighter and burn fewer calories.

Another factor that influences metabolism is your body composition. Metabolic speed is affected by how your body is composed, i.e., the percentage of fat, bone and muscle in your body. Generally, the more muscle mass you have the more calories your body will burn when you’re at rest. Metabolism researchers call this your fat-free mass, and it’s a major predictor of your how many calories your body burns when you’re at rest. A pound of muscle burns around 6 calories a day; a pound of fat burns just 2 calories per day.

Size

Your body size matters, also. You may think skinnier people have faster metabolisms, but that’s not true. Bigger people need more energy and burn more calories, regardless of how much of their size is due to muscles and due to fat.

Why? It takes more energy for weight-bearing activities like walking for someone who is heavier. Even at rest, people with more body mass tend to have a faster metabolism. This may explain why people who lose weight find it harder to continue losing weight; their bodies are lighter and burn fewer calories.

The caloric difference can add up over time, especially as we age. Because we tend to lose muscle mass at the same time our metabolism is slowing, the typical result is weight gain.

How We Measure Metabolism?

Your body needs a minimum number of calories to function, and this is called your Basal Metabolic Rate. This is how many calories you need to consume to maintain things like digest food, breathe and maintain your body temperature. 

Your Resting Metabolic Rate or RMR is another way metabolism is measured. This includes the calories you need for sustainability like BMR plus the calories you need for things like eating, walking, drinking coffee and sweating.

What is Basal Metabolic Rate (BMR)?

The body composition test can also tell you know how much muscle mass and how much fat you carry. Since muscles burn more calories than fat, this can affect your BMR.

BMR and RMR are very similar measurements (and often used interchangeably). Together they account for 60 to 70 percent of all the calories you burn. For example, it’s estimated that the energy your brain, kidneys, liver and heart use account for 60 percent of your calories a day – even though they only constitute 5 percent of your body weight. The rest of your calories come from exercise, climbing stairs, getting in and out of the car and walking your dogs. 

What You Eat Affects Your Metabolism: Calories In, Calories Out

If you’re part of an MDVIP-affiliated practice, chances are your doctor uses a body composition scale such as an InBody Scale to measure your BMR or RMR every year. This can give you a baseline to maintain weight or work to modify it. 

When you add up the calories you burn from activity and the calories burned by your resting or basal metabolism, these should match the calories that you eat. If you want to lose weight, you’ll need to consume fewer calories than you burn. Let’s say your BMR is 1,400 calories a day. If you burn 600 calories a day doing other things – from riding a bicycle to gardening to walking to work, your total calories burned is 2,000. You’ll need a 2,000-calorie diet to maintain your weight. 

How Exercise Helps with Your Metabolism

Knowing this can help you select the right type of exercise to build more lean muscle – and hopefully burn some of the fat. Some examples include weightlifting, Pilates, heavy gardening, stair- and hill-climbing, cycling and yoga.

How Do You Know If Your Metabolism is Slow?

  • Constantly tired
  • Headaches
  • Weight gain
  • Depression

How Can You Speed Up Your Metabolism?

  • Eat plenty of protein
  • Drink water
  • Lift weights
  • Quality sleep

If you are Concerned About your Metabolism, Schedule a Visit with your Primary Care Doctor 

Metabolism is complicated science. Even after many decades of study, we’re still learning what makes our bodies tick and why some people can maintain their body shape without doing anything and why others struggle to manage and lose weight. 

If you’re struggling with your weight, talk to your primary care doctor. Understanding your body composition will help you better understand your metabolism – and that can help you better understand what you need to do to maintain a healthy weight.


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