Editor’s Note: This post was updated on October 16, 2026 , for accuracy and comprehensiveness. It was originally published on March 18, 2020.
Metabolic rate is how fast your body burns energy, measured in calories, to keep you alive and moving.
At rest, that baseline is your Basal Metabolic Rate (BMR): the minimum calories your body needs for breathing, circulation, brain activity, and every other process that runs without you thinking about it. Add the energy you burn moving and digesting, and you get your Total Daily Energy Expenditure, the full number of calories you burn in a day.
Most people think about metabolism in terms of speed: "my metabolism is fast," or "my metabolism is slowing down." But the more useful question isn't how fast it runs. It's what sets the rate in the first place, and that's mostly your body composition.
Metabolism is linked with weight because it's the biological process that turns what you eat into energy. The Mayo Clinic defines it as the process by which the body converts food and drink into energy, combining calories with oxygen to release the energy the body needs to function.
Notice it says nothing about the speed you process food. That’s digestion, which people often confuse with metabolism.
What Is Basal Metabolic Rate (BMR)?
Basal Metabolic Rate (BMR) is the minimum number of calories the body needs to perform basic functions at rest. BMR is usually expressed in terms of calories. Your Basal Metabolic Rate also has another interesting quality: the more Lean Body Mass (which includes muscle, water, and minerals) you have, the greater the BMR tends to be.
This is the whole reason body composition and metabolism are the same conversation: lean mass is the biggest thing setting your BMR, and it's the one input you can actually change.
The common assumption is only partly accurate. Metabolic rate does not drop sharply in a person's 30s. A landmark 2021 study in Science tracking more than 6,400 people found that metabolic rate holds remarkably steady from about age 20 to 60, with no separate dip for menopause. So for most of adult life, age is not the main factor.
After 60, metabolism does start to decline, by a little under 1% a year. Part of that is lost muscle, since muscle burns more calories than fat, and part is your cells simply slowing down. The muscle part is the part you can do something about, which is exactly why body composition sits at the center of this.
Regardless of your age, weight, or body type, knowing how your metabolism works gives you more control than the "it just slows down" story suggests.
Metabolism 101
Let’s take a deeper look at what you might call a “slow” metabolism. Far from being an issue of fastness or slowness, weight gain is almost always the result of a caloric imbalance that goes unchecked over a long period of time.
To clarify, your Basal Metabolic Rate is not the only factor that plays into your overall caloric needs, and it’s not the total amount of calories you need in a day. There are two other major influencers, which are:
Your energy level, meaning how active you are
The thermic effect of food, meaning the energy your body uses to digest it
These, taken together with your Basal Metabolic Rate, provide your Total Daily Energy Expenditure (TDEE). This is the number of calories your body burns in a day.
BMR is a necessary piece of information to estimate TDEE. Although they’re not exact, equations exist for estimating your TDEE based on your activity level and BMR. These are based on multiplying BMR by an activity factor, a number between 1 and 2, that increases with activity level.
How Many Calories Do You Need?
You’re probably familiar with the 2000-calorie diet. This is a range set by the U.S. Food and Drug Administration (FDA) in 1993 for use on nutrition labels for packaged food.
So what are your actual caloric needs? A good way to start is by using a BMR calculator, which will determine the number of calories your body burns each day to perform its basic, life-sustaining functions. This includes all the involuntary processes in your body such as breathing, digesting food, pumping blood, brain activity, and much more. There is no shortage of online resources and apps that will provide you with a BMR calculator. Certain medical/fitness devices also feature BMR as an output during body composition analysis. However, there are a few things you should know about metabolism calculations before diving into the first option you find. Your caloric needs can be calculated in a couple of different ways and with a few different equations, including the revised Harris-Benedict equation and the Mifflin-St. Jeor equation. These equations calculate BMR using your weight, with some adjustments for height, age, and gender. However, if you fall outside average assumptions for height, age, and gender (if you’re an athlete, for example), these formulas may not accurately produce your metabolic rate.
For people who do fall outside the assumed ranges for height, age and gender, there is a third option: use the amount of lean body mass you have to determine your metabolic rate. This is what the J.J. Cunningham equation will do. Using this method as a BMR calculator has a couple of benefits:
It won’t give you results that have been influenced by estimations derived from the typical representative member of your age and gender
As you increase lean body mass by developing your skeletal muscle mass, your caloric needs will increase, and the Cunningham equation will account for this.
Once you have your BMR in hand, you’re ready for the next step.
Total Calories and Dieting
Remember, your BMR is just the number of calories your body burns at rest and does not account for the calories you need to walk, talk, exercise, etc. When thinking about your caloric needs for a meal plan, you’ll need to convert your BMR to your Total Daily Energy Expenditure (TDEE). You can do this by multiplying your BMR by a factor that represents your estimated energy level.
So, let's take the example of a 171.1-pound male with 133.6 pounds of lean body mass and assume he is moderately active. Using the Cunningham equation, this person would have a BMR of around 1,833 Cal/day. Multiply that by the appropriate conversion, and you get about 2,840. This is how many calories this person needs to maintain his weight.
This also means the diet has to match the current goal – losing fat mass and/or gaining lean body mass. Fitness and meal plans that work against each other tend to produce mixed results.
The most classic example is this: “I want to get in shape, so I am going to diet (eat less) and work out more (increase energy use).”
This isn’t a bad plan – if you’re looking to lose fat. If you’re looking to build muscle and get stronger, it’s very unlikely that you will achieve this by eating less than your TDEE while increasing your activity level beyond what you’re accustomed to.
As with any dietary plan, you will expect to see changes appear over time.
How long results take varies by individual. Weight can be checked every one to two weeks, though body composition testing distinguishes what that weight is made of.
Another important factor to consider: your BMR. Since your BMR is closely linked to your lean body mass, any changes will affect the number of calories you burn.
For example, if your plan is to gain lean body mass, and over a period of time you are successful in doing so, your energy needs are going to increase. This is why it is so important to be measuring body composition.
A diet is much more than creating a calorie deficit. It’s important to use a BMR calculator or body composition analyzer to understand how much energy your body needs.
Without this information, adjusting intake is guesswork. With it, changes can be matched to measured energy needs.
Improving and Increasing Your Metabolism
Exercise and dietary patterns both influence metabolic rate, largely through their effect on body composition.
Because your body needs more energy to support itself when it has more Lean Body Mass, working to increase your Lean Body Mass can actually increase your Basal Metabolic Rate, which can have a huge impact on your TDEE once you factor in your activity level.
For many people, maintaining metabolic rate and avoiding a slowdown is a goal in itself. As covered above, that decline arrives later and more gradually than most people expect, and part of it is tied to the muscle a person keeps. So what protects metabolic rate?
In short: by maintaining the Lean Body Mass that you already have. That means maintaining your Skeletal Muscle Mass.
Your Skeletal Muscle Mass isn’t the same as your Lean Body Mass, but it is the overall biggest contributor to it. It’s the muscle that you can actually grow and develop through exercise, and increases/decreases in SMM have a strong influence on increases/decreases in Lean Body Mass.
Skeletal Muscle Mass is best developed through strength training and resistance exercise along with a proper diet. A regular exercise plan that includes strength training and resistance exercise will help you maintain your Skeletal Muscle Mass.
This can be especially important as you age. As people become older and busier, activity levels tend to drop and a proper diet can become harder to maintain as responsibilities increase. Poor diet and nutrition can lead to loss of Lean Body Mass over time, which leads to a decrease in overall metabolism–not a slowdown.
So what influences metabolic rate? The largest modifiable factor is lean body mass.
Metabolic Flexibility?
Metabolic flexibility is your body's ability to switch between burning carbs and fat for fuel. Someone with good flexibility burns carbs when they eat them and taps into fat when they don't, which makes it easier to use stored body fat between meals. People with lower metabolic flexibility often report low energy between meals and have more difficulty using stored fat.
Exercise is the most reliable way to build it. If you're mostly sedentary, adding activity trains your body to reach for fat as fuel; if you already train, adding variety (strength, intervals, endurance) nudges it further. Intermittent fasting works on the same principle. Diet plays a role too, and a flexible metabolism is one more thing a healthy body composition supports, alongside a lower risk of metabolic syndrome.
Other Things That Affect Your Metabolic Rate
A few other factors nudge your metabolic rate, though none rivals lean mass. Cold exposure activates brown fat, a calorie-burning tissue that generates heat, and regular exposure can increase how much of it you carry. Prolonged heat, like hot yoga in a heated room, raises your BMR temporarily as your body works to cool itself. These effects are measurable but small relative to lean mass.
Wrapping Up
Your metabolic rate isn't a fixed trait or a mystery that unravels with age. For most of adult life it holds steady, and the biggest thing setting it is your lean body mass, the one input you can actually build. Maintaining muscle is closely tied to maintaining metabolic rate. Getting your body composition accurately tested is a solid first step.
Frequently Asked Questions
What is metabolic rate?
It's how fast your body burns calories. Your resting baseline is your Basal Metabolic Rate (BMR); add movement and digestion and you get your Total Daily Energy Expenditure, the calories you burn in a full day.
What is BMR?
Basal Metabolic Rate is the minimum number of calories your body needs at complete rest to keep basic functions running: breathing, circulation, brain activity, cell repair. For most people it's the largest share of daily calories burned.
Does your metabolism really slow with age?
Yes, but later than most people think. It's stable from about 20 to 60, then declines a little under 1% a year. Some of that later decline comes from losing muscle, which is the part you can slow by staying active.
How can you increase your metabolic rate?
The most consistent factor is building and keeping lean muscle, since it raises your BMR. Strength training plus enough protein does more for your resting rate than any single food or supplement.
