What is Metabolism?

This word “metabolism” is thrown around a lot these days.

You know that if yours is too slow you might gain weight. But what exactly does this all mean?

Well technically “metabolism” is the word to describe all of the biochemical reactions in your body. It’s how you take in nutrients and oxygen and use them to fuel everything you do.

Your body has an incredible ability to grow, heal, and generally stay alive. And without this amazing biochemistry, YOU would not be possible.

Metabolism includes how the cells in your body:

  • Allow activities you can control (e.g. physical activity).
  • Allow activities you can’t control (e.g. heartbeat, wound healing, processing of nutrients & toxins).
  • Allow storage of excess energy for later

So when you put all of these complex processes together, you can imagine how they could work too quickly, too slowly, or just right.

Which brings us to the “metabolic rate”.

Metabolic rate

Your metabolic rate is how fast your metabolism works and it is measured in calories (yup, the word calories refers to a measurement of energy).

The word Calorie comes from Latin, which means heat. It is a unit of energy.

In science-y terms, 1 calorie is the amount of energy it would take to raise the temperature of 1 kilogram of water by 1 degree Celsius. It takes 1000 of these small calories to raise the temperature of water by 1 degree Celsius.

A calorie is often used to measure the energy value of foods.

The calories you eat can go to one of three places:

  • Work (i.e. exercise and other activity).
  • Heat (i.e. from all those biochemical reactions).
  • Storage (i.e. extra leftover “unburned” calories stored as fat).

As you can imagine, the more calories you burn as work or creating heat, the easier it is to lose weight and keep it off because there will be fewer “leftover” calories to store for later.

There are a couple of different ways to measure metabolic rate. One is the RMR or “Resting Metabolic Rate” which is how much energy your body uses when you’re not being physically active. You know – just breathing and sleeping. It takes a surprising number of calories to keep your blood pumping and brain working.

Here is a breakdown of the percentages of RMR used by different parts of the body:

  • Liver: 27%
  • Brain: 19%
  • Skeletal Muscle: 18%
  • Kidneys: 10%
  • Heart: 7%
  • Other organs: 19%

GEEK ALERT! RMR is often used interchangeably with BMR but they are slightly different. RMR is responsible for approximately 60 to 70% of the total number of calories you burn in a day, and calculations exclude physical activity. Having a high RMR means you get to eat like a teenager and not gain weight. Your RMR depends on a variety of factors like your age, height, gender, hormone health, and muscularity. For example, a 25-year-old man might burn more than 1800 calories a day (at rest) while a 40-year-old woman might burn 1400 calories a day (at rest).

What affects your metabolic rate?

In a nutshell: a lot!

Your Hormones

The first thing you may think of is your thyroid. This gland at the front of your throat releases hormones to tell your body to “speed up” your metabolism. Of course, the more thyroid hormone there is, the faster things will work and the more calories you’ll burn. If you’re hypothyroid, like me, you have a slow thyroid and require medication so that you don’t sleep like a sloth throughout the day. Another hormone that can impact your metabolic rate is your testosterone, and women and men both experience a natural drop in testosterone levels with age.

But your hormones are not the only thing that affects your metabolic rate.

Your Size

How big you are also counts.

Larger people have higher metabolic rates, but your body composition is crucial!

Your Muscle Mass

This is exactly why weight training is often recommended as a part of a weight loss program. The more muscle you carry, the more calories you burn at rest.

When people lose weight their metabolic rate often slows down which you don’t want to happen. You can offset this by adding more muscle mass to your frame.

What’s not to love about muscles? They let you eat more food, they give you shape and sex appeal, and they make you feel strong and powerful. Win!

Where was I? Oh yeah, things that affect your metabolic rate….

Your Activity Levels

Aerobic exercise also temporarily increases your metabolic rate. Your muscles are burning fuel to move so they’re doing “work”. Remember: Calories are expended through the production of heat and work (e.g. activity). Unused calories go into storage.

How active you are in general matters – a lot! People like my husband who fidget a lot and can’t stand still burn more calories than others, through a process known as NEAT, or “Non-Exercise Activity Thermogenesis”. NEAT activities increase our metabolic rate and account for the vast majority of our non-resting calorie needs.

In science circles, all forms of movement that aren’t categorized as sports-like exercise are considered NEAT (Non-Exercise Activity Thermogenesis). Examples include typing, walking to work, loading the dishwasher, and even fidgeting. NEAT is a key component in how we maintain, lose, or gain bodyfat. If you know someone with a “fast metabolism”, it might just be that they move and fidget a lot more, or have a more active job, and thus burn several hundred more calories a day than you!

Interestingly, NEAT naturally drops as calories are restricted. Have you ever been on a really low-calorie diet and had zero energy? This is why you don’t want to drop your calories too much. Too few calories will cause your body to shut down and reduce NEAT, which will mean you burn less calories in a day overall. It’s a vicious circle….If you are on a diet and you find yourself parking closer to the grocery store, or taking the elevator instead of the stairs, and just not wanting to do normal activities, treat it as a sign that you may need more food energy.

The other thing that affects your metabolism is the “Total Daily Energy Expenditure” (TDEE). Total Daily Energy Expenditure is the sum of your resting metabolic rate (RMR) as well as the energy used for activity throughout a 24-hour period. This activity is referred to as the Thermic Effect of Physical Activity (TEPA). This includes planned exercise and nonplanned movement such as NEAT.

As you can imagine, muscles that actively move and do work need more energy than fat does. So the more lean muscle mass you have, the more energy your body will burn and the higher your metabolic rate will be.

But there are other things that affect your metabolism…

Your Diet

The type of food you eat also affects your metabolic rate!

Your body actually burns calories to absorb, digest, and metabolize your food. This is called the “thermic effect of food” (TEF). On average, your body uses about 10% of its daily energy just digesting food, but this percentage changes depending on the type of food you eat.

If you’ve ever heard the expression, “not all calories are created equal”, it’s because of this process. Once you understand how your body metabolizes foods differently, you can use the thermic effect of food to your advantage.

Fats, for example, increase your TEF by 0-3%; carbs increase it by 5-10%, and protein increases it by 20-30%. By trading some of your fat or carbs for lean protein you can slightly increase your metabolic rate.

Another bonus of protein is that your muscles need it to grow. By working them out and feeding them what they need, they will help you to lose weight and keep it off.


To recap, Total daily energy expenditure (TDEE) is the amount of energy your body burns on a daily basis. It is calculated from:

  • Resting Metabolic Rate (influenced by your hormones, muscle mass, age, gender, etc.).
  • Thermic Effect of Physical Activity (how much you move and exercise in a day, includes NEAT).
  • Thermic Effect of Food.

To calculate your caloric needs for a day:


If you would like to take a deeper dive into this topic, and calculate your own TDEE, click here. There is a link at the bottom of the article to a calculator which will help you figure out your daily calorie needs. If you would like to learn more about

This is just the tip of the iceberg when it comes to metabolism and how so many different things can work to increase (or decrease) your metabolic rate. Stress and sleep can also impact the metabolic rate, but that’s for another discussion.

The good news? Most of these factors are under our control, which is empowering information for those who struggle to stay lean!




Want to learn how to use macros and flexible dieting to lose fat, maintain your weight, or build muscle? Check out our Eat For Your Goals Course.

We have a Macro Calculator that you can use. Access it HERE. Let us know below what you think about the calculator and your results!

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