Metabolic rate is the rate at which your body burns calories. You can separate the types of calories your body burns into two categories: resting calories or activity calories.
While you’re just sitting on your couch or at your desk working, your body is burning a certain amount of baseline calories. This is your body at rest. In fact, the calories your body uses for basic biological functions at rest account for about 60–75 percent of the total amount of energy you burn.
While muscle mass burns more than fat mass, your organs use up most of the calories you need for breathing, heart beating, regulating body temperature, digestion, and so on.
| Organ | Select Functions | % Expenditure |
|---|---|---|
| Liver | Processing and storing energy, fighting infections, clearing blood of drugs/toxins | 27 |
| Brain | Controlling thought, memory, motor actions, senses, every action that regulates the body | 19 |
| Heart | Pumping oxygen-rich blood to all cells | 7 |
| Kidneys | Filtering system | 10 |
| Skeletal muscle | Moving the body | 18 |
| Other organs | Performing all other internal functions | 19 |
Based on estimates by M. Elia in Organ and Tissue Contribution to Metabolic Weight (Raven Press, 1992).
Although resting metabolic rate, RMR, accounts for the majority of calories your body burns, another 25–40 percent of calories are being expended on a daily basis. This energy is called thermogenesis. It’s the energy used for things like eating, walking around the block, and competing in a triathlon. Adding together RMR and thermogenesis make up the Total Energy Expenditure (TEE), or your total burn rate.
There are three types of thermogenesis:
Exercise thermogenesis: Physical or daily activities like going to the gym
Non-exercise activity thermogenesis: Non-structured activity, like shivering or fidgeting
Diet-induced thermogenesis or the Thermic Effect of Food: Calories burned when breaking down food as you eat it
Depending on how active you are, exercise and movement account for about 15–30 percent of your burn, with about 10 percent from chewing, swallowing, digesting, absorbing, and storing the food you eat. Your body is constantly burning food, which releases units of heat (known as calories but technically called kilocalories, shortened to kcal).
The Law of Thermodynamics states that energy isn’t created or destroyed. It is simply transferred from the food you eat to the fuel your body uses to function.
If you aren’t eating enough, your body senses deprivation. Your metabolism slows when your diet isn’t giving you all the nutrients/calories you need. Adaptive thermogenesis is your body’s way of protecting you from losing too much energy if you’re suddenly dropped onto a desert island.
Then when you eat regularly or too much, your body would tend to hold on to those calories because it thinks you’re still on a desert island and you’re just having a rare feast.
Part of maximizing your metabolism involves ensuring that your body doesn’t feel deprived so that it keeps on burning calories as effectively as possible.
The degree to which your body burns calories varies based on many factors like what’s on your plate and what your diet’s been like in the past. Not all calories are created equal either: Protein has a greater thermic effect than fat, for example, meaning protein burns faster.
Not only does fat give you more calories per gram (9 calories per gram compared to carbs and protein offering 4 calories per gram), but fewer of those calories will be burned through the thermic effect.
To actually measure the amount of heat you’ve lost — to measure your metabolic rate — is a very expensive and complicated procedure. So, most scientists use something called indirect calorimetry instead, which measures oxygen consumption and carbon dioxide production. The oxygen you breathe in helps fuel your furnace, as it does when it keeps a fire burning.
For each liter of oxygen you take in, a certain amount of calories are expended, and your respiratory exchange ratio (ratio of carbon dioxide to oxygen) determines that amount.
