The human body requires energy to function. We gain energy by eating, and reduce it by living and functioning. When these two are balanced, body composition remains the same. Let’s look at the average body composition and how weight is distributed across different types of tissue.
The muscles and the brain are major sources of energy expenditure. The food you eat uses some energy to digest, but then also adds a lot of new energy to the system. Energy is an abstract concept that you can’t directly see, but we can measure it in common units like calories or joules.
This is similar to how a car works. Gasoline, for example, has about 31,000 calories per gallon. The food we eat, like fats or sugars, have slightly less energy but are still quite energy dense. Unlike your car, which only uses gas when the engine is on, your body is constantly on. Even while you sleep, the system is constantly using energy — your heart is beating, your brain is functioning, you are breathing and digesting, etc. Even though you don’t think about these things much, they require significant energy. This base level of energy is called Basal Metabolic rate.
Where does the energy get used?
A large amount of your total energy goes towards your basal metabolic rate. This distribution is often counter to people’s expectations. Exercise activity, though the most memorable, is often the lowest percentage. For example, just sleeping for 9 hours requires more calories than to run a mile. The energy needed just to exist for a few hours is more than it takes to move your body a mile. Even though it feels like doing nothing, your heart would have to beat more than 30,000 times, your brain activates millions of neurons and consolidates memories, your body creates and metabolizes a wide variety of chemicals, your lungs transport large amounts of oxygen and CO2, and so much more.
This chart can give you a rough idea of the magnitude of different ways your body spends energy.
You could think of it like spending money. The BMR is like paying rent and taxes, a constant but high amount required just for basic utilities. Other expenditure is like going out for food at a restaurant. Though the price may be more noticeable and under your control to change, generally it will be a small percentage of your total spending. This is similar to exercising or running to burn some calories.
Where is the energy stored?
Before we move into calorie counts, which are an abstract measurement of energy, let’s look at what we are actually measuring under the hood. There are three main energy storage systems — you can think of these like batteries in your body. Most energy expenditure will come from a combination of these fuel sources.
Optimizing the exact energy source at any time is not usually necessary, as they are all connected.
Calculating your calorie numbers
Let’s start with the basics of what we know for sure.
We know the human body uses more than a few hundred calories a day to power itself, even for just basic functions like breathing and having your heart beat. The energy usage is also less than 10,000 calories. If the human body was so inefficient that it burned that many calories, we would likely be extinct because finding that much food in a day would be impossible for most people. So the true answer is somewhere in between, generally around 1,000–3,000 for most people.
That helps to inform our mental models, but is still quite a wide range. Can we narrow it down even further? How many calories did you likely burn yesterday?
The most correct answer
The only true way to know how many calories you used is to measure intake over a long period (which is hard), and then also measure fluctuations in bodyfat (also challenging). At a perfect energy balance, body composition would remain identical (same weight, bodyfat levels and muscle mass). The body generally prefers this state, and often it can self-manage your eating and energy usage to keep it exactly the same even over many weeks or months. In a healthy metabolic system, it can do this by automatically adjusting your hunger levels, activity levels, etc. so they all add up at the end of the day. Measuring intake is not trivial or precise, but that approach would be much more tailored to your exact current usage than any statistical estimates or guesses from another app.
To do this, you should have a detailed weight history over many days or weeks. That allows for typical fluctuations to get smoothed out and results in a more correct trend line.
Independently from measuring your energy balance, we can estimate how much energy your body uses daily by a few well known equations. They essentially try to approximate it based on gender and size.
The following equations are some ways to measure BMR, which is generally about 70% of total energy usage. From BMR, your total daily energy expenditure can then be estimated based on diet and activity levels. Being more active will burn slightly more calories through NEAT and EAT, while eating a higher protein diet will burn more calories through thermic effect of food and the energy needs of digesting different items.
Original Harris–Benedict equation (1918)
Typically, a larger person will use more energy than a smaller person, and this trend is especially related to amount of muscle. On average, men also seem to use more energy than women, partly due to differences in muscle mass and body composition. These patterns were incorporated by Harris–Benedict into a simple BMR equation, which uses weight and height to output how many calories are used daily for basal metabolic rate.
Almost anyone should be able to calculate these, as the individual variables are very easy to measure. You likely know how tall you are, and how old you are. The only other thing needed is knowing how much you weigh, which can be done with any scale! Of course, the simplicity of these measurements also means the accuracy is limited, but it is good enough for our purposes of getting a rough idea of calorie burn numbers.
As we get older, our body’s energy usage changes slightly, and muscle mass is often lost as well. These trends are taken into account by subtracting a multiple of the age.
Revised Roza & Shizgal equation (1984)
Since 1918, we have learned a bit more about the human body and made more measurements. In 1984 (also long ago, but relatively more recent) the above formula was revised by Roza & Shizgal and published. You should note that this is just an estimate that gets you within 5–10% of the likely true measure, rather than a precise number to fixate on.
Updated Equation by Mifflin & St Jeor (1990)
In 1990, it was further improved and simplified by people called Mifflin & St Jeor. This iteration of the same formula is now the most commonly used calorie estimation.
We recommend thinking in terms of a range of possible values, with this number as the midpoint, rather than assuming it will be exactly that precise amount to an exact decimal. For example, if your number from the equation was hypothetically 1,937, that actually means the equation thinks it is somewhere between 1,800 to 2,100.
In the 90s when these equations were made, most people didn’t have bodyfat scales at their house, and obesity was also less prevalent. This equation should work well for average body compositions, but makes some assumptions about what percent of the body was muscle vs bodyfat. A more precise count could be determined incorporating your specific lean mass measurements (subtracting the bodyfat from total weight, essentially).
Cunningham or Katch–McArdle variations
If you track your weight and bodyfat consistently in Gyroscope, we will be able to estimate your lean mass and use it for these equations.
With Lean Body Mass (bodyweight minus the fat percentage), the formula can be potentially more precise, especially for someone who is lower or higher in bodyfat than the average person.
Most of the other variables in the previous equations, like gender or height, are there to simply approximate lean body mass. If we can more precisely measure our lean body mass, the formula can become more simple and precise. According to this simplification, each pound of additional muscle uses about 10 calories per day.
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