THE FOOD · INVESTIGATION
What actually
is a calorie?
You've probably heard the word calorie hundreds of times.
But what exactly is it?
Calories are on food labels, in fitness apps, on restaurant menus and all over social media.
We're constantly told how many calories we should eat, burn, save or avoid.
And somehow, this little number has become one of the most important ways we talk about food and body weight.
But what exactly does the number mean?
And why does something as simple as a number have such a huge role in the way we talk about food and body weight?
Let's start from the beginning.
What is
a calorie?
A calorie is a unit of energy. In nutrition, the word usually refers to a kilocalorie, abbreviated as kcal.
Strictly speaking, a kilocalorie is 1,000 calories. However, nutrition labels and everyday conversations usually use the word "calorie" when they mean a kilocalorie.
So when a food label says that something contains 200 calories, it means 200 kcal.
actually represent?
Food contains chemical energy stored in the bonds between its molecules.
When you eat, your digestive system breaks food down into smaller molecules that your body can absorb and use.
Your cells can then extract energy from these molecules and use it to power the processes that keep you alive — from maintaining your body temperature and sending signals through your nervous system to contracting your muscles and building new molecules.
How do we actually
measure the energy
in food?
This is where things get slightly more interesting.
The energy content of food can be measured using a device called a bomb calorimeter.
The name sounds dramatic, but the principle is relatively simple.
A sample of food is burned in a sealed container surrounded by water. The heat released by the combustion warms the surrounding water, and the change in temperature can be used to calculate how much energy was released.
Your body obviously doesn't burn food in exactly the same way as a calorimeter.
Digestion, absorption and metabolism are biological processes, and not all of the energy in food becomes available to your body.
The number is an estimate of the amount of metabolizable energy the food provides.
In other words, calories are useful — but they're not a perfect measurement of exactly what your body will do with a particular food.
Where does the
energy in food
come from?
The energy in food mainly comes from three macronutrients: carbohydrates, fats and proteins.
Your digestive system breaks these nutrients down into smaller molecules that can be absorbed and used by your body.
Carbohydrates are broken down into simple sugars, including glucose. Fats are broken down into fatty acids and other molecules, while proteins are broken down into amino acids.
Your body can use these nutrients in different ways depending on what it needs.
Some molecules can be used as sources of energy, while others are used to build and repair tissues or stored for later.
And this is why the three macronutrients don't provide the same amount of energy per gram.
Alcohol also provides energy, although it isn't considered a macronutrient in the same way as carbohydrates, fats and proteins.
FOLLOW THE ENERGY
Energy isn't
just one thing.
When we talk about "energy" in nutrition, it can sound like energy is one single thing.
Energy can exist in different forms, including chemical, thermal and mechanical energy.
Chemical energy is stored in the bonds and molecular structures of substances. Food contains chemical energy, just like fuels such as gasoline.
Thermal energy is associated with the movement of particles and is what we experience as heat.
Mechanical energy is associated with movement and physical work.
The important part is that energy can be transferred from one form to another.
Your cells aren't
tiny car engines.
But the analogy helps.
Take a car.
Gasoline contains chemical energy. When the fuel is burned in the engine, that energy is released largely as thermal energy. The engine then uses that energy to ultimately produce mechanical work: moving the car.
Your cells also transform energy, but they do it differently.
When your body breaks down nutrients, the chemical energy stored in those molecules can be transferred into another form of chemical energy: ATP, short for adenosine triphosphate.
ATP is often described as the cell's "energy currency."
Instead of using the chemical energy in food directly for every cellular task, cells transfer some of that energy into ATP, which can then power processes that require energy.
Your muscles use ATP to contract. Cells also use it to transport substances across membranes and to build and maintain molecules such as proteins.
Cells also release some of that energy as heat along the way.
One of the main processes involved in producing ATP is cellular respiration: a series of chemical reactions that allows cells to extract energy from nutrients and transfer it into forms they can use.
Your cells aren't tiny car engines. But the basic idea is useful: food contains chemical energy, and your cells transform it into forms that can power the work of living.
Are calories
"good" or "bad"?
We often talk about calories as if they have a moral value: "good calories," "bad calories," or foods that are "too high-calorie."
a measurement.
It isn't healthy or unhealthy by itself.
What matters is the food that provides those calories and what that food contributes to your overall diet.
Two foods can contain a similar number of calories while providing very different amounts of protein, fibre, vitamins, minerals and other compounds that affect health.
At the same time, a food doesn't have to be exceptionally nutrient-dense to have a place in your diet.
Food also provides pleasure, culture, convenience and social connection.
Think of calories as information, not a moral score.
So why do calories
matter for body weight?
If a calorie is simply a unit of energy, why does the amount of calories we eat have anything to do with changes in body weight?
energy balance.
Your body is constantly taking in energy from food and drinks while also using energy to keep you alive and carry out everything you do.
When, over time, your body receives more energy than it uses, the excess energy can be stored in the body.
When your body uses more energy than it receives, it has to draw on stored energy to make up the difference.
That's what we mean by energy balance.
But this doesn't mean body weight is controlled by a simple calculator where you can perfectly predict what will happen after one day of eating and exercise.
They are influenced by things such as body size, muscle mass, physical activity, digestion, hormones and many other physiological processes.
And importantly, the amount of energy you eat can also influence how much energy your body uses.
Changes in body weight can affect energy requirements, while changes in hunger, fullness and spontaneous movement can influence how much energy you consume and expend.
WHERE THE ENERGY GOES
Your body uses
energy for much
more than exercise.
Your total daily energy expenditure, often abbreviated as TDEE, is the total amount of energy your body uses over the course of a day.
Resting energy expenditure
The energy your body needs to stay alive and maintain basic functions such as breathing, circulation and body temperature.
Physical activity
The energy used for movement — from exercise and walking to standing, cleaning and climbing stairs.
Thermic effect of food
The energy your body uses to digest, absorb and process the nutrients you eat.
These components don't stay perfectly constant. They can change from day to day and over time as your body, activity levels and food intake change.
How does your energy
expenditure change?
is not a fixed number.
It changes with your body, your activity and your energy intake.
One important factor is body composition.
Different tissues have different metabolic demands. Organs such as the brain, liver, heart and kidneys use substantial amounts of energy even when you're resting.
Muscle tissue also requires energy to maintain itself, although its energy use per kilogram is lower than that of several of these organs.
This means that, all else being equal, having more lean mass generally contributes to a higher resting energy expenditure than having the same body weight with less lean mass.
Your overall body size matters too. A larger body generally requires more energy to maintain, and moving a larger body also requires more energy.
Your energy expenditure can change over time as well. If you lose body mass, for example, your resting energy expenditure generally decreases because you have less tissue to maintain and less body mass to move.
Changes in activity can also raise or lower your daily expenditure.
They reflect the body you have and what that body is doing.
SO, WHAT DID I ACTUALLY LEARN?
Calories are useful.
But they don't tell
the whole story.
A calorie is simply a unit of energy.
Food contains chemical energy, and your body transforms that energy into forms it can use to keep cells functioning, move, build and repair tissues, and maintain the body.
The calories in a food tell you how much energy it provides.
They don't tell you whether that food is nutritious, filling, enjoyable or appropriate for a particular situation.
Your body is also using energy constantly — not just when you exercise.
Your resting metabolism, physical activity and the processing of food all contribute to your total energy expenditure.
And that expenditure isn't fixed.
It changes with your body size, body composition, activity and physiological state.
What does this food actually bring to the table?
AND MAYBE THAT'S THE POINT
Calories are
information.
They can tell you something useful about the energy contained in your food.
But they don't tell you everything about that food.
And they certainly don't tell you whether you were "good" or "bad" for eating it.
not a moral score.
Your body is much more complicated than a number on a food label.
And honestly, I think that's a much more interesting thing to learn about.
Don't fear a whole macronutrient over one bad decade.
READ THE EVIDENCE
Sources
- Nordic Nutrition Recommendations 2023. Nordic Council of Ministers.
- Food and Agriculture Organization / World Health Organization. Human energy requirements.
- National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes.