January 6


What is a Calorie and a Kilojoule? How to Measure your Energy Budget

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We hear about calories all the time. How many calories are in this cookie? Or how many do we burn in this exercise class, or with 20 minutes on the cross-trainer? But what actually is a calorie anyway? And how many of them do we need each day?

Essentially, calories are a way for us to keep track of our daily energy budget.


When we're in a healthy balance, we put in (i.e. eat) about the same amount of energy as we use. Over time, if we regularly put more energy into our bodies than we use, we'll gain weight. Our body will gradually store the excess energy in our fat cells.

On the other hand, if we continually use up more energy than we take in, we lose weight.

It's useful then for us to be able to measure the energy we are taking in and using. This is where the calorie (and kilojoules) comes in.

Essentially, the calorie is just the unit we use to measure energy

You'll see calories listed on nutrition labels as kcal.

By definition, 1 calorie is the amount of energy it takes to raise the temperature of 1 kg of water by 1 degree Celsius.

Kilojoules work similarly, they just use a different method to quantify energy. Some countries show kJ on nutrition labels rather than kcal.

Technically, the joule (or kilojoule) is the Standard International Unit for measuring energy. One Joule is defined as the work required to exert a force of 1 Newton over a distance of 1 metre.

Okay, that's enough of the physics, but you might like to know that calories and kilojoules are linked. Their scales are proportional so you can convert one to the other and vice versa. To convert calories into kilojoules, multiply by 4.184. On the flip side, to convert kJ into kcal, divide by 4.184.

Everything we eat (and drink) has a calorie count

Basically, it's a measure of how much energy is stored within the food




Commercial Carrot Cake (100g slice)

400 kcal

1675 kJ

375ml Can Coca Cola

192 kcal

803 kJ

Chicken Breast (100g)

105 kcal

438 kJ

Granny Smith Apple

74 kcal

309 kJ

Large Egg

56 kcal

235 kJ

During digestion, our body releases energy from food and stores it in other molecules. When we need to use some, these molecules get broken down to provide energy.

Typically, our bodies use this energy in 3 main ways:

  • Around 10% fuels the digestion process itself
  • Another 20% enables physical activity
  • The largest chunk, about 70%, keeps our basic functions in our organs and tissues going.

That last part is what's known as your basal metabolic rate. Essentially, it's the number of calories you'd need to live if you were laying perfectly still and not eating.

Doughnut chart showing how calories are used in the body, split between TEF, Activity and BMR

How energy is typically used within our body

On top of this, if you include the amount of movement you do and digestion, you'll get your total calorie requirements for an average day.

It makes sense that the more movement you do, the more energy your body needs.

If you're taking part in an extreme endurance event like the Tour de France, your body might need up to 9,000 kcal each day.

Pregnant women generally require a little more calories than normal. Elderly people on the other hand tend to have a slower basal metabolic rate. Their bodies burn energy more gradually, so they need less overall.

Before you start counting calories, here's something else you should know

Those nutrition labels on food items show how much energy that food contains. They don't indicate the amount of energy your body will actually absorb from it.

Some foods simply take more work and energy for our bodies to digest. For example, fibrous foods like celery and whole grains require a little more effort from our digestive system.

However, some foods release their calorie content fairly easily.

In other words, your body will take in less energy from a 100kcal serving of celery compared to a 100kcal serving of potato chips.

It's worth noting too that particular foods offer nutrients over and above the energy they provide. Nutrients like protein, vitamins and minerals - essential for good health.

On the other hand, some foods provide far less valuable nutrition. Eating too much of these types of food is likely to leave you overweight and even malnourished.

Since no two bodies are identical, there are also slight differences in energy production and usage

There will be natural variation in enzyme activity, our gut microbiome and even the length of our intestines. This means how our body processes the food we eat will be individual to each one of us.

Essentially, even with the same amounts and types of food, different people might not absorb the same number of calories.

So to sum up, the calorie is a useful way to measure energy.

However, to calculate exactly how many we actually require, we need to take into account several factors. Factors like exercise, what we do for work, the types of food we eat and how our body processes energy.

That's where an accredited sports nutritionist can become vital. We can help you with these calculations and devise an appropriate meal plan to assist your energy balance.

About the author

Paul Stokes

Paul Stokes BSc (Hons) is a Certified Personal Trainer, Accredited Sports Nutritionist, qualified Exercise to Music Instructor, Precision Nutrition coach, Massage Therapist and teaches 8 of the Les Mills Group Exercise programs.

He currently works in the Oil & Gas industry as a Wellness Coach, imparting his vast knowledge and experience to improve the quality of life of several hundred offshore workers.

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