What is a calorie?

I often start my talks by asking this question: “what is a calorie?” and it always confuses people. Everyone knows what it is but they can rarely define the concept. We use the word all the time without giving it much thought. It’s on every food wrapper and many takeaway menus from large food chains will tell you the number for each dish. 

The best answer I tend to get is “energy”. And that is correct, a calorie is a measure of energy just like a meter is a measure of distance. To be exact, 1 calorie is the amount of energy it takes to raise the temperature of 1 g water with 1 degree Celsius. That isn’t much, which is why we tend to speak in kilocalories or Calories with a capital C, so times 1000.

It’s also not the only measure, energy is also measured in Joules or, if you are speaking about commodities as natural gas, this is measured per million British Thermal Units in the US. It measures the same thing: energy. Like all living things, we need energy to survive and function. Our energy expenditure (EE): how many calories we need, depends on 3 main factors:

The largest chunk of your calorie needs is determined by your Basal Metabolic Rate (BMR)

This sometimes comes as a surprise, but you are always burning calories because every cell in your body needs a constant supply of energy to stay alive. Your BMR is the amount of calories you need to exist at rest, not digesting any food and not moving. It’s the energy you need to just keep your heart beating, your lungs breathing and fuelling your cells and all other unconscious bodily processes.

This makes up about 60-70% of energy expenditure in most people. BMR is affected by body size and composition: bigger bodies have more cells and need more energy. It also matters what your body is made of. Muscle burns more calories than fat tissue which is why 2 people of the same weight can have very different energy requirements if one person has more body fat than the other.

But also what you eat affects how many calories you absorb

The thermic effect of food (TEF) is the energy you need to digest and absorb nutrients from the food you eat. This makes up about 10-15% of your calorie requirement depending on the type of foods you eat.

Some nutrients are harder to digest than others. Protein has a TEF of about 25%, this means that if you eat 100 kcal of protein, you will lose about 25% digesting it. For carbohydrate this number is 5-10% and fat 0-3%.

Eating lots of processed foods also means you’ll be at the lower end of this spectrum and you won’t lose many calories digesting and absorbing food. The clue is in the name here: processing food does the digesting for you. If you eat a diet based on whole foods your body needs to do all the work and therefore TEF will be higher, which means you’re effectively absorbing fewer calories from your food.

The final 25-40% of your calorie needs is determined by how much you move: PAL

The more active you are, the more calories you need. Physical Activity Level or PAL can be further divided between EAT: Exercise Activity Thermogenesis and NEAT: Non-Exercise Activity Thermogenesis, it’s basically the exercise you do on purpose and just living your life. EAT is your training session in the gym or going for a run. NEAT constitutes any other movement you do, so that’s walking to the tube, fiddling around or cycling to work.

The impact of EAT is usually overestimated and the impact of NEAT underestimated. People tend to think that if they exercise they don’t need to pay as much attention to their food intake because they’ll “burn it off” anyway. However, exercise doesn’t always burn as many calories as you hope. Even if your spin class burns 400 calories (bearing in mind most cardio equipment massively overestimates the calories burned) you need to take into account the calories you’d have burned regardless without doing anything.

If you hadn’t done the class you may still have burned 100 calories, so this class really only adds 300 calories to your Energy Expenditure. That’s only a little more than the average chocolate bar people down mindlessly with their coffee. If you spend the rest of the day sat at a desk and travel by tube like many people in London working in an office job do, your NEAT is going to be low.

On the other hand, someone who is on their feet all day and works as a waiter for example, but never set foot in a gym, may have a higher EE because his EAT may be zero, but his NEAT is high. Because we spend more time outside of the gym than inside, the effect of NEAT is often stronger. This is not to say exercise is useless but it is something to keep in mind if your goal is fat loss and you are making an estimation of your calorie needs. If you exercise but are not active during the rest of the day, be conservative with this estimation.

The energy balance determines the consequences of your intake 

So how do we put a number on these factors? Several formulas have been developed to make an estimation of calories, some are more accurate than others. Here’s a handy website that I often use to make an estimation of calories. The most accurate to date is the Mifflin St. Jeor Equation. The formulas are below:

Men

10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5

Women

10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161

The number that comes out of this equation is how many calories you need to sustain your body at its current weight and composition: maintenance calories. Whether you should eat this amount is depending on your goal. To describe the relationship between calories burned and eaten we use the term energy balance. If you burn as much as you eat the balance is 0 or neutral. You’re not gaining or losing any weight.

If you are trying to lose weight you’ll need to eat less calories than maintenance. Your body will then have a shortage and rely mainly on your body’s reserves: body fat, but also muscle tissue for energy to compensate. The energy balance is then negative and you’d be in an energy deficit. A positive energy balance means you’re eating more than you burn and you’ll gain weight. This weight gain will be a combination of muscle and fat. Gaining muscle generally goes best in an energy surplus but because muscle growth is a slow process you only need to be in a slight surplus. Eating well above maintenance may only gain you body fat.

Calorie calculations are never exact science 

Please note that these calculations of your maintenance calories, deficit or surplus are always estimations. It is really difficult, maybe even impossible, to accurately calculate how many calories you’re burning at any given time. It’s not exact science. You’d need a device called a metabolic chamber to measure this precisely and not even every university or research lab has that luxury. 

You should treat these numbers as what they are: estimations, a starting point from which you adjust as needed depending on progress you make but also how you feel. Say for example you see great fat loss results on 2000 kcal but feel sluggish all day, then it’s probably a good idea to increase your intake a little. So go for 2200 and see how you feel and progress with that intake. 

We get our energy in 3 main macronutrients: carbohydrates, fats and protein. There’s a 4th macronutrient which is alcohol but this isn’t considered an essential part of our diet so it’s often left out. Each macronutrient provides a different number of Calories per gram. Protein and carbohydrates provide 4, fats 9 and alcohol 7 kcal per gram. Even though alcohol is often conveniently left out of the equation, it can contribute significantly to your calorie budget.

The energy balance is what determines whether you lose or gain weight. Not so much where you are getting your calories from in terms of macros (protein, carbohydrates, fats) or foods (sugars, dairy, bread), when you are having your meals (I promise you can eat after 8PM and lose weight) or whether you have had your shot of apple cider vinegar in the morning (I promise this makes no difference).

The next part in the Nutrition Basics range explores the first macronutrient: carbohydrates.

References:

Barr, S. and Wright, J., 2010. Postprandial energy expenditure in whole-food and processed-food meals: implications for daily energy expenditure. Food & nutrition research, 54(1), p.5144.

McArdle, W.D., Katch, F.L. & Katch V.L. (2014). Exercise Physiology: Nutrition, Energy and Human Performance. Lippincott Williams and Wilkins; (7th edition).