The 3 FactorsThat Determine Calorie Requirements

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Calories = energy. When we train and exercise, we require adequate calories to allow our body to produce the energy needed for the movements in the training session. But how much calories are required to not only produce enough energy, but to optimally perform?

Likewise, if you've ever tried to gain or lose weight, you've probably heard someone say "it's all about calories in (food consumed), and calories out (through exercise)". If you have MORE calories coming in through your diet than you expend during exercise, then you have a caloric surplus, and you're going to gain weight. Likewise, you've probably been told that if you consume LESS calories than you use during your workout, then you have a caloric deficit and will thus lose weight. While this is true in a broad sense, it doesn't tell us how much calories are REQUIRED not only from our diet and exercise, but how our body physiologically carries out its day-to-day processes. So how much calories do you need?

Everybody's energy requirements are different. The number of calories that are needed depends on body size, training status, age, sex, genetics, among other things.

This is important in understanding that all athletes and individuals require different amounts of energy to exercise, live, and perform. However, more than just calories in and calories out, there are THREE main factors that influence your energy (calorie) requirements more than anything else. The great thing about these THREE things is that they are physiologically the same for EVERYONE. Caloric requirements are different for each person...the factors that determine your energy requirements remain the same everyone.

SO what are the 3 factors that determine calorie requirements?

Factors Influencing Energy Requirements:
1. Resting Metabolic Rate
2. Thermic Effect of Food
3. Amount of Physical Activity 

But what does it all mean, Basil? Let's break these factors down a little bit...

1. Resting Metabolic Rate
Of all the factors, resting metabolic rate (or RMR for short) is the largest contributor to total energy expenditure, and therefore requirements. It is the amount of energy required by your body to maintain normal body functions such as heart rate, blood blow, thermoregulation, breathing etc. It is what is commonly referred to as "metabolism". If you have a high resting metabolic rate (i.e. metabolism), you burn more energy at rest than a lower RMR. Things that increase your RMR include an increase in lean muscle mass, high body temperature, being young, and something called hyperthyroidism. Factors that will decrease your RMR, and thus slow your metabolism down are a low calorie diet, loss of lean muscle mass, and age. 

In general, more physically fit individuals have a higher RMR and therefore require more calories through their diet to meet the body's demands. Likewise, if you are trying to lose weight for a healthier lifestyle, instead of restricting your calories (which decreases your RMR), you might want to increase exercise and eat a balanced diet throughout the day to increase your metabolism and thus amount of energy burned (see number 2 for WHY!). 

Determining your RMR can be done using simple online calculators (like THIS one) which use algorithms to estimate your energy needs. 

Resting Metabolic Rate is the largest contributor to your energy requirements and is commonly referred to as your "metabolism"

2. Thermic Effect of Food
This factor is one of the more unknown ones that people often never think of. Thermic effect of food is the increase in the amount of energy that is expended by your body above the RMR following a meal. This includes the energy requirements for your body to digest, absorb, metabolize, and store the food that it has just consumed. You might not think about it, but different foods require different amounts of energy to do these processes. For example, dietary fat is easy for the body to process and therefore results in very little thermic effect, while proteins are harder to digest and absorb and thus require greater amounts of energy to do so. 

I won't go into great detail in this article about that, but what you need to understand is that:

WHAT you put in your body is more important than HOW MUCH

Think about your daily diet and the macronutrient (carbohydrate, fat, and protein) breakdown. Do you eat more protein than carbohydrates (which you SHOULDN'T do...but again...let's stay on topic), and thus have a greater thermic effect of food? Because if you do, then you might require more overall calories to fill the void where the digestion of these high energy requirement foods took over. Make sense? Think about it...

3. Amount of Physical Activity
This is one of the more obvious ones. The amount of physical activity will (obviously) play a major role in determining how much energy (or calories) you require. If you have a high activity level, you will burn more calories and thus require more in your diet to replenish your stores. This is the most variable among individuals but is also the easiest to monitor and track. However, the TYPE of exercise as well as individual will also determine how much energy is required. 

For example, the highest energy cost of any type of exercise is long distance aerobic exercise by large athletes. Generally, the larger the individual, the more energy that is spent. Conversely, the lowest energy requirements are activities that use skill or power exercises, which is interesting (at least, interesting to me). You might associate high speed power exercises as needing more energy to perform, which they do on a rep-by-rep basis. However, these types of exercises are performed at a low-rep intensity. Cardio training is done over a longer period of time, and thus requires more energy. 

For an estimated daily calorie needs by activity level, see the table below:

Activity Level
Calories/kg body weight
Calories/kg body weight

Our energy requirements are supplied by the calories that we consume. However, as we now know, everyone is different and it's not nearly as simple as "calories in, calories out". Knowing these 3 factors that determine calorie requirements (above) can help YOU not only understand the different ways your body needs energy, but also how it can be used to optimize performance. 

Got questions? Let me know!

Kalan Anglos, Msc, cscs

Kalan is an Exercise Physiology & Sports Performance Expert who brings an evidence-based approach with practical-based knowledge to the world of combat sports and human performance. He is a PhD candidate and is recognized as a Certified Strength & Conditioning Specialist (CSCS) with the National Strength & Conditioning Association (NSCA), and has obtained his MSc. Kinesiology degree at the University of Victoria. Through his masters thesis research, Kalan has established and implemented the KFit Test Battery for Combat Sport Athletes which is used by both Karate BC & Karate Canada as their standard fitness test for  athletes across the country. Additionally, Kalan is an exercise physiology lab instructor at the University of Victoria, and trains individuals (including athletes) every day to help meet their fitness needs and goals. He has many years of experience both as an elite athlete and high performance coach and is knowledgeable in the many fields surrounding fitness and training for sports performance.