Ask the Experts: Understanding Calories

Words by Taylor Thomas at Thomas Endurance Coaching

When it comes to sports nutrition there are a million different formulas being presented to athletes. Whether it’s specific diets, “must have” supplements, or the next performance enhancer, there’s plenty of options for athletes to choose from. While ultimately an athlete’s diet is a personal decision when it comes to nutrient intake, there are some concrete scientific concepts that, if understood, will help to inform overall nutrition strategies. Understanding the foundation of nutrition, and the role these things play in performance, fueling, and recovery will lead to a better approach overall, regardless of an athlete’s high-level strategy.

What is a calorie, Calorie and Kilocalorie?

One of the building blocks of our modern understanding of nutrition and fueling for sport is the calorie. While we’re inundated with this word, it’s easy to overlook or miss what an actual calorie is. At the very basic level, a calorie is a unit of measure that is used to measure energy in a laboratory. The Calories that are listed on food packages like gels and energy bars are actually kilocalories, or 1,000 calories (as measured in a lab). A Calorie (as found on food) is the amount of energy needed to raise 1 kilogram of water 1 degree Celsius. This measure of food energy can also be expressed in kilojoules where 1 kilocalorie (kcal) equals 4.184 kilojoules (kj). So, the Calorie on a package of food is 1,000 times larger than that used in chemistry and physics.

Historically the kcals in a particular food product where calculated based on the direct amount of energy they could produce. This was measured using a complex system where the food was burned and the resulting rise in water temperature was measured. Now, food manufacturers use an estimation system where standard values for fat, carbohydrate, protein, and alcohol are used to provide Calories. These average values are 4 kcal/g for protein, 4 kcal/g for carbohydrate, 9 kcal/g for fat, and 7 kcal/g for alcohol. So, the label on an energy bar that contains 10 g of protein, 20 g of carbohydrate and 9 g of fat would read 201 kcals or Calories.1 With this understanding of Calories it’s easy to see the direct link between the food we take in, and the energy it provides to fuel our activities.

How are Calories Used?

The number of Calories in food is a measure of how much potential energy that food possesses. As discussed above each macronutrient carries a certain amount of energy per gram. When food is ingested into the body, this potential energy is unlocked (burnt) to fuel us. The body needs Calories (energy) for everything it does. Our basal metabolic rate, simply the energy needed to keep us alive, needs fuel, along with digestion, physical activity, and more. The Calorie is a vital building block of our body’s ability to maintain proper function.

Our bodies “burn” the Calories in food through metabolic processes where enzymes break the carbohydrates into glucose and other sugars, the fats into glycerol and fatty acids and the proteins into amino acids. These broken down molecules are then transported through the bloodstream to the cells where they are used for immediate use as fuel, or sent to the final stages of the metabolic process where they react with oxygen to release their stored energy.2

Does the source of Calories matter? While the decision to eat certain types of foods, and where those foods come from, is a largely personal conviction, it is worth looking at the comparison between “whole” and processed foods as seen from the body’s perspective. The primary concept to understand is that it takes energy to eat. Chewing, swallowing, churning the stomach, making stomach acid, producing enzymes, it all takes energy. Scientists have three names for this fact: dietary induced thermogenesis (DIT), thermal effect of food (TEF), or specific dynamic action (SDA). What’s important to note about these processes as it relates to athletes and fueling is that processed food takes less energy to digest and absorb, thus there is a greater net Calorie gain when compared to whole foods that use more energy when digested.3 If weight maintenance or loss is a goal this could be a particularly vital consideration when planning intake for any time during a training cycle.

Balancing Burn and Intake for Athletes

The concept of “Calories in, Calories Out” takes on a much different meaning for athletes who are looking to maximize their performance. There must be an appropriate amount of Calories available to fuel training sessions and recovery. However, too much and weight gain may become an issue which could negatively impact performance. Knowing exactly how many Calories one needs isn’t a precise science, as every athlete is different in regards to their unique energy systems, body composition, and training regiment. Although, there are some guidelines that can help to better understand the body’s energy needs in relation to training. To better estimate caloric requirements athletes can categorize training sessions based on intensity.

  • Mild activity or rest day: 12 -14 Calories per pound of body weight.
  • Up to 1 hour of moderate exercise: 15-17 Calories per pound.
  • High activity = 1-2 hours of moderate exercise: 18-24 Calories per pound
  • Very High = 3+ hours of training 24-29 Calories per pound

You can also estimate the number of Calories you need on a particular day by adding together the amount needed for your estimated or measured Resting Metabolic Rate (RMR), the amount needed for activity when not training, and the amount needed for the training time for your specific sport. RMR is typically estimated at 11-12 Calories per pound, although this can change dramatically based on any number of individual scenarios.4 When all of these factors are taken into consideration it’s easy to see how an athlete in training could need dramatically more Calories to sustain their activity than those less active, or non-active individuals. Fueling for performance is key for those that are looking for a healthy and sustainable approach to training.

An individual’s approach to diet and nutrition can vary much in the same way that every athlete takes a unique approach to their training. While many of these concepts are a personal decision in regards to fueling and what an athlete puts into their body, there are foundational principles that all athletes should be aware of. Understanding what a Calorie is, how it’s used, and how they should be adjusted in relation to training goals is paramount. Everybody needs energy from food to simply sustain life. Factor in caloric burn from training volume and intensity and the approach should begin to look very different.

Taylor Thomas is the founder and head coach of Thomas Endurance Coaching (TEC) and has more than a decade of experience in the endurance sports industry as an athlete, coach, race promoter, and team organizer. TEC provides expert level coaching to athletes of all ability levels and specializes in both a scientific and metrics-based approach to endurance sports. They guide athletes in all disciplines of both running and cycling. For more information on their personal coaching and training plan options visit TEC at @endurance_coach.


Painter, Jim “ How Do Food Manufacturers Calculate the Calories of Packaged Foods?” Scientific American, May 2003

Julia Layton “How Calories Work” 9 November 2007. <> 11 December 2018

Helen Kollias “Research Review: A Calorie Isn’t a Calorie” <>

Ryan, Monique “ Sports Nutrition for Endurance Athletes” Second Edition, Velo Press 2007 81-84

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