Calories are complicated, let's break it down.

Welcome to Fueling Field Notes, where I’m documenting my journey to preparing for the CSSD (Certified Specialist in Sports Dietetics) exam. These posts are equal parts study notes, science breakdowns, and practical takeaways from the field.

Let’s start with the basics, because even the word “calorie” is more complicated than it looks.

Scientists measure energy using a unit called the joule. But a joule is tiny, so in practice we use kilojoules (kJ), which are 1,000 joules grouped together.

In nutrition, we use calories instead. A calorie (lowercase c) measures the heat required to raise the temperature of 1 gram of water by 1 degree Celsius. Like the joule, it’s also a very small unit, which is why you’ll never see it on a food label.

What you do see on a food label is the kilocalorie (kcal), sometimes written as Calorie with an uppercase C. One kilocalorie equals 1,000 calories, and it represents the heat required to raise 1 kilogram of water by 1 degree Celsius. This is the unit the nutrition world actually runs on.

For the sake of conversion: 1 kcal equals 4.184 kJ.

For the rest of Fueling Field Notes, when I say calorie, I mean kilocalorie. That’s the standard in sports nutrition, and it’s what’s on your food label.

The thermal energy of the food is directly measured by a process called direct calorimetry. This analysis of food involves a device called a bomb calorimeter. The “bomb” is a metal container in which the food sample is burned in a pressurized, pure oxygen atmosphere. It’s surrounded by an insulated water bath. As the food sample burns, temperature changes in the surrounding water bath are recorded to determine the thermal energy of the food.

The calorie values you see assigned to macronutrients:

Carbohydrates: 4 calories/gram

Protein: 4 calories/gram

Fats: 9 calories/gram

Alcohol: 7 calories/gram

These are known as the Atwater general factors, named after an American chemist who developed them over 100 years ago.

Atwater measured how much energy food releases when burned in a bomb calorimeter, and recognized that the human body doesn’t absorb everything it consumes. So he adjusted the raw combustion values to account for digestive losses. The result was a practical set of values that could be applied to any food to estimate its available energy.

Here’s how he arrived at the final numbers:

Raw combustion values from the bomb calorimeter:

Carbohydrates: 4.1 calories/gram, protein: 5.6 calories/gram, fat: 9.4 calories/gram.

After adjusting for digestive losses, the values were rounded to the familiar numbers we use today.

Carbohydrates: 4 calories/gram

Protein: 4 calories/gram

Fat: 9 calories/gram

Note: The reason protein drops from 5.6 to 4 calories per gram is that protein contains nitrogen, and the body cannot fully burn nitrogen for energy the way it does carbon and hydrogen. Instead, nitrogen is excreted as urea in the urine, which represents energy that leaves the body unused.

The coefficient of digestibility is the percentage of a food that is actually absorbed and used by the body after digestion.

Individual foods can vary.

Highly processed and refined foods tend to have higher digestibility, meaning more of their energy is absorbed. Whole foods with more fiber and complex food matrices tend to have lower digestibility, meaning some energy passes through without being absorbed.

This is also why, mid-exercise, it’s ideal to select foods/fuel sources with higher digestibility so we can access all the energy it has to offer to support performance. To the processed-food police on the internet, it’s not broccoli that’s getting athletes across the finish line.

There are a variety of ways we can measure the amount of energy we expend. But it’s difficult. Even the most accurate measurement techniques have limited use with athletes because of their impracticality.

Here are just a couple of methods used in research:

Direct calorimetry measures the heat the body produces directly.

Remember the bomb calorimeter? The same principle applies here, except instead of burning food, a person sits inside a thermally insulated chamber, and the heat they release is measured. It’s the most accurate method we have, but also the most impractical. You’re essentially asking someone to sit in a very sophisticated box. Useful for research, not useful in the field.

Indirect calorimetry is the more practical option and the gold standard for measuring energy expenditure. Instead of measuring heat directly, it measures oxygen consumption and carbon dioxide production.

The logic behind it: The body burns fuel to produce energy, and that process requires oxygen and produces carbon dioxide. By measuring how much oxygen you consume and how much carbon dioxide you exhale, we can calculate how much energy your body is using.

Energy is needed for various bodily processes, such as basal functions (proprioceptive and conscious movements), digestion, absorption, metabolism, food storage, and expended energy during exercise.

Refers to the energy needed to perform normal daily body functions, aka: the energy needed to keep our body functioning even if you rested all day long.

RMR has become a more popular measurement, and it’s influenced by age, sex, body composition (height, weight, muscle mass), thyroid hormones, and genetic factors.

Interesting fact: In a lean adult, organs account for ~75% of resting energy expenditure, though they make up only 10% of total body weight. Although skeletal muscle accounts for only 20% of resting metabolic rate, it represents ~40% of total body weight.

RMR seems to decrease with age (2-3% every decade). Men generally have a higher RMR than women because of their larger body size and greater muscle mass.

RMR is often calculated using a formula/prediction calculator to provide estimates.