What Is a Calorie, Really? Understanding 'Energy In and Out'

You’ve probably heard the phrase “calories in, calories out” thrown around as a simplistic explanation for weight gain or loss. But what is a calorie, how is it measured, and how accurate are those numbers we see on nutrition labels and fitness trackers? Let’s break it down...

Photo by Artur Łuczka on Unsplash.

Calories: A Unit of Energy

A calorie is a unit of energy. Specifically, one calorie (small “c”) is the amount of energy needed to raise the temperature of 1 gram of water by 1°C. But when we talk about food, we’re actually referring to kilocalories (kcal), or 1,000 calories—enough energy to raise 1 kilogram of water by 1°C (FAO, 2003). So when a granola bar label says it has 200 calories, it actually contains 200 kilocalories.

How Calories in Food Are Measured

Originally, food calories were measured using a bomb calorimeter. This device combusts the food in a sealed container surrounded by water and measures the temperature increase to calculate total energy content. However, this method measures gross energy—some of which we can’t actually use due to losses during digestion and absorption (Merrill & Watt, 1973).

Today, the more common method for estimating calorie content is based on the macronutrient composition of food using the Atwater system:

• Carbohydrates: 4 kcal/g

• Proteins: 4 kcal/g

• Fats: 9 kcal/g

• Alcohol: 7 kcal/g

"Skim Milk Nutrition Facts" by Jamesmcq24 from Getty Images Signature via Canva.

These values are averages. The actual usable energy can vary depending on the food matrix, fiber content, and individual digestion efficiency (Livesey, 2001). For example, nuts are often overestimated in calorie content because of their incomplete digestibility (Novotny et al., 2012).

📝 BIG NOTE: In his thought-provoking paper, "Dissolution of Calories and Introduction to Modern Nutrition," Auston W. Cherbonneaux argues that the traditional calorie model — based on 19th-century combustion science — is outdated and insufficient for explaining how the human body actually processes energy. Cherbonneaux gives an overview of the way nutrients fuel the body not through literal combustion but through complex biochemical pathways involving molecular structure, electron movement, and cellular signaling. He contends that calories fail to capture this nuance and proposes replacing “calories in, calories out” with a more accurate, biology-informed framework, proposing an alternative "Intake, Call, Use" model as a more practical and personalized way of understanding how different bodies respond to food and exercise, offering a more meaningful approach to nutrition and health.

How the Body Burns Calories

Once food enters the body, its energy is used in several different ways. Let’s break down where your daily energy expenditure goes.

❤️ Basal Metabolic Rate (BMR)

This is the energy your body needs at rest to maintain vital functions like breathing, circulating blood, and regulating temperature. BMR accounts for 60–70% of most people’s total energy expenditure (Heymsfield et al., 2018). Factors influencing BMR include age, sex, body size, lean muscle mass, and genetics.

🍎 Thermic Effect of Food (TEF)

Digesting, absorbing, and metabolizing food also costs energy—typically about 10% of your daily calorie intake (Westerterp, 2004). Protein has the highest thermic effect (~20–30%), followed by carbohydrates (~5–10%), and fat (~0–3%) (Halton & Hu, 2004). So yes, your body does “burn” calories while eating.

🧹 Non-Exercise Activity Thermogenesis (NEAT)

NEAT includes all the energy you burn doing everything that isn’t sleeping, eating, or formal exercise — things like walking to the fridge, typing, or fidgeting. NEAT can vary dramatically — by up to 2,000 kcal/day between individuals of similar size—making it a key factor in weight maintenance and gain (Levine et al., 1999).

🏃 Exercise Activity

Intentional exercise contributes to total energy expenditure, but usually less than people think. A one-hour workout might burn 300–800 kcal, depending on intensity and body weight. But this is often overestimated, and your body may compensate by reducing NEAT or increasing hunger (Melanson, 2017).

How much energy you burn during exercise also depends on which metabolic pathway your body is using:

• Aerobic metabolism occurs in the presence of oxygen and is used during lower-intensity, longer-duration activities like walking, jogging, or cycling. It primarily burns fat and carbohydrates. Aerobic metabolism is highly efficient, generating about 36 ATP (adenosine triphosphate, the body’s energy currency) per molecule of glucose and over 100 ATP from one molecule of fat (McArdle et al., 2015).

• Anaerobic metabolism kicks in during short, high-intensity efforts like sprinting or heavy lifting, where energy demand outpaces oxygen supply. This system rapidly breaks down glucose into lactate, yielding only 2 ATP per glucose molecule. It’s less efficient but much faster (Brooks et al., 2005).

• Ketosis occurs during carbohydrate restriction or fasting. In this state, the liver converts fat into ketone bodies, which many tissues (including the brain) can use for energy. Ketone oxidation yields slightly less ATP than glucose or fat under aerobic conditions but offers a steady, long-lasting fuel source for endurance activities (Phinney & Volek, 2011).

Each system serves a purpose: anaerobic pathways offer rapid energy with a trade-off in efficiency and fatigue, while aerobic and ketogenic metabolism support sustained activity and fat oxidation.

Also worth noting is the afterburn effect, formally called excess post-exercise oxygen consumption (EPOC). This refers to the elevated calorie burn that continues after intense workouts as your body replenishes oxygen stores, clears metabolic byproducts, and repairs muscle tissue. While EPOC can contribute an additional 6–15% energy burn post-exercise (LaForgia et al., 2006), its overall contribution is modest compared to total daily activity.

Why Calorie Counting from Exercise Is Often Misleading

Calorie counters on fitness trackers and cardio machines are notoriously inaccurate — often off by as much as 20–40% (Lee et al., 2014).

💡 To estimate the calories burned during exercise, many caluclators use MET (Metabolic Equivalent of Task) value of the activity, your body weight, and the duration of the exercise. The formula is: Calories burned = (MET * body weight in kg * duration in minutes) / 200.

Fitness trackers with heart rate monitors will use that metric as as a variable in their calorie burn calculation, utilizing an equation like: Calories burned per minute=(Age×0.074)−(Weight (kg)×0.05741)+(Heart rate×0.4472)−20.4022.

These calculations can give you a rough estimate, but they aren't always accurate. How correct they are depends on how well the formula or device considers individual differences. For example, two people with the same heart rate might burn different calories because their body types and metabolism vary.

MET is the ratio of your working metabolic rate relative to your resting metabolic rate. It's standardized so that the value can be used for all groups of people, irrespective of their age, sex, and genetics. The value makes it easier to compare different activities to each other. The MET formula provides a very broad estimate, and it will not provide the exact number of calories burned during a particle activity. If you want to get an accurate number, the only way is to go to a lab that can do the work for you. They will attach you to machines that will calculate all the numbers correctly from your maximum oxygen uptake (VO2 max) to determine the calories burned during exercise.

Meanwhile, heart rate is a common and fairly reliable way to measure how hard you're exercising — but it has its limits. Factors besides exercise intensity — like hyration, tempatrue, emotional state, and medications — can affect your heart rate.

Plus, exercise-induced calorie burn is only one small part of your total daily energy expenditure. That’s why focusing on overall activity level — including steps, movement throughout the day, and strength training — is more useful than trying to fine-tune calories burned per workout.

Rather than aiming for precision (which is mostly illusory), a better approach is to establish sustainable habits: moving regularly, eating whole foods, and adjusting intake based on long-term trends in body composition, energy levels, and performance.

How Useful Are Nutrient-Based Calorie Estimates?

The Atwater system is a decent approximation, but it glosses over complexities in digestion and metabolism. Calories from protein are less efficiently stored as fat than those from carbohydrates or dietary fat. Fiber reduces net caloric availability. Highly processed foods tend to have more bioavailable calories than whole foods (Hall et al., 2019).

This means two 500-calorie meals — one of ultra-processed snacks and one of vegetables, lean protein, and whole grains — may have different impacts on hunger, metabolism, and fat storage. Quality matters.

The Takeaway

Calories are a helpful starting point: they represent "energy in" and "energy out." But the human body is not a bomb calorimeter, and not all calories are created equal. Understanding where your energy comes from and where it goes can empower you to make better choices — but don’t get lost in the math. Focus on patterns, not perfection.

REFERENCES

Brooks, G. A., Fahey, T. D., & Baldwin, K. M. (2005). Exercise physiology: Human bioenergetics and its applications (4th ed.). McGraw-Hill.

FAO. (2003). Food energy – methods of analysis and conversion factors. Food and Agriculture Organization of the United Nations. https://www.fao.org/3/y5022e/y5022e00.htm

Hall, K. D., Ayuketah, A., Brychta, R., et al. (2019). Ultra-processed diets cause excess calorie intake and weight gain: An inpatient randomized controlled trial of ad libitum food intake. Cell Metabolism, 30(1), 67–77.e3. https://doi.org/10.1016/j.cmet.2019.05.008

Halton, T. L., & Hu, F. B. (2004). The effects of high protein diets on thermogenesis, satiety and weight loss: A critical review. Journal of the American College of Nutrition, 23(5), 373–385. https://doi.org/10.1080/07315724.2004.10719381

Heymsfield, S. B., Peterson, C. M., Thomas, D. M., Heo, M., & Schuna Jr, J. M. (2018). Why are there race/ethnic differences in adult body mass index–adiposity relationships? A quantitative critical review. Obesity Reviews, 19(2), 187–210. https://doi.org/10.1111/obr.12629

LaForgia, J., Withers, R. T., & Gore, C. J. (2006). Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. Journal of Sports Sciences, 24(12), 1247–1264. https://doi.org/10.1080/02640410600552064

Lee, J. M., Kim, Y., & Welk, G. J. (2014). Validity of consumer-based physical activity monitors. Medicine & Science in Sports & Exercise, 46(9), 1840–1848. https://doi.org/10.1249/MSS.0000000000000287

Levine, J. A., Eberhardt, N. L., & Jensen, M. D. (1999). Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science, 283(5399), 212–214. https://doi.org/10.1126/science.283.5399.212

Livesey, G. (2001). A perspective on food energy standards for nutrition labeling. British Journal of Nutrition, 85(3), 271–287. https://doi.org/10.1079/BJN2000252

McArdle, W. D., Katch, F. I., & Katch, V. L. (2015). Exercise physiology: Nutrition, energy, and human performance (8th ed.). Wolters Kluwer Health.

Melanson, E. L. (2017). Exercise, energy balance and body weight regulation. Progress in Molecular Biology and Translational Science, 135, 129–152. https://doi.org/10.1016/bs.pmbts.2015.07.018

Merrill, A. L., & Watt, B. K. (1973). Energy value of foods: Basis and derivation. United States Department of Agriculture. https://naldc.nal.usda.gov/download/CAT87209099/PDF

Novotny, J. A., Gebauer, S. K., & Baer, D. J. (2012). Discrepancy between the Atwater factor predicted and empirically measured energy values of almonds in human diets. The American Journal of Clinical Nutrition, 96(2), 296–301. https://doi.org/10.3945/ajcn.112.035782

Phinney, S. D., & Volek, J. S. (2011). The art and science of low carbohydrate performance. Beyond Obesity LLC.

Westerterp, K. R. (2004). Diet induced thermogenesis. Nutrition & Metabolism, 1, 5. https://doi.org/10.1186/1743-7075-1-5