Calories Explained: Energy Units and Body Use

Published January 2026

What Is a Calorie?

A calorie is a unit of energy. Specifically, one calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius. In nutrition, we typically use "kilocalories" (kcal), though they're commonly referred to simply as "calories."

When we talk about the calories in food, we're discussing how much energy that food contains. This energy is what powers all biological processes in the body—from the beating of your heart to the thinking happening in your brain right now.

How Calories Are Measured

Scientists measure the caloric content of foods using a method called bomb calorimetry. A food sample is placed in a sealed chamber surrounded by water. The food is combusted, and the resulting heat raises the temperature of the water. By measuring how much the water temperature increases, scientists can calculate exactly how much energy that food contains.

This is a precise, reproducible method that's been used for over a century to establish nutritional data. Different foods release different amounts of energy when combusted—fats release the most energy per gram, while carbohydrates and proteins release less.

How the Body Uses Energy

The human body uses the energy from food for three main purposes:

1. Basal Metabolic Rate (BMR)

This is the energy required for basic biological functions at rest. Your heart must beat continuously, your lungs must breathe, your cells must repair themselves, and your body must maintain body temperature. These processes account for a significant portion of daily energy use—often 60-75% of total daily expenditure for sedentary individuals.

2. Thermic Effect of Food (TEF)

The body uses energy to digest, absorb, and process nutrients from food. This accounts for roughly 10% of daily energy expenditure. Different foods require different amounts of energy to process—protein requires more energy to digest than carbohydrates or fats, a phenomenon called the "thermic effect of protein."

3. Physical Activity

Movement requires energy. Exercise, sports, daily activities, and even fidgeting all consume calories. The amount varies greatly depending on the intensity and duration of activity and individual factors like body size.

Energy Balance

The body's energy balance is determined by the relationship between energy intake (calories consumed) and energy expenditure (calories used). When intake equals expenditure, weight remains stable. When intake exceeds expenditure, excess energy is stored, primarily as fat tissue. When expenditure exceeds intake, the body draws on stored energy reserves.

This is a fundamental biological principle, though it's more complex than a simple equation because many factors influence both sides of this balance.

Individual Variation

While caloric values of foods are measurable and consistent, the amount of energy different people use varies significantly. Two people of the same size, age, and activity level may have different metabolic rates due to genetics, hormonal factors, previous dieting history, and other variables.

This is why generic calorie calculations are approximations. They provide useful starting points for understanding energy balance, but individual needs require individual assessment.