What Are Calories, Exactly? Do Calories Make You Fat? It’s Complicated

How calories are defined and measured

By Michael Ormsbee, PhDFlorida State University
Edited by Kate Findley and proofread by Angela Shoemaker, Wondrium Daily

When we want to determine if a food will cause us to gain weight, we often look at the calorie count. How effective is this practice, though? Professor Ormsbee investigates.

Calorie concept
Actually, a kilocalorie, or kcal, is defined when describing a calorie as a unit of heat measurement defined as the amount of energy required to raise the temperature of one kilogram (kg) of water by 1℃. Photo By Vitalii Vodolazskyi / Shutterstock

What Is a Calorie?

Food contains energy that has the potential to be converted into useful work once metabolized or broken down by your body. That energy is measured in units called calories. What is a calorie and how do we know how many calories are in each food that we eat?

This is where things can get confusing.

Many people don’t know that by using this definition, you are actually describing what is also called a kilocalorie, or kcal. This is the typical calorie that you see on Nutrient Fact labels. 

“In reality, calories are sort of made up,” Professor Ormsbee said. “You don’t really eat calories; you eat different kinds of food with different amounts of nutrients in them.” 

Measuring Calories

However, we give foods a calorie count by burning them in something called a bomb calorimeter and measuring how much heat is given off. Scientists put food into this machine; and then they ignite the food. 

As heat is given off, it heats up the chamber that the food is in. The air leaves the chamber and flows through water that surrounds the chamber. Scientists then measure the change in water temperature, and from that change in temperature, they calculate how many calories were in the food.

Each gram (g) of carbohydrate equals four calories; 1 g of protein also equals four calories, and 1 g of fat is nine calories. You may be wondering how these ratios are calculated. 

Actually, when you burn 1 g of carbohydrate, 4.18 calories are released. When you burn 1 g of protein, 5.65 calories are released, and when you burn 1 g of fat, 9.44 calories are released. Alcohol releases 7.09 calories when combusted.

These numbers can vary by the type of carbohydrate, protein, and fat that you burn as well. What is interesting is that these numbers are slightly different from the four, four, and nine that are commonly taught in nutrition and exercise science classes. 

This is because the digestion and absorption rates of nutrients are less than 100% in most foods. This results in a reduced energy intake than when measured directly in a bomb calorimeter.

Gross Energy Vs. Net Energy

Another way of looking at this is gross energy versus net energy. The gross energy value is assigned to a food after being burned in a bomb calorimeter. The net energy value takes into account something called the coefficient of digestibility, or the percent of the ingested food that actually makes it through the gut and into your bloodstream.

For example, dietary fiber reduces the percent of the food that is usable by the body for energy production. This makes sense—we often hear of fiber as a nutrient that promotes regularity or regular bowel movements.

It turns out that many factors impact how many calories you actually can use in your body. Some of these might be how the food is grown—like the soil conditions, what the animal’s diet contained, how ripe the food was when harvested.

Even how you cook and prepare the meal makes a difference. In general, the macronutrient with the highest coefficient of digestibility is carbohydrates at 97%, followed by fats at 95%, and finally proteins at 92%. An interesting note is that there can also be variation even within a food category.

Thus, as you can see, the process of determining the calories in foods is probably more extensive than you imagined. It may also be a shock to see how inexact the process is. 

In fact, the calories listed on food labels are really just approximations because you can be sure that the data from the bomb calorimeter has a degree of variability. Research has estimated an error of up to 25% may be seen in the typical nutrition databases that you might use to calculate your calorie intake. We just don’t know how each individual will respond to the food and how much energy it will take to digest, absorb, transport, and ultimately excrete these foods. 

“Now you can probably see how calculating calories might give you a snapshot of the ‘Energy In’ part of the energy balance equation, but there are still some kinks in the system,” Professor Ormsbee said. “It does give you an idea of how things change over time, and I think it is useful to check in with your total calorie intake to keep those things honest.”

Consider the quality of the foods you eat, though, and not simply the quantity. In many cases, you can eat a lot more quality food and still maintain or lose weight.

Dr. Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University.

Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.

This article was edited by Kate Findley, Writer for Wondrium Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for Wondrium Daily.