Energy: How a Calorie Works

What is a calorie? 

Two types of calories exist: the small calorie and the kilocalorie, also known as the large or food calorie.  The small calorie is generally measured only by scientists measuring reactions between chemicals.  The large calorie, or kilocalorie, is equal to 1,000 small calories.  To distinguish between a small calorie and a large calorie, writers sometimes capitalize “Calorie” when referring to the large calorie. 

When we discuss food and calorie intake as it is related to weight gain, weight loss, and nutrition, we are talking about the kilocalorie.  Most of the time when we talk about food, the word “calorie” is not capitalized. 

A calorie (whether a kilocalorie or a small calorie) as we generally understand it is a unit of heat energy.  Specifically, a kilocalorie is the amount of heat it takes to increase the temperature of one kilogram of water one degree. One kilocalorie produces 4.2 kilojoules of energy.

Where do calories come from?

Humans obtain their calories from food and beverages.  Nearly all foods and most beverages contain some amount of calories.  The nutritional makeup of the food determines how many calories the food will have.  

The calories in our food and drink come from three sources: carbohydrates, fats, and proteins.  Each source provides the body with a different number of calories.  One gram of carbohydrates contains 4 calories, one gram of protein also contains 4 calories, and one gram of solid fat contains 9 calories.     

In the past few years, there has been a debate about how our body processes or metabolizes the calories that from carbohydrates, fats, and proteins.  This debate is discussed below in “Is a calorie really a calorie?” 

What happens when we “burn calories”?

When we eat, our bodies process the calories and produce energy.  In other words, our bodies “burn calories.”  The energy derived from the food we eat allows us to live.

We use the energy to grow when we are children, to breathe, to digest food, to keep the heart beating, to keep the kidneys functioning, to develop new red and white blood cells, to contract muscles, and to digest food.  

The number of calories the body burns in this maintenance mode is called the basal metabolic rate.  The amount of energy required to digest food is called the thermal effect of food. 

In addition to helping our bodies grow and function on a daily basis, calories are also responsible for weight gain.  To lose one pound, we must burn 3,500 more calories than we eat.  If we want to gain one pound, we must eat 3,500 more than we burn. 

When we digest or metabolize food, the enzymes in our stomach change carbohydrates into glucose, fats into fatty acids, and proteins into amino acids.  If the glucose, fatty acids, and amino acids are not immediately used by the body, they are stored as fat.  It is when we are not able to use all of our energy and it is stored that we gain weight. 

Each of our bodies will metabolize or burn calories at a different rate.  Our sex, weight, amount of muscle, and age all determine our metabolic rate.  Men, people with more muscles, and younger people have higher metabolic rates, even when they are at rest, than do women and older people.  We can increase our metabolic rate for a short period of time by exercising.    

Is a calorie really a calorie?

According to scientists, a calorie is a calorie.  No matter where the calorie comes from, the energy produced from that calorie is exactly the same amount as the energy produced by any other calorie.  As discussed above, one kilocalorie will heat one kilogram of water one degree.    

During the past thirty years, some nutritionists, however, have started to investigate whether or not the body metabolizes calories from carbohydrates, proteins, and fats differently.  These studies have led to an extended debate of the benefits of high-protein, low-carb diets over low-fat diets. 

It is important to note that there are studies that come out on both sides of the argument.  For example, a study published in the American Journal of Clinical Nutrition found that dieters who followed a diet high in protein and low in carbohydrates experienced more pronounced weight loss than those who ate a low-fat diet. The scientists, however, were careful to note that they did not yet have a scientific reason for this difference in weight loss because the calories eaten produce the same amount of energy regardless of the source.