A calorie is a unit of energy. In nutrition, calories refer to the energy people get from the food and drink they consume, and the energy they use in physical activity.

Calories are listed in the nutritional information on all food packaging. Many weight loss programs center around reducing the intake of calories.

This MNT Knowledge Center article focuses on calories associated with food and drink, as well as the way the human body uses energy. MNT covers what a calorie is, how many calories humans need each day, and how to get calories in a way that benefits overall health.

Fast facts on calories

  • Calories are essential for human health. The key is consuming the right amount.
  • Everyone requires different amounts of energy each day, depending on age, sex, size, and activity level.
  • People in the United States consume more than 11 percent of their daily calories from fast food.
  • Foods high in energy but low in nutritional value provide empty calories.

Man looks at nutritional information while shoppingShare on Pinterest
The nutritional information on all food packaging will advise how many calories it contains.

Most people only associate calories with food and drink, but anything that contains energy has calories. 1 kilogram (kg) of coal, for example, contains 7,000,000 calories.

There are two types of calorie:

  • A small calorie (cal) is the amount of energy required to raise the temperature of 1 gram (g) of water by 1º Celsius (º C).
  • A large calorie (kcal) is the amount of energy required to raise 1 kilogram (kg) of water by 1º C. It is also known as a kilocalorie.

1 kcal is equal to 1,000 cal.

The terms "large calorie" and "small calorie" are often used interchangeably. This is misleading. The calorie content described on food labels refers to kilocalories. A 250-calorie chocolate bar actually contains 250,000 calories.

The United States government states that the average man needs 2,700 kcal per day and the average woman needs 2,200 kcal per day.

Not everybody needs the same number of calories each day. People have different metabolisms that burn energy at different rates, and some people have more active lifestyles than others.

The recommended intake of calories per day depends on several factors, including:

  • overall general health
  • physical activity demands
  • sex
  • weight
  • height
  • body shape

Here is a more detailed breakdown from the U.S. Department of Agriculture (USDA) on how many calories are needed for different body types.

The human body needs calories to survive. Without energy, the cells in the body would die, the heart and lungs would stop, and the organs would not be able to carry out the basic processes needed for living. People absorb this energy from food and drink.

If people consumed only the number of calories needed every day, they would probably have healthy lives. Calorie consumption that is too low or too high will eventually lead to health problems.

The number of calories in food tells us how much potential energy they contain. It is not only calories that are important, but also the substance from which the calories are taken.

Below are the calorific values of three main components of food:

  • 1 g of carbohydrates contains 4 kcal
  • 1 g of protein contains 4 kcal
  • 1 g of fat contains 9 kcal

As an example, here is the breakdown of how a person would get calories from one cup of large eggs, weighing 243 g:

Fat: 23.11 g

23.11 g x 9 kcal = 207.99 kcal

Protein: 30.52 g

30.52 x 4 kcal = 122.08 kcal

Carbohydrate: 1.75 g

1.75 x 4 kcal = 7 kcal

243 g of raw egg contains 347 kcal. 208 kcal comes from fat, 122 kcal is taken from protein, and carbohydrate provides 7 kcal.

Fast food in American diets

Researchers from the Centers for Disease Control and Prevention (CDC) published a report in 2013 showing that 11.3 percent of calories consumed by people in the U.S. come from fast foods.

Nutritionists and healthcare professionals say this figure is too high.

As people get older, they tend to get fewer of their daily calories from fast foods. Fast foods make up only 6 percent of the daily calorie intake of older adults.

However, with the number of highly calorific meals served in restaurants or aimed at younger individuals, it is important that people pay close attention to where they get their calories.

When should you eat?

Share on Pinterest
A hearty breakfast can help the body to use calories more effectively.

The time of day at which a person eats can shape how effectively their body uses calories.

Researchers at Tel Aviv University wrote in the journal Obesity that a large breakfast containing approximately 700 kcal is ideal for losing weight and lowering the risk of diabetes, heart disease, and high cholesterol.

A large breakfast may help to control body weight. When people eat matters as much as what they eat.

Empty calories are those that provide energy but very little nutritional value. The parts of food that provide empty calories contain virtually no dietary fiber, amino acids, antioxidants, dietary minerals, or vitamins.

According to ChooseMyPlate.gov, a diet management tool from the USDA, empty calories come mainly from solid fats and added sugars.

  • Solid fats: Although these exist naturally in many foods, they are often added during industrial food processing, as well as during the preparation of certain foods. Butter is an example of a solid fat.
  • Added sugars: These are sweeteners that are added to foods and beverages during industrial processing. They are filled with calories. In the U.S., the most common types of added sugars are sucrose and high-fructose corn syrup.

Added sugars and solid fats are said to make foods and drinks more enjoyable. However, they also add many calories and are major contributors to obesity.

Alcohol can also contribute empty calories to the diet. One normal serving of beer can add 153 kcal to a person's intake for the day.

If beer is not your drink of choice, you can use this calorie calculator provided by the National Institute on Alcohol Abuse and Alcoholism to work out how many calories alcohol adds to your diet.

Sources of empty calories

The following foods and drinks provide the largest amounts of empty calories:

Solid fats and added sugars

  • ice cream
  • donuts
  • pastries
  • cookies
  • cakes

Solid fats

  • ribs
  • bacon
  • hot dogs
  • sausages
  • cheese
  • pizza

Added sugars

  • fruit drinks
  • sports drinks
  • energy drinks
  • soda

Sugary drinks are the leading source of empty calories for people in the U.S.

Share on Pinterest
Sugary drinks are the main source of empty calories for people in the U.S.

More than half of all people in the U.S. have at least one sugary drink each day. Approximately 5 percent of people in the U.S. aged 2 years or older consume at least 567 kcal per day from sugary drinks. This is the equivalent of more than four cans every day.

There are ways of sourcing products with less solid fat or empty sugars. Rather than choosing the standard hot dog or a fatty cheese, for example, a person could choose low-fat options for either.

However, even the lower-fat options are no replacement for calories consumed from sources that also provide nutritional benefit. Rachel Johnson, a spokesperson for the American Heart Association (AHA), shared the following with MNT:

"Sugar-sweetened beverages are the number one single source of calories in the American diet and account for about half of all added sugars that people consume.

Most Americans don't have much room in their diets for a completely nutrient-void beverage. One recent study showed that drinking more than one sugar-sweetened beverage a day increases your risk of high blood pressure.

It's better if you can avoid them altogether and instead consume water, fat-free or 1 percent fat milk, 100 percent fruit juice, and low-sodium vegetable juices."

The intake of empty calories can be avoided or dramatically reduced by incorporating fresh, healthy food and drink into the diet.

Calories seem to be linked only to weight gain and obesity, but they are vital for health. They only pose a health risk when people consume more than the recommended amount.

When thinking about calories, you should not be considering just your diet but also your level of physical activity. A high intake of calories can be countered with regular, high-intensity exercise.