How Do They Calculate Calories on Food Labels?

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Do you ever find yourself standing in line at a coffee shop or restaurant with a large display board that lists all the available foods and beverages along with their calorie content, trying to decide if you really want the large size fries with 490 calories or whether the medium size with 320 calories will feel satisfying? Or asking yourself: How can a 16 ounce caramel macchiato have THAT many calories??!! (For the record, it’s 280 calories).

We think about calories in terms of body weight: eat fewer calories to lose weight, eat more calories to gain weight, run on the treadmill to burn more calories. But just what is a calorie?

What are calories?

A calorie is a measure of energy. More specifically, one calorie is the amount of energy it takes to heat one kilogram of water 1 degree Celsius at sea level. When scientists first started to measure the energy, or calories in food, they used a bomb calorimeter. A sample of food like that large portion of fries is placed into a metal container called a bomb. The bomb is filled with oxygen and placed in another container where it is surrounded by water. The researchers ignite the food with an electric current, the water absorbs the heat released as the food burns, and a thermometer measures the changes in temperature in the water. Finally, the calories are determined by calculating the change in water temperature multiplied by the volume of water.

Calories on food labels

The 1990 Nutrition Labeling and Education Act (NLEA) for the first time required that food manufacturers put the amounts of nutrients and calories on the package label. Instead of using the bomb calorimeter method to establish calories in foods, manufacturers began using an easier process: the Atwater method.

The Atwater method

In the 1800s, William O. Atwater developed a process to determine the average number of calories in the three macronutrients in food: protein, fat, and carbohydrate. His 4-9-4 method came up with an average of 4 calories per gram of protein, 9 calories per gram of fat, and 4 calories per gram of carbohydrate that is still in use today. For example, using the Atwater method, a box of crackers that contains per serving...

  • 5 grams of fat (5 x 9 = 45 calories)
  • 22 grams of carbohydrate (22 x 4 = 88 calories)
  • 2 grams of protein (2 x 4 = 8)

...should contain approximately 140 calories.

It’s important to recognize that 4-9-4 is an average, and not an exact amount. For example, 1 gram of fat in one food may yield 8.34 calories while 1 gram of fat from another food yields 9.7 calories. The same thing happens with carbohydrate and protein: the 4-9-4 is an average, not an exact amount. Also, the NLEA labeling rules require that the calories from carbohydrate, fat and protein are rounded to the nearest whole number.

However, the Atwater method doesn’t take into account that the number of calories actually absorbed and used by the body can vary depending on the type of food, and even the individual person. A study published in the September 2008 Journal of Nutrition found that the calories in whole nuts are poorly absorbed due to nuts having very tough cell wall. If the cell walls aren’t broken down by chewing, they pass through the gastrointestinal tract without releasing all of the fat inside the cell walls. That means that some of the fat and the corresponding calories are excreted in feces and aren’t utilized for energy. Additional research shows that energy values using traditional Atwater factors overestimate the energy value of almonds and pistachios by 32% and 5%, respectively.

Our tips:

We want to believe that the information on food labels is 100% accurate, but in reality, it is a compilation of best available data rounded to whole numbers. That doesn’t mean the information isn’t useful, because there are many other variables when it comes to calculating calorie intake from foods. Most of us eyeball the portion size of foods we eat, while the calorie information on the food label is based on an accurate measurement. We tend to underestimate the amount we eat and it's very easy to forget to add in the teaspoon of butter added to the green beans at dinner or the tablespoon of mayonnaise on our lunchtime ham sandwich. How often do we actually measure the amount of salad dressing, guacamole, or whipped cream we’re using?

Use calorie information to your advantage:

  • Compare calories in similar foods, such as different brands of granola, crackers, or pasta to aid in making a choice of which one to purchase.
  • Use the calorie information on restaurant menus to choose meals and beverages that fit into your overall daily food needs. If I want an appetizer at the Cheesecake Factory, I could choose the 100 calorie Edamame, 410 calorie Crispy Crab Bites, or 1150 calorie Korean Fried Cauliflower. Who knew there could be such a difference?!

By Lynn Grieger, RDN, CDE, CPT, CHWC


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  4. Nutrition Education and Labeling Act of 1990. accessed 1-14-20; published 11-8-90
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  6. Novotny JA, Gebauer SK, Baer DJ. Discrepancy between the Atwater factor predicted and empirically measured energy values of almonds in human diets. Am J Clin Nutr. 2012 Aug;96(2):296-301. doi: 10.3945/ajcn.112.035782. Epub 2012 Jul 3.
  7. Baer DJ, Gebauer SK, Novotny JA. Measured energy value of pistachios in the human diet. Br J Nutr. 2012 Jan;107(1):120-5. doi: 10.1017/S0007114511002649. Epub 2011 Jun 28.
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