Friday, May 6, 2011

How are calories in foods determined?

Think back to your high school chemistry class and a very fun lab when you got to burn a piece of candy to determine the amount of calories released (I know Conestoga does it, and I hope you were so lucky to experience this lab!). As you may have learned from this lab, a calorie is a unit of energy and it is defined as the amount of energy needed to raise one kilogram of water by one degree Celsius. In terms of nutrition labeling, the calorie we know is actually a kilocalorie since it is 1,000 times larger than what chemists and physicists use in their line of work. If you have ever come in to see me for a nutrition consultation you may notice I write things as "40 kcal, 100 kcal" (kcal= kilocalorie) since that is the way we had to write the units during my nutrition classes and other sciences courses in college.

Previously, calories had been determined by the FDA similar to the way we did it in high school labs, with a bomb calorimeter. The food was burned all the way and the rise in water temperature was then measured. However, this is an outdated method which is not incredibly accurate.

Fast forward to the present method: In 1990 the George H.W. Bush signed the Nutritional Labeling and Education Act of 1990 (NLEA) which required most foods regulated by the FDA have nutrition information on their food label and that all nutritional claims ("low fat", "high fiber") meet the FDA regulations. This law also changed the way the calorie content of foods are determined. Instead of burning the foods, indirect calorimetry is done using the Atwater System... which basically breaks down the food into the energy containing nutrients (fat, protein, alcohol, and carbohdyrate). Fat= 9 kcal/gram, Protein= 4 kcal/ gram, Alcohol= 7 kcal/gram, and Carbohydrate= 4 kcal/gram. Knowing the calorie content per gram of macronutrient, scientists are able to calculate the caloric content of foods. This system is much more complicated than I will explain, but it gives a pretty good idea of the total available calories in a food... the human digestive tract is very efficient and will obtain most of the calories present (unless high amounts of dietary fiber are consumed).

I hope this little science lesson makes you more aware of where the calories come from in your food, I would go into much greater detail of how they determine the specific amount of each macronutrient (e.g: Nitrogen content -> protein estimation), and digestibility calculations, but I don't want to bore you. 

***EXCITING NEWS!!!: If you are a member of the Independence Blue Cross Personal Choice PPO Plan, you can have 6 free nutrition counseling sessions with me over the course of a year! To schedule an appointment, email me at:

"How do food manufacturers calculate the calorie count of packaged foods." Scientific American. 2006, July 31.

Agriculture and Consumer Protection. “The Relationship Between Food Composition and Available Energy.” 1981, Oct. 17.

Maynard, J. The Atwater System of Calculating the Caloric Value of Diets. 1944, Dec. 1.


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