How do they measure Calories in food?
The Food and Drug Administration (FDA) oversees the process of measuring Calories in food. Food laboratories test the nutrient composition of a food using specialized equipment for chemical analyses. Food samples are often frozen (with liquid nitrogen) and blended into a fine powder to be analyzed. A ‘Kjeldahl analysis’ is used to calculate the amount of protein in a sample. The building blocks of protein, amino acids, contain nitrogen, a substance essential for life. Using the analysis a laboratory technician can extract the nitrogen from the food powder and use it to calculate how much protein is in the food.
Another analysis, known as a ‘hexane extraction’ can measure the amount of fat in a food sample. Finally, carbohydrate content is typically measured by the difference, or what is left over after the other nutrients are tested. The United States Department of Agriculture (USDA) has published a nutritional database that lists the calorie content for thousands of food items. This resource is now available online through the National Institute of Standards and Technology for a fee. The database is huge and covers every food imaginable.
Currently, the information that appears on food labels is dictated by The Nutrition Labeling and Education Act of 1990 (NLEA). The NLEA requires the Calorie content to be printed on packaged food labels.
Most of the calorie values in the USDA database (for packaged foods) are calculated using the Atwater system, an indirect calorie estimation. This measurement system does not require foods to be chemically analyzed in a laboratory setting. It is very basic. Total calories in a food item are calculated by multiplying the amount of grams of each of the three macronutrients (and alcohol) by the number of calories each provides per gram and then adding those numbers all together.
Do calories from fiber count?
Most carbohydrate foods contain dietary fiber. Dietary fiber is, in general, not digested and used by the body so that particular component is subtracted from the total carbohydrate content before it is calculated and added to the total.
- Drummond, K.E. & Brefer, L.M.: Nutrition for Foodservice & Culinary Professionals, 7th ed. John Wiley & Sons, Inc., New York, New York, 2010.
- J Painter. How do food manufacturers calculate the calorie count of packaged foods? Scientific American. July 31, 2006. http://www.scientificamerican.com/article.cfm?id=how-do-food-manufacturers
- National Institute of Standards and Technology: http://www.nist.gov/index.html