Primary Energy Sources

Calories are a measure of food energy and are expressed in kilocalories or kilojoules. Published values, including those in the USDA nutrient database, usually are calculated values derived from general or specific fuel factors based on Atwater coefficients. Food manufacturers most commonly apply only the general energy factors to calculate calorie content for the product label, although other methods are allowed including use of the specific Atwater factors, general factors with subtraction of insoluble dietary fiber, specific factors approved by the FDA, or bomb calorimetry. The general Atwater factors of 4 kcal/g protein, 9 kcal/g fat, 4 kcal/g carbohydrate, and 7 kcal/g alcohol are applied to all foods regardless of their type, while specific Atwater factors vary by food group. Examples of specific energy factors used for major groups of food are shown below:

Protein factor kcal/g Fat factor kcal/g CHO factor kcal/g
Meat, fish, poultry 4.27 9.02
Eggs 4.36 9.02
Dairy products 4.27 8.79 3.87
Fats, animal 8.93
Cereals 3.91 8.37 4.12
Legumes and nuts 3.47 8.37 4.07
Vegetables 2.62 8.37 3.90
Fruits 3.36 8.37 3.60
Sugars 3.87
Fats, vegetable 8.84

Kilocalories can be converted to kilojoules (kJ) using the following:

kJ = 4.184 x kcal

Total Fat is a mixture of triglycerides, phospholipids, sterols and similar compounds. Values are determined by chemical analyses and are expressed in grams.

Total Carbohydrate values are calculated per 100 gm as the difference between 100 and the sum of the percentages of water, protein, fat, ash and alcohol. Total carbohydrate is approximately equal to the sum of available carbohydrates (starch and sugars) and dietary fiber. Values are expressed in grams.

Available Carbohydrate includes sugars and starches and is calculated as the difference between total carbohydrate and dietary fiber for most foods. (For foods containing large amounts of organic acids, such as citric acid in powdered drink mixes, available carbohydrate is calculated as the sum of sugars and starch.) Values are reported in grams.

available carbohydrate = total carbohydrate – total dietary fiber.

Protein values are calculated from total nitrogen content adjusted for non-protein nitrogen times a conversion factor specific for each food group. Values are expressed in grams.

Animal Protein is protein from animal products, including meats, eggs and dairy foods. Values are given in grams.

Vegetable Protein is the amount of protein contributed by plant products and is expressed in grams. The sum of animal and vegetable protein in a food equals the total protein of that food.

Alcohol volume is converted to weight by use of conversion factors based on the specific gravity of alcohol. Amounts are expressed in grams.

The percent of calories contributed by each macronutrient (protein, fat, carbohydrate, alcohol) is calculated by the NDSR software using the following method:

  1. Energy factors (e.g., general or specific Atwater factors) for each macronutrient and specific to each individual core food are incorporated into the database.
  2. The gram weight of each macronutrient for each food is multiplied by its energy factor to determine the calories provided by each macronutrient for that food.
  3. The calories contributed by each macronutrient are summed for all foods in the diet.
  4. The total calories per macronutrient are divided by the total calories in the diet and multiplied by 100 to equal the percent calories contributed by each macronutrient.

This method, beginning with NDSR 2008, provides the most accurate measure of both total calories and percent of calories from each macronutrient.