Protein
The Nutrient Data Laboratory (NDL) derived the values for protein, which were calculated from the level of total nitrogen (N) in the food, using the conversion factors recommended by Jones (1941).

The specific factor applied to each food item is provided. The general factor of 6.25 is used to calculate protein in items that do not have a specific factor. There is no factor for items prepared using the recipe program of the NDBS or for items where the manufacturer calculates protein.

Protein values for chocolate, cocoa products, coffee, mushrooms, and yeast were adjusted for nonprotein nitrogenous material. The adjusted protein conversion factors used to calculate protein for these items are as follows: chocolate and cocoa (4.74), coffee (5.3), mushrooms (4.38), and yeast (5.7). When these items were used as ingredients, such as chocolate in chocolate milk or yeast in bread, only their protein nitrogen content was used to determine their contribution to the calculated protein and amino acid content of the food.

Protein calculated from total nitrogen, which may contain nonprotein nitrogen, was used in determining carbohydrate by difference. This unadjusted protein value is not given in the Nutrient Data file; rather, it is given as a footnote in printed sections of "Agriculture Handbook 8."

For soybeans, nitrogen values were multiplied by a factor of 5.71 (Jones, 1941) to calculate protein. The soybean industry, however, uses 6.25 to calculate protein. The protein content of soy flours, soy meals, soy protein concentrates, and soy protein isolates is expressed both ways. The item calculated using the 6.25 factor is identified as "crude protein basis."

Total Lipid
The total lipid (fat) content of most foods was determined by gravimetric methods, including extraction methods such as those that use ether or a mixed solvent system of chloroform and methanol, or by acid hydrolysis. Total lipid determined by extraction is reported as Nutrient No. 204. It is sometimes referred to as "crude fat" and includes the weight of all lipid components, including glycerol, soluble in the solvent system. Nutrient No. 204 may not be identical to the fat level declared on food labels under the NLEA, where fat is expressed as the amount of triglyceride that would produce the analytically determined amount of lipid fatty acids and does not include other lipid components not soluble in the solvent system. The term "NLEA fat" is commonly referred to as "total fatty acids expressed as triglycerides."

Carbohydrate
Carbohydrate, when present, was determined as the difference between 100 and the sum of the percentages of water, protein, total lipid (fat), ash, and, when present, alcohol. Total carbohydrate values include total dietary fiber. Carbohydrate in beer and wine was determined by methods 979.06 (27.1.21) and 985.10 (28.1.18) of AOAC International (AOAC 2003), respectively. Total dietary fiber content was determined by enzymatic-gravimetric methods 985.29 and 991.43 of the AOAC (2003). Total sugars is the term used for the sum of the individual monosaccharides (galactose, glucose, and fructose) and disaccharides (sucrose, lactose, and maltose). Analytical data for individual sugars were determined using AOAC methods (2003), either high-performance liquid chromatography (HPLC) or gas-liquid chromatography (GLC). When analytical data for total sugars were unavailable for items in the FNDDS, values were imputed or obtained from manufacturers and trade associations. Starch was analyzed using the AOAC method 966.11 (2003). Because the analyses of total dietary fiber, total sugars, and starch are performed separately and reflect the analytical variability inherent to the measurement process, the sum of these carbohydrate fractions may not equal the carbohydrate-by-difference value.

Food Energy
Food energy is expressed in kilocalories (kcal) and kilojoules (kJ). One kcal equals 4.184 kJ. The data are for physiological energy, which is the energy value remaining after losses from digestion and metabolism, are deducted from gross energy. Calorie values, with the exception of multi-ingredient processed foods, are based on the Atwater system for determining energy values. Derivation of the Atwater calorie factors is discussed in "Agriculture Handbook 74" (Merrill and Watt, 1973). For multi-ingredient processed foods, calorie values (source codes 8 or 9; for more information on source codes, see p. 25) generally reflect industry practices (as permitted by NLEA) of calculating calories from 4-4-9 calories per gram of protein, carbohydrate, and fat, respectively, or from 4-4-9 calories per gram of protein, carbohydrate minus insoluble fiber, and fat. The latter method is often used for high-fiber foods.

Calorie factors for protein, fat, and carbohydrates are included in the Food Description file. For foods containing alcohol, we used a factor of 6.93 to calculate calories per gram of alcohol. No calorie factors are given for items prepared using the recipe program of the NDBS. Instead, total calories for these items equal the sums of the calories contributed by each ingredient after adjustment for changes in yield, as appropriate. For multi-ingredient processed foods, if the calories calculated by the manufacturer are reported, no calorie factors are given.

Calorie factors for fructose and sorbitol, not available in the Atwater system, were derived from the work of Livesay and Marinos (1988). Calorie factors for coffee and tea were estimated from those for seeds and vegetables, respectively.

Related topics include Minerals, Vitamins, Amino Acids, and Fatty Acids.