Total carbohydrate values for foods in the USDA National Nutrient Database for Standard Reference (SR) are determined as the difference between 100 and the sum of water, protein, total lipid, ash, and alcohol content. Under the National Food and Nutrient Analysis Program, a NIH‐USDA collaboration, foods are nationally sampled and analyzed for dietary fiber (enzymatic gravimetric methods AOAC 985.29 or 991.43 (2010)), individual sugars; liquid chromatography (AOAC 982.14) or previously, AOAC methods (2010) with either high performance liquid chromatography (HPLC) or gas‐liquid chromatography (GLC)), and starch (enzymatic colorimetric AOAC 966.11 (2010)) or a polarometric method (The Feedings Stuffs Regulations 1982). Component analyses are performed separately and reflect the analytical variability inherent to each measurement process, so the sum of these carbohydrate fractions may not equal carbohydrate‐by‐difference (CBD). CBD values were compared to the sum of analytical carbohydrate fractions (total sugars, starch, and dietary fiber) for about 1,000 foods (e.g., baked products, cereal grains, fruits and vegetables) in SR; for commercially processed mixed dishes (with 6.1‐76.3g carbohydrate/100g), the sum of carbohydrate fractions was 91‐109% of CBD. In foods containing medium‐chain carbohydrates (e.g., dextrins, non‐digestible carbohydrates other than the mono‐ and di‐saccharides), the analyzed sum and CBD frequently differ. Analyses of these carbohydrates, included in some newer definitions of dietary fiber, “normalization” techniques, and ongoing analyses of carbohydrate fractions are essential.
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Pehrsson et al. (2015) studied this question.