Comments on the methods used to determine the energy values of carbohydrates: Dietary fibre, sugar alcohols and other bulking agents

Methods to evaluate the availability of food energy from difficult to digest carbohydrates are described. Emphasis is given to problems of interpreting results from energy determinations and to the accuracy and precision of the methods used. Consideration is given to determination of the heats of combustion of food components; determination of the metabolisable energy of diets and both apparent and partial metabolisable energy values of dietary components; and derivation and determination of net energy values of carbohydrates including methods to assess the availability of carbohydrates using enzymatic models, the ileostomy model, intubation, the labelled CO2 breath test, the hydrogen (H2) breath test, and isotopic tracer methods. The use and misuse of animals and indirect calorimetry in humans are also considered. Energy evaluation methods are classified into those which make direct determinations and those which determine indices for which established factors are needed for conversion into energy values. The direct methods appear less precise and potentially less accurate than methods that measure indices and use factors. In the latter, the precision (standard error of the determination) of several procedures is about 1 kJ/g (0.25 kcal/g) but in none is the accuracy established to be better than 2.5 kJ/g (0.6 kcal/g). Such precision is sufficient to allow inter-laboratory, inter-method and intersubstrate studies to be made. Discrimination between difficult to digest carbohydrates in food labelling could easily be justified at the 2 kJ/g (0.5 kcal/g) level provided that inter-substrate trials were undertaken, but, without such trials, discrimination greater than 3.5 kJ/g (1 kcal/g) would not seem warranted. The most precise and accurate approach to obtain energy values in humans appears to be one that couples the determination of apparent metabolisability with the determination of availability via the small intestine.