Male Wistar rats were fed a defined formula diet free from non‐starch polysaccharides and either containing no additive or supplemented with gum arabic or cellulose or starch. Food and faeces were analysed by bomb calorimetry both to assess the effect of these substances on the apparent digestibility of dietary gross energy and to ascribe digestible energy values to the supplements. The former was not affected by starch and was decreased more by cellulose than by gum arabic. The energy values obtained were 17.4 ± 0.4 kJ/g starch, 1.7 ± 0.6 kJ/g cellulose and 14.7 ± 0.5 kJ/g gum arabic. The latter is consistent with the high apparent digestibility of gum arabicin vivoand casts doubt on the validity of growth assay procedures that suggest a near‐zero energy value for gum arabic.