Can L-sugars contribute to whole body energy metabolism? Energy balance studies were undertaken in rats fed L-sugars at a rate of 10 g/100 g basal diet. Partial metabolizable energy values (MEVp) during the last 28 of 56 d while consuming the L-sugar diets showed that L-glucose contributed no energy (0.3 +/- 0.9 kJ/g, P > 0.5, mean +/- SEM), whereas L-fructose and L-glucose contributed 6.9 +/- 0.9 (P < 0.001) and 8.8 +/- 1.8 (P < 0.001) kJ/g, respectively. Over periods from 0 to 28 and 0 to 56 d of L-sugar treatment, measurements were made of energy intake, the average lean mass of animals, fat and lean mass deposition. Using these measurements and a computational model, estimates were made of each supplement's net metabolizable energy value for maintenance (NEVs). These estimates confirmed the supply of energy from L-fructose, with NEVs of 5.6 +/- 3.2 (P < 0.32) and 6.1 +/- 1.7 (P < 0.01) kJ/g over 28 and 56 d, respectively, and L-gulose with NEVs of 10.2 +/- 3.4 (P < 0.01) and 11.4 +/- 1.8 (P < 0.001) kJ/g over 28 and 56 d, respectively. A lack of energy supply from L-glucose was confirmed with NEVs of -2.6 +/- 3.2 (P > 0.5) and 0.0 +/- 1.6 (P > 0.5) kJ/g over 28 and 56 d, respectively. As reference substrates, sucrose and cellulose gave expected energy values in all determinations. Methods comparison analysis showed no differences between MEVp and NEVS. The assimilation of energy from L-fructose and L-gulose is probably via large bowl microorganisms. These observations show that L-gulose and L-fructose contribute to whole body energy metabolism while L-glucose does not.