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Whole Body Metabolism Is Not Restricted to D-Sugars because Energy Metabolism of L-Sugars Fits a Computational Model in Rats^1,2,

Institute of Food Research, Norwich Laboratory, Colney, Norwich NR4 7UA, United Kingdom

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 (MEV_p) during the last 28 of 56 d while consuming the L-sugars diets showed that L-glucose contributed no energy (0.3 ± 0.9 kJ/g, P > 0.5, mean ± SEM), whereas L-fructose and L-gulose 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 (NEV_s). These estimates confirmed the supply of energy from L-fructose, with NEV_s of 5.6 ± 3.2 (P < 0.02) and 6.1 ± 1.7 (P < 0.01) kJ/g over 28 and 56 d, respectively, and L-gulose with NEV_s 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 NEV_s 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 MEV_p and NEV_s. The assimilation of energy from L-fructose and L-gulose is probably via large bowel microorganisms. These observations show that L-gulose and L-fructose contribute to whole body energy metabolism while L-glucose does not.

KEY WORDS: • L-sugars • food energy value • body composition • metabolic modeling • rats

¹ Support by Lev-O-Cal, c/o Biospherics Inc., Beltsville, MD.

² The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 28 November 1994. Revision accepted 10 July 1995.