1. Laboratorio de Bioquimica Nutricional e de Alimentos, Departamento de Bioquimica, Instituto de Quimica, Universidade Federal do Rio de Janeiro, 21949-900 Rio de Janeiro, Brazil, and Department of Animal Nutrition and Physiology, Research Centre Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark