1. Metabolic Research Laboratories, Wellcome-Medical Research Council Institute of Metabolic Science, University of Cambridge, Cambridge CB2 0QQ, U.K.
2. Department for Health, Centre for Nutrition, Exercise, and Metabolism, University of Bath, Bath, Somerset BA2 7AY, U.K.
3. Department of Biology and Biochemistry, University of Bath, Bath, Somerset BA2 7AY, U.K.