1. Medical Unit (Section of Metabolism and Endocrinology), The London Hospital Medical College, London E1 2AD, U.K.
2. Department of Perinatal and Child Health, M.R.C. Clinical Research Centre, Harrow HA1 3UJ, U.K.
3. M.R.C. Biomedical NMR Research Centre, N.I.M.R., Mill Hill, London, U.K.