1. Institute of Human Nutrition, College of Physicians and Surgeons, Columbia University, 630 West 168th Street PH 1512, New York, New York 10032, Department of Physiology and Biophysics, Department of Medicine, Boston University, 715 Albany Street, W-W302, Boston, Massachusetts 02118, and Department of Pediatrics, Columbia University, 630 West 168th Street, New York, New York 10032