[1] St Michaels Hosp, Dept Med, Div Gastroenterol, Toronto, ON M5S 1A8, Canada
[2] Univ Toronto, Toronto, ON M5S 1A8, Canada
[3] Tufts Univ, Vitamin Metab Lab, Jean Mayer USDA Human Nutr Res Ctr Aging, Medford, MA 02155 USA
[4] Tufts Univ, Sch Med, New England Med Ctr, Dept Pathol, Medford, MA 02155 USA
[5] Harvard Univ, Sch Med, Massachusetts Gen Hosp, Dept Pathol, Cambridge, MA 02138 USA
[6] Tufts Univ, Sch Med, New England Med Ctr, Dept Internal Med,Div Gastroenterol, Boston, MA 02111 USA
[7] Tufts Univ, Sch Med, New England Med Ctr, Dept Internal Med,Div Clin Nutr, Boston, MA 02111 USA
[8] Univ Toronto, Dept Pathol, Toronto, ON, Canada