Fructose-induced Intestinal de Novo Lipogenesis

8 healthy volunteers, 8 pre-bypass obese subjects, 8 subjects between 12-18 months post-bypass, and 8 subjects matched to the post-bypass patients will be studied in two sub-studies Sub-study 1) after ingestion of two or three different of the following test-meals: A)cream and whey protein B)cream, whey protein and fructose + 13C6 fructose C)test meal with cream, whey protein, fructose and glucose + 13C6 fructose Their intestinal de novo lipogenesis will be estimated by measuring 13C-palmitate on triglyceride-rich lipoprotein particles associated with apoB48 in blood Sub-study 2) after ingestion of two or three different of the following test-meals: A)cream labelled with 13C6 palmitic acid and whey protein B))cream labelled with 13C6 palmitic acid, whey protein, fructose and glucose Their intestinal de novo lipogenesis will be estimated by measuring 13C-palmitate on triglyceride-rich lipoprotein particles associated with apoB48 in blood in sub-study 1 Their exogenous lipid absorption kinetics will be assessed by measuring 13C palmitate on triglyceride-rich lipoprotein particles associated with apoB48 in sub-study 2