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

Inclusion Criteria:

• age 18-50 years
• sex male or female
• less than 2 kg body weight change over the past 3 months subjects

Exclusion Criteria:

• antidiabetic and hypolipemic drugs
• alcohol consumption >20g/day
• severe eating disorders
• severe psychological problems
• vegetarian diet or other specific diet
• consumption of illicit substances
• pregnancy
• weight gain or weight loss > 1 kg in the last 3 months

8 non-obese healthy male and female subjects with BMI <25 8 obese male and female subjects with BMI> 35 8 post-gastric bypass male and female subjects 8 healthy subjects matched to post-gastric bypass subjects for age, gender and weight