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Background: Roux-en-Y gastric bypass (RYGB) restricts food intake, and when the Roux limb is elongated to 150 cm, the procedure is believed to induce malabsorption.

Objective: Our objective was to measure total reduction in intestinal absorption of combustible energy after RYGB and the extent to which this was due to restriction of food intake or malabsorption of ingested macronutrients.

Design: Long-limb RYGB was performed in 9 severely obese patients. Dietary intake and intestinal absorption of fat, protein, carbohydrate, and combustible energy were measured before and at 2 intervals after bypass. By using coefficients of absorption to measure absorptive function, equations were developed to calculate the daily gram and kilocalorie quantities of ingested macronutrients that were not absorbed because of malabsorption or restricted food intake.

Results: Coefficients of fat absorption were 92 ± 1.3% before …