Composition of weight loss in morbidly obese patients after gastric bypass

Optimal treatment of morbid obesity requires reduction of excess body fat while minimizing lean tissue loss. In order to determine if gastric bypass surgery satisfies both these requirements, we have studied changes in body composition and weight loss in adult (mean age = 33 years) morbidly obese patients (14 males, 68 females) after gastric bypass surgery. The preoperative weight of this cohort was 136 ± 27 (SD) kg, which was 228 ± 37% of ideal body weight by actuarial standards. Lean body mass (LBM) was estimated from potassium-40 measurements obtained from a wholebody potassium-40 counter and by urine creatinine excretion on a subgroup of 20 patients. The average weight loss at 3, 6, and 12 months after surgery was 25, 35, and 46 kg, respectively. The weight loss phase lasted approximately 1 year, at which time 34% of the initial weight was lost and the percentage of loss of excess weight above ideal body weight plateaued at 61%. LBM accounted for 32% of the total weight loss at 1 month, 11% at 6 months, and only 3% at the end of 1 year. The percentage of body weight represented by LBM increased from 35% prior to surgery to 51% by 18 months after gastric bypass. Actual LBM content increased steadily after the first month following surgery. Mean total weight loss following three different surgical procedures for gastric bypass did not differ significantly. Gastric bypass surgery appears to satisfy the objectives of reducing excess weight contributed by fat while minimizing lean tissue loss in morbidly obese patients.