The ‘gold standard’ modern bariatric procedures – gastric bypass, sleeve gastrectomy and gastric band – are generally very safe and do not lead to significant calorie malabsorption.
Weight loss after these procedures is achieved via a combination of restriction and post-operative hormonal changes that lead people to eat less.
Some ideas of why people lose weight after surgery have been tarnished by what are now largely historical weight loss operations – performed more than 60 years ago – that acted very differently to modern day procedures.
Operations such as the ‘bilio-pancreatic diversion’ and ‘jejunal-ileal bypass’ could lead to significant malabsorption of calories and nutrients. These had potentially dangerous side effects and are now generally not performed routinely.
[There are some specialised centres, however, who still perform a variation of these ‘malabsorptive’ procedures for ‘super’ obese patients. These procedures (the commonest being the ‘duodenal switch’) are done in specialised Bariatric units with extremely close monitoring of post-operative nutrition and close patient follow-up.]