A sleeve gastrectomy removes roughly three-quarters of the stomach, leaving a narrow tube. A Roux-en-Y gastric bypass leaves the stomach in place but creates a small pouch at the top and reroutes the small intestine to it. Both restrict how much can be eaten. Only one of them changes how food is absorbed, and that difference drives most of the decision.

Sleeve
Removes part of the stomach. No rerouting of the intestine
Bypass
Small stomach pouch, intestine rerouted. Restrictive and malabsorptive
Both
Laparoscopic in most cases; usually 1–3 nights in hospital
Both require
Lifelong dietary change and, for bypass especially, lifelong supplements

What the sleeve does

The greater curvature of the stomach is removed and the remainder is stapled into a tube. The restriction is obvious. Less obvious is the hormonal effect: the removed portion is where most ghrelin is produced, the hormone most associated with appetite, and appetite after a sleeve is often reduced in a way pure restriction would not explain.

It is a simpler operation than a bypass, with no intestinal join, and nothing is rerouted — which matters if the stomach may need to be examined by endoscopy later, or if the patient takes medicines whose absorption is hard to predict after a bypass.

What the bypass does

Food enters a small pouch and passes directly into the small intestine, skipping the rest of the stomach and the first section of the bowel. So there is restriction and there is reduced absorption, and there are hormonal changes in the gut that affect blood sugar regulation quickly — often before much weight has been lost at all.

That last point is why bypass is frequently favoured for patients with type 2 diabetes, and why the literature on metabolic surgery reports higher rates of diabetes remission after bypass than after sleeve. Remission is not cure: it can relapse, particularly if weight is regained.

Reflux: the factor that decides many cases

A sleeve can worsen gastro-oesophageal reflux, and can cause it in people who did not have it. A bypass usually improves it. For a patient with significant existing reflux or a hiatus hernia, this single fact often settles the choice — and it is the most common reason a sleeve is later converted to a bypass.

What has to be done for the rest of your life

  • Vitamin and mineral supplements — particularly B12, iron, calcium and vitamin D. After a bypass this is not optional and deficiencies can be serious.
  • Blood tests at intervals, for years, to catch a deficiency before it causes symptoms.
  • Protein-first eating, in small volumes, separated from drinking.
  • Follow-up with the surgical or bariatric team, not only in the first months.
Which one loses more weight?

Averaged across large studies, bypass tends to produce somewhat greater weight loss than sleeve, and the gap narrows over the years. Both ranges overlap heavily, and an individual result depends far more on what happens after the operation than on which operation it was.

Is the sleeve reversible?

No. Stomach tissue is removed and cannot be replaced. A sleeve can be converted to a bypass, but that is a further operation, not a reversal.

Is a bypass reversible?

Technically it can be reversed because nothing is removed, but reversal is a major operation done for complications, not something to rely on as an exit.

Will I need surgery for loose skin afterwards?

Many people who lose a large amount of weight consider it. It is a separate operation, usually considered only once weight has been stable for a period, and it is not part of the bariatric procedure.