Bariatric surgery has published eligibility criteria, and any clinic that operates without applying them is not offering a treatment, it is selling an operation. The criteria are a starting point rather than the whole assessment, but they are the starting point.

Long-standing threshold
BMI 40 and above, or 35 and above with an obesity-related condition
Increasingly recognised
BMI 30–35 with poorly controlled type 2 diabetes, in some guidelines
Also required
Documented attempts at non-surgical weight management
Assessed by
Surgeon, dietitian, and usually a psychologist and a physician

Why BMI is not the whole answer

BMI is a crude measure. It does not distinguish muscle from fat, it behaves differently across ethnic groups — several guidelines apply lower thresholds for people of South Asian and East Asian descent — and two people at the same BMI can have entirely different metabolic risk.

So the conditions matter as much as the number. Type 2 diabetes, obstructive sleep apnoea, high blood pressure, fatty liver disease and severe joint disease all weigh into the decision, and some of them lower the threshold at which surgery is considered reasonable.

What the assessment is actually looking for

  1. Medical

    Conditions that change the risk of anaesthesia and surgery, and anything that needs treating first — sleep apnoea in particular.

  2. Endoscopy

    Looking at the stomach and oesophagus. Reflux, hiatus hernia or gastritis can change which operation is appropriate.

  3. Nutritional

    Existing deficiencies, current eating patterns, and whether the post-operative diet is realistic for your circumstances.

  4. Psychological

    Eating disorders, untreated depression, alcohol use, and whether expectations match what surgery does.

That third and fourth item are where a hurried process shows. An operation booked from photographs and a deposit, with no dietetic or psychological assessment, has skipped the parts that most determine whether the result lasts.

When surgery is not appropriate

  • Untreated eating disorders, particularly binge eating or bulimia.
  • Active substance misuse, including alcohol — and alcohol absorption changes markedly after a bypass.
  • A medical condition that makes the anaesthetic risk unacceptable until it is treated.
  • Pregnancy, or planning pregnancy in the following 12–18 months, when rapid weight loss and nutritional stress are not wanted.
  • An inability to commit to lifelong supplements and follow-up, which after a bypass is not a preference but a requirement.
Do I have to try dieting first?

Most guidelines expect documented attempts at non-surgical management. That is not a moral test — it establishes the pattern, and dietetic contact beforehand is also what prepares you for eating after.

Is there an age limit?

Not a fixed one. Both adolescent and older adult surgery are done in appropriate cases, with additional assessment at both ends. Age changes the assessment rather than closing the door.

What if my BMI is just under the threshold?

Talk to a bariatric physician rather than looking for a clinic that will not check. Criteria exist because the balance of benefit and risk changes, and a provider willing to ignore them for you is willing to ignore them generally.

What about the new weight-loss medications?

GLP-1 medications have changed the landscape and are a genuine alternative for some people. They are also a long-term treatment rather than a course, and they belong in the same conversation as surgery rather than being compared against it in isolation.