Image copyright SPL Image caption Many cases of type 2 diabetes can be managed by diet changes, but some patients may need insulin

Weight-loss surgery can cut the risk of diabetes and heart attacks as well as keep fat off, a UK study suggests.

It is the largest comprehensive investigation of bariatric surgery - spanning around four years in nearly 8,000 patients.

The health benefits of the surgery are clear and substantial for people who are severely overweight, the authors told PLoS Medicine.

They say 1.4m people in England could benefit.

Currently, around 8,000 people a year receive the treatment on the NHS.

If all 1.4m were offered bariatric surgery, the researchers estimate it would avert nearly 5,000 heart attacks and 40,000 cases of type 2 diabetes over four years.

All surgery carries risks, however, and so people should only be offered surgery if attempts to lose weight through healthy eating and physical activity have already been tried and not worked, they add.

Experts said surgery should not be seen as a 'quick fix'.

What is bariatric surgery?

Bariatric surgery, also known as weight loss surgery, is used as a last resort to treat people who are dangerously obese (having a body mass index of 40 or above or 35 plus other obesity-related health conditions).

Guidelines for the NHS all patients with a BMI of 35 or over who have recent-onset type 2 diabetes should be assessed for surgery. Patients must have tried and failed to achieve clinically beneficial weight loss by all other appropriate non-surgical methods and be fit for surgery.

The two most common types of weight loss surgery are:

Sleeve gastrectomy or gastric bypass , where some of the stomach is removed or the digestive system is re-routed past most of the stomach

, where some of the stomach is removed or the digestive system is re-routed past most of the stomach Gastric band, where a band is used to reduce the size of the stomach so a smaller amount of food is required to make someone feel full

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For the study, the researchers compared 3,882 patients who underwent weight loss surgery with an identical number who did not.

They looked at gastric bypass, sleeve gastrectomy and gastric banding. All led to a dramatic and sustained weight loss of between 20kg and 48kg. And the weight stayed off.

This, in turn, significantly lowered the person's risk of developing type 2 diabetes, high blood pressure, angina and heart attacks.

Among those who already had diabetes, their condition improved substantially and many (60%) were able to come off medication altogether.

Lead researcher Dr Ian Douglas, from the London School of Hygiene and Tropical Medicine, said: "The results are really encouraging. Obviously we would love to help people lose weight in other ways, through exercise and healthy diets, but that's difficult. Diets do not always work well for everyone.

"We are not saying surgery is right for everyone, but it can be really effective."

Dr Alasdair Rankin of Diabetes UK warned bariatric surgery should not be seen as a one-stop solution for type 2 diabetes and obesity.

"It should be offered along with ongoing support and clear plans for long term follow up."