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On average people lost 5 kg more in each of the first four months after weight loss surgery than those of a similar weight who did not undergo surgery. The risks of developing type 2 diabetes and high blood pressure were reduced. Surgery was linked to reduced risk of heart disease and sleep-related breathing problems. Those entering the trial had an average BMI of 44.7 kg/m2.

Rates of obesity are high in the UK, around a quarter of the adult population is obese, and rising every year. Obesity is linked with health conditions such as type 2 diabetes, which is expensive to manage and has a big impact on people’s quality of life. Reducing the number of people with obesity-related health conditions after surgery would give economic benefits alongside the health benefits to the individual.

This study provides a compelling clinical case for weight loss surgery for appropriately selected people.

Why was this study needed?

Around 13 million people, a quarter of the UK adult population, is currently obese (with a BMI of 30 kg/m2 or more), and over a million of those are morbidly obese (BMI more 40 kg/m2 or more). Obesity is linked to a range of health problems, which are expensive to manage and impact on people’s quality of life.

In the UK weight loss surgery is recommended for morbidly obese people and obese people with other related health conditions or recent onset type 2 diabetes.

Weight loss surgery is being offered to increasing numbers of people, but evidence about the effectiveness of different types of weight loss surgery was scarce. This study compared weight loss and health outcomes of morbidly obese people who underwent weight loss surgery with similar people who did not.

What did this study do?

This study used data from the Clinical Practice Research Datalink database to identify 3,882 morbidly obese people who had undergone weight loss surgery, then matched (by age and sex) them with 3,882 similar people who had not undergone surgery. The database includes information from GP practices covering around 8% of the population. Although not comprehensive, the database has been assessed as representative of the UK population.

Cohort studies of surgery can be prone to confounding, with those who are most likely to benefit being selected for surgery. The researchers considered this in their study design, using a propensity scoring system to match the groups and reduce confounding. A randomised study might provide a more accurate estimate of weight loss following surgery, but the size and consistency of the results in this study suggest we can broadly trust this result and have confidence that it reflects real life practice.

What did it find?

  • Amongst patients with an average BMI of 44.7 kg/m2 on average people lost 4.98 kg each month in the first four months after surgery.
  • Although this weight loss slowed after four months, it was estimated that over four years people would lose 38 kg after gastric bypass, 31 kg after sleeve gastrectomy and 20 kg after a gastric band. There were corresponding reductions in BMI following surgery. There was no overall change in weight or BMI over four years amongst people who did not have surgery.
  • Weight loss surgery was associated with a statistically significant reduction in the likelihood of heart attack (hazard ratio [HR] 0.28, 95% CI 0.10 to 0.74), developing type 2 diabetes (HR 0.68, 95% confidence interval [CI] 0.55 to 0.83), high blood pressure (HR 0.35, 95% CI 0.27 to 0.45), angina (HR 0.59, 95% CI 0.40 to 0.87), or sleep-related breathing problems (HR 0.55, 95% CI 0.37 TO 0.82).
  • Surgery was associated with increased likelihood of resolution of existing type 2 diabetes or high blood pressure. Forty five per cent of people who had type 2 diabetes prior to surgery saw their blood sugar reduce to a point where they would no longer be classified as diabetic (defined by HbA1c – the long term measure of blood sugar control – less than 6.0%).
  • Surgery was linked to a significant reduction in the chance of requiring treatment with an oral antidiabetic drug (HR 0.26, 95% CI 0.18 to 0.37) or insulin (HR 0.22, 95% CI 0.11 to 0.43).
  • Surgery had no effect on the likelihood of stroke, fractures, cancer or death overall.

What does current guidance say on this issue?

NICE 2014 guidance on the identification and management of overweight and obesity covers a range of interventions from lifestyle interventions such as changes to diet and physical activity, to surgery. Treatment approach would generally be considered in a stepped fashion – from the least to most intensive – depending on the person’s BMI and associated health characteristics. Surgery is recommended for people with a BMI 40 kg/m2 or more (morbid obesity) or a BMI between 35 and 40 kg/m2 (severe obesity) with related health conditions, such as type 2 diabetes or high blood pressure. Obese people with a BMI between 30 and 34.9 kg/m2 can also be offered surgery if they have recently developed type 2 diabetes.

What are the implications?

Weight loss surgery costs between £5,000 and £15,000, depending on the procedure. Calculating the exact cost of obesity is challenging because many of the costs are indirect – stemming from other medical conditions that obesity causes. This study suggests that weight loss surgery could reduce the number of obese people with diabetes, high blood pressure, heart disease and the sleep-related breathing condition, obstructive sleep apnoea. This would reduce the cost of managing these conditions. For diabetes alone, this could reduce the cost of the drugs, regular specialist appointments, and the management of complications such as foot care, amputations and retinal screening. The NIHR produced a similar study, using the same database, which also found that weight loss surgery reduced the risk of obese people developing type 2 diabetes.

This evidence makes a compelling case for increasing the number of eligible people receiving weight loss surgery.


Citation and Funding

Douglas IJ, Bhaskaran K, Batterham RL, Smeeth L.Bariatric Surgery in the United Kingdom: A Cohort Study of Weight Loss and Clinical Outcomes in Routine Clinical Care.PLoS medicine. 2015;12(12).



Booth H, Khan O, Prevost T, et al. Incidence of type 2 diabetes after bariatric surgery: population-based matched cohort study. Lancet Diabetes Endocrinol 2014;2(12):963-968. This project was funded by the National Institute for Health Research HS&DR Programme (project number (12/5005/12).

Diabetes UK. Diabetes: Facts and Stats. London: Diabetes UK; 2014.

Diabetes UK. The cost of diabetes. London: Diabetes UK; 2014.

HSCIC. Statistics on obesity, physical activity and diet: England 2015. Leeds: Health & Social Care Information Centre; 2015.

NHS England. Joint report on commissioning obesity services published. London: NHS England; 2014.

NICE. Obesity: identification, assessment and management of overweight and obesity in children, young people and adults. NICE CG189. London: National Institute for Health and Care Excellence; 2014.

NICE. Preventing type 2 diabetes: risk identification and interventions for individuals at high risk. NICE PH38. London: National Institute for Health and Care Excellence; 2012.

NICE. Surgery for obese adults. London: National Institute for Health and Care Excellence; 2014.

Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre


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Almost all (99.5%) of surgeries performed for people in this study were gastric band, gastric bypass or sleeve gastrectomy. All of these surgeries reduce the size of the stomach so that people feel full after ingesting less food. Gastric band surgery uses keyhole surgery to fit an adjustable band around the stomach to reduce its size. Gastric bypass uses staples to create a smaller pouch at the top of the stomach, which is connected to the intestines using keyhole surgery. Sleeve gastrectomy involves removing up to 75% of the stomach via keyhole surgery and is not reversible.

Body mass index (BMI) is a measure of weight relative to height. Sub groups of people according to their BMI are:

  • normal weight: 18.5 to 24.9 kg/m2
  • overweight: 25.0 to 29.9
  • simple obesity: 30.0 to 34.9
  • severe obesity: 35.0 to 35.9
  • morbid obesity: 40.0 to 44.9
  • super obesity: 45.0 or greater


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