Obese and overweight patients are less likely to die after a heart operation than those who are a healthy weight, research suggests.
The new study throws NHS rationing in some parts of the country - where obese patients have been told to slim down before they can have surgery - into doubt.
Experts funded by the British Heart Foundation (BHF) analysed data for the number of people who died in hospital following heart surgery, according to their weight.
The results showed that people who were a healthy weight were almost twice as likely to die as those who were obese or overweight, while those who were underweight had the highest risk of death.
The study, published in the journal Circulation, included 401,227 adults who had undergone heart surgery between 2002 and 2013.
Overall, 11,511 patients died. Some 4.4% people of a normal weight died, compared to just 2.8% who were overweight and 2.7% who were obese. Meanwhile, 8.5% people who were underweight died.
The University of Leicester researchers took into account factors that could influence the results, such as people's age or whether they had other health conditions.
But the study still found that being fat - including suffering complications of obesity such as high blood pressure, diabetes and artery disease - seemed to offer protection,
The researchers also reviewed data from 557,720 further patients included in studies across Europe, the United States and Asia, and found similar results.
Professor Gavin Murphy, BHF professor of cardiac surgery at the University of Leicester, said: "Obesity is a reason often given for not offering patients surgery.
"With this study, we show that, for cardiac patients at least, being obese should not be a reason to turn patients away from surgery.
"These results also raise questions as to whether there may be attributes of obesity that directly protect patients.
"Understanding these processes may open the door to new prevention strategies or treatments."
Professor Jeremy Pearson, associate medical director at the BHF, said: "Survival rates for heart surgery are now extremely high, thanks to many years of research.
"We always recommend a healthy waistline, which significantly reduces lifetime risk of heart disease and therefore a person's risk of needing cardiac surgery.
"However, this large study strongly suggests that being overweight can give patients added protection when facing major heart surgery, reducing their chance of complications or death before leaving hospital.
"To properly understand why, we need to fund more research into the mechanisms involved. This could lead to new strategies to improve survival rates in normal and underweight patients following surgery."