NEW YORK (NYTIMES) - A large new study found that people who lost significant amounts of weight through bariatric surgery gained a striking benefit: Their likelihood of developing cancer fell sharply.

The study, published this June in the Journal of the American Medical Association, followed more than 30,000 adults with obesity for about a decade.

It found that those who underwent weight loss surgery had a 32 per cent lower risk of developing cancer and a 48 per cent lower risk of dying from the disease, compared with a similar group of people who did not have the surgery.

On average, the people who had weight loss surgery lost about 25kg more than those who did not over the course of the study. The researchers found that the more weight people lost, the more their cancer risk fell.

Obesity is a strong driver of cancer risk. Health authorities have linked excess weight to higher rates of many cancers. The new study is among the largest and most rigorous to suggest that the increased risk can be reversed in people who lose substantial amounts of weight.

While the new research focused on weight loss through bariatric surgery, the study's authors speculated that the benefit would apply to weight loss through other methods as well, such as diet and exercise or the use of weight-loss medications.

Dr Steven E. Nissen, a co-author of the study, said the findings "provide one more reason why people who are obese should lose weight".

"It's an important public health message," added Dr Nissen, chief academic officer of the Heart, Vascular and Thoracic Institute at the Cleveland Clinic. "I think a lot of people do not understand or realise that obesity is such a strong risk factor for cancer, and they certainly don't understand that it's reversible."

One expert not involved in the study, Dr Ania Jastreboff, said it demonstrates that treating obesity and achieving "clinically meaningful" weight reductions can improve health outcomes. "That's a really important finding," said Dr Jastreboff, an associate professor at the Yale School of Medicine and the director of weight management and obesity prevention at the Yale Stress Centre.

According to the Centres for Disease Control and Prevention (CDC), obesity is a leading cause of preventable cancers, along with smoking, heavy drinking and exposure to ultraviolet radiation. People who are overweight or obese have a higher risk of developing 13 types of cancer, including endometrial, breast and liver. The CDC estimates that the 13 types of cancer linked to obesity make up 40 per cent of all cancers diagnosed in the United States each year.

It is not entirely clear why obesity seems to promote cancer, but one potential factor is inflammation. Studies show that fat cells spew into the bloodstream a variety of inflammatory proteins known as cytokines, which have been found to stimulate cancer cells. Many scientists believe this chronic state of inflammation caused by fat cells is also one of the reasons people with obesity have higher rates of complications from Covid-19.

According to the CDC, people with obesity tend to have higher levels of hormones such as insulin and insulin-like growth factor-1 (IGF-1), which can stimulate the development of colon, kidney, prostate and endometrial cancers.

Bariatric surgery can lead to substantial weight loss. Though some patients end up regaining some of the weight they lost, studies show that most people who undergo bariatric surgery are able to sustain a more than 20 per cent reduction in their body weight a decade after their procedure. That is much greater than what people with obesity can typically lose and maintain through diet and exercise alone.

For the new study, Dr Nissen and his colleagues wanted to see how that amount of weight loss would affect cancer rates. They recruited 5,053 people with obesity who had undergone bariatric surgery at the Cleveland Clinic and followed them. Each patient was "matched" to five other patients who were similar in many respects. They were about the same age, sex and race. They also had similar medical histories and body mass indexes, but did not undergo weight loss surgery.

All told, there were more than 30,000 participants in the study, which included 25,265 people in the control group. Both groups had a median age of 46 and a median body mass index of 45, which the CDC categorises as "severe" obesity. That is the equivalent of someone who is 1.72m tall and weighs about 136kg.

After a decade, the patients who had the surgery had lost an average of about 28kg, while those in the control group (who were advised by their doctors to try to lose weight on their own) had lost an average of 2.7kg. Slightly less than 3 per cent of patients who had surgery developed cancer, compared with roughly 4.9 per cent of those in the non-surgical group - equivalent to a 32 per cent reduction in risk for those who had surgery.

In general, the data suggested that patients needed to lose a large amount of weight - at least 20 to 25 per cent of their body weight - to see a beneficial change in their cancer risk, said Dr Ali Aminian, the lead author of the study and the director of the Bariatric and Metabolic Institute at the Cleveland Clinic.

The reduction in risk was particularly significant for endometrial cancer, which has the strongest association with obesity.

Dr Aminian and several of the study's authors reported receiving research grants or consulting fees from Medtronic, a company that makes devices used in weight-loss surgery. Dr Nissen disclosed receiving research funding from companies that develop weight loss drugs, including Novartis and Eli Lilly.

He said that while the new study focused on bariatric surgery, the message was that people should seek to lose weight in whatever way they could, whether through diet and exercise or with help from medical therapies.