People can be "fat but fit" and should focus on exercise rather than dieting for a longer life, experts have said.

In a new review collating numerous studies, researchers said doing more exercise and improving fitness is more effective than just shedding pounds when it comes to getting healthy and cutting the risk of dying early.

Writing in the journal iScience, Professor Glenn Gaesser, from Arizona State University, and associate professor Siddhartha Angadi, from the University of Virginia, claimed that applying a "weight-neutral" approach to the treatment of health issues caused by obesity would also cut the health risks associated with yo-yo dieting.

Prof Gaesser said: "We would like people to know that fat can be fit, and that fit and healthy bodies come in all shapes and sizes.

"We realise that in a weight-obsessed culture, it may be challenging for programmes that are not focused on weight loss to gain traction.

"We're not necessarily against weight loss; we just think that it shouldn't be the primary criterion for judging the success of a lifestyle intervention programme."

The researchers said multiple studies have shown how people around the world have been trying to lose weight over the past 40 years, and yet obesity has continued to rise.

They said "a weight-centric approach to obesity treatment and prevention has been largely ineffective", adding: "Moreover, repeated weight loss efforts may contribute to weight gain, and is undoubtedly associated with the high prevalence of weight cycling (yo-yo dieting), which is associated with significant health risks."

The pair pointed to studies suggesting that exercise was better for a longer life than just losing weight.

They argued that "many obesity-related health conditions are more likely attributable to low physical activity and cardiorespiratory fitness rather than obesity per se".

"Epidemiological studies show that cardiorespiratory fitness and physical activity significantly attenuate, and sometimes eliminate, the increased mortality risk associated with obesity," they said.

Studies have also found that "increasing physical activity or cardiorespiratory fitness is consistently associated with greater reduction in risk of all-cause and cardiovascular mortality than intentional weight loss".

However, the researchers said that adopting a weight-neutral approach "does not mean that weight loss should be categorically discouraged", particularly when so many people do desire to lose weight.

"But shifting the focus away from weight loss as the primary goal, and instead focusing on increasing physical activity to improve cardiorespiratory fitness, may be prudent for treating obesity-related health conditions," they said.

In one study (the Nord-Trondelag Health Study), during a follow-up of almost 16 years of adults with coronary heart disease, sustained low physical activity was associated with 19% lower risk of dying from any cause and sustained high physical activity was associated with a 36% lower risk of death, when compared with people who were inactive.

In comparison, weight loss was associated with a 30% increased risk of death but there was no increased risk of death caused by weight gain.