For many, it's a lifelong battle - a never-ending nightmare of quick-fix diets, exercise fads and obsessing over the bathroom scales.

But what if the fight against fat was making you sick? What if the excess kilos you've been desperately trying to shed were actually protecting you against premature death? Could you get off the weight-loss treadmill and lengthen your life?

This is the theory put forward in a contentious new book that is ruffling feathers in the health sector.

At a time when obesity and its associated burden of chronic diseases is a growing global problem, the book, to be released in Australia this week, suggests our obsession with thinness is courting disaster.

In The Obesity Paradox: When Thinner Means Sicker and Heavier Means Healthier, US cardiologist Carl Lavie says our modern culture has been duped into thinking excess body fat is bad.

He says the key to optimal health for millions of overweight and obese people may be staying the size they are.

''Fat has been demonised by our society, and our research shows fat is not always the devil,'' he told Fairfax Media from his home in Louisiana - the fattest state in the US. ''You can be heavy and amazingly healthy. Fitness is a lot more important than fatness.''

Lavie, who works at the John Ochsner Heart and Vascular Institute in New Orleans, said a growing body of evidence, including his own research over a decade, shows that while excess fat can lead to risk factors for chronic illness such as cardiovascular disease and type two diabetes, once these diseases develop heavier people have better outcomes.

''This is the obesity paradox,'' he says. ''Among the patients who have heart disease, the overweight and moderately obese are actually doing considerably better, sometimes 30 to 50 per cent lower mortality rates, than the lean people who have the same diseases.

''In no way am I promoting obesity, but for the people who have been losing the battle of the weight, if they can at least become fit, then they can have a very good prognosis and good overall health.''

Lavie says it is unclear how the paradox works, but it may be that heavier people might never have developed chronic disease without weight gain, whereas thinner people may be genetically likely to become sick and have a poorer prognosis.

But Associate Professor Tim Gill, principal research fellow at the University of Sydney University's Boden institute of obesity, nutrition and exercise and eating disorders, says he is suspicious of the theory, arguing that some overweight people might have better survival rates because thinness is often linked to smoking, and that fat reserves can be useful if the body is under stress such as when undergoing surgery or fighting cancer or immune system diseases.

''As a message to the whole community the idea of an obesity paradox is counterproductive, because it's saying that being overweight is good for you and don't do anything about it, when really this observation only applies to a small group of people at a particular point in their life,'' Gill says.

Lavie stresses that for the morbidly obese, losing weight is still the best option, but says fixating on numbers such as waist circumference or body mass index - a measure that divides a person's weight by the square of their height to determine obesity levels - gives a false impression of health.

Frances Lockie, a Sydney public servant who writes a ''fat acceptance'' blog, is 92 kilos and considered obese, according to her BMI.

She believes our cultural obsession with leanness and weight loss causes stigma. ''It gives people a licence to be rude and make offensive comments because it falls under the banner of being concerned for your health,'' she said.

''I work out, I eat healthily and I see my doctor regularly, but if I was to try and get into the 'healthy' weight range I'd need to lose about 20 to 30 kilos and I genuinely don't know how I'd do that without chopping off a leg.''