EXCESS sugar and carbohydrates - not physical inactivity - are behind the surge in obesity, health experts have claimed.

Writing in the British Journal Of Sports Medicine, they said poor diet now generates more disease than physical inactivity, alcohol and smoking combined.

They point out that while obesity has rocketed in the past 30 years, there has been little change in physical activity levels.

"This places the blame for our expanding waistlines directly on the type and amount of calories consumed," they wrote.

The "false perception" that exercise matters more than healthy eating is due to how the food industry is marketed, they argue, describing it as "chillingly similar" to the tobacco industry.

They use the example of Coca-Cola associating its products with sport, "suggesting it is OK to consume their drinks as long as you exercise".

They claim the public health messaging around diet and exercise, and their relationship to the epidemics of type 2 diabetes and obesity, has been corrupted by "vested interests".

Celebrity endorsements of sugary drinks and the association of junk food and sport must end, they said.

"Changing the food environment - so that individuals' choices about what to eat default to healthy options - will have a far greater impact on population health than counselling or education," they argue.

"Healthy choice must become the easy choice. Health clubs and gyms therefore also need to set an example by removing the sale of sugary drinks and junk food from their premises."

Lead author Dr Aseem Malhotra, a cardiologist at Frimley Park Hospital in Surrey, said that up to 40% of those with a normal body mass index will harbour metabolic abnormalities typically associated with obesity, which include hypertension, dyslipidaemia, non-alcoholic fatty liver disease and cardiovascular disease.

"However, this is little appreciated by scientists, doctors, media writers and policymakers, despite the extensive scientific literature on the vulnerability of all ages and all sizes to lifestyle-related diseases," he said.

Research has revealed that for every excess 150 calories of sugar, there was an 11-fold increase in the prevalence of type 2 diabetes, in comparison to an identical 150 calories obtained from fat or protein, independent of the person's weight and physical activity level, he said.

Cutting down on carbs was also found to be the single most effective intervention for reducing all the features of the metabolic syndrome and should be the first approach in diabetes management, with benefits occurring even without weight loss, he said.

Instead, fat appears to be the ideal fuel for most exercise.

"It is abundant, does not need replacement or supplementation during exercise, and can fuel the forms of exercise in which most participate."

Dr Malhotra, an adviser to the campaign group Action on Sugar, said there are growing concerns that insulin-resistant athletes may be at risk of developing type 2 diabetes if they continue to eat very high-carbohydrate diets.

"It is time to wind back the harms caused by the junk food industry's public relations machinery," the editorial concludes.

"Let us bust the myth of physical inactivity and obesity. You cannot outrun a bad diet."