Being dangerously overweight is all down to bad diet rather than a lack of exercise, according to a trio of doctors who have reopened the debate about whether food, sedentary lifestyles or both are responsible for the obesity epidemic.

In an article for a leading health journal the authors – who include British cardiologist Dr Aseem Malhotra, an outspoken critic of the food industry – accuse food and drink firms such as Coca-Cola of having wrongly emphasised how physical activity and sport can help prevent people becoming very overweight.

The truth, they say, is that while physical activity is useful in reducing the risk of developing heart disease, dementia and other conditions, it “does not promote weight loss”.

“In the past 30 years, as obesity has rocketed, there has been little change in physical activity levels in the western population. This places the blame for our expanding waistlines directly on the type and amount of calories consumed.”

The authors add: “Members of the public are drowned by an unhelpful message about maintaining a ‘healthy weight’ through calorie counting, and many still wrongly believe that obesity is entirely due to lack of exercise.”

That “false perception”, they claim in the British Journal of Sports Medicine, “is rooted in the food industry’s public relations machinery, which uses tactics chillingly similar to those of big tobacco … denial, doubt, confusing the public and even buying the loyalty of bent scientists, at the cost of millions of lives.”

Given the worsening scale of obesity “let us bust the myth of physical activity and obesity. You cannot outrun a bad diet”, say Malhotra and his co-authors.

They challenge conventional wisdom further by arguing that those who want to avoid excess weight gain should adopt a diet that is high in fat but low on both sugar and carbohydrates.

Athletes and others about to do exercise should ditch high-carbohydrate intake regimes and instead eat more fat, they say, because “fat, including ketone bodies, 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.”

In a broadside against food industry practices, they also urge celebrities to stop promoting sugary drinks, call on health clubs and gyms to stop selling them and denounce “manipulative marketing” for sabotaging government efforts to introduce taxes on those drinks and to ban the advertising of junk food.

But their comment piece was dismissed by the food industry and divided opinion among experts in diet, obesity and health.

“The benefits of physical activity aren’t food industry hype or conspiracy as suggested. A healthy lifestyle will include both a balanced diet and exercise, as Change4Life summarises: eat well, move more, live longer”, said Ian Wright, director general of the Food and Drink Federation, a trade association which represents producers and retailers.

Catherine Collins, of the British Dietetic Association, said the doctors had downplayed the metabolic and physical health benefits of undertaking even moderately intense exercise and had used “incomplete evidence” to make their case.

Professor Susan Jebb, professor of diet and population health at Oxford University, who also chairs the food network of the government’s Responsibility Deal, said: “The authors fail to note that weight loss programmes which combine diet and physical activity are the most successful route to weight loss in both the short (three to six months) and medium term (12 months)”.

However, Tam Fry, of the National Obesity Forum, said: “The junk food and drinks industry has known for years just what it has do to make its products healthy, but persists in not doing it. The coalition has so far colluded with this through its inept attempts to challenge the producers to be responsible.

“The next government has to crack down on junk if obesity is to be halted and the NHS not brought to its knees. Whitehall could also crack down on commercial sponsorship of sport – but it won’t. Funding sport makes corporations feel good about themselves and they know it’s good for business. They have the cash and the public purse doesn’t”, added Fry, who is also an expert adviser to the Action on Sugar campaign group.