What, for instance, if air conditioning or lack of sleep helped make you fat? Or what if obesity were caused by a microbe -- what if, bang, you caught an unlucky sneeze and this made you chub out?

These ideas challenge the mainstream view that the bulging waistlines of an advancing society can be overwhelmingly pinned to diet and lifestyle.

The World Health Organisation (WHO) last September warned that a billion people were overweight and obese, and the toll could rise to 1.5 billion by 2015, driven by low- and middle-income countries.

The WHO accepted there were "a number of factors" for this increase, but especially blamed "a global shift in diet towards increased energy, fat, salt and sugar intake, and a trend towards decreased physical activity due to the sedentary nature of modern work and transportation, and increasing urbanisation."

Some worry that this view is dangerously monolithic.

Writing on Tuesday in the International Journal of Obesity, a team of US public-health experts caution against focussing obsessively on the "Big Two" -- a slower lifestyle and modern food marketing.

"This has created a hegemony whereby the importance of the Big Two is accepted as established and other putative factors are not seriously explored," they say. "The result may be well-intentioned but ill-founded proposals for reducing obesity rates."

They contend the evidence against junk food, supersize-me portions and high-calorie corn syrup is "equivocal and largely circumstantial" and offer some intriguing ideas of their own for other drivers of the obesity tsunami.

Among them:

-- Industrial chemicals called endocrine disruptors that disturb metabolism, encouraging the formation of fat.

-- Giving up smoking: people who give up cigarettes very often gain weight.

-- Air conditioning, which establishes a comfortable temperature zone. In temperatures above this zone, people eat less. The rise in number of air-conditioned homes in the United States virtually mirrors the increase in the US obesity rate.

-- Fat people marry other fat people. These individuals may be genetically vulnerable to obesity, a trait that could handed on to their children.

Another hypothesis is that lack of sleep jolts the metabolic system into demanding doses of instant energy.

University of Chicago researcher Esra Tasali notes that waistlines in modern societies started to expand when people started to sleep less. Today, the "sleep deficit" is about two hours per night compared with 40 years ago.

In work unveiled at an obesity conference last October, Tasali recruited a group of healthy young adults and divided them into three groups. One group had eight hours' sleep; another had their sleep regime extended to 12 hours; and the third was limited to only four hours.

The sleep-deprived group swiftly developed cravings for high-calorie sweets, and their metabolisms were akin to those of diabetics.

Meanwhile, Nikhil Dhurandhar of the Pennington Biomedical Research Center at Louisiana State University believes obesity could be caused by a bug.

At least 10 different pathogens are known to cause obesity in animals, causing dramatic changes to the metabolic system so that more energy gets converted into fat.

Dhurandhar believes that something similar may happen among humans exposed to cousins of the common cold.

He tested the stored blood of 500 Americans and found that 30 percent of obese people had antibodies for Ad-36, an adenovirus which causes coughing, sneezing and cold-like symptoms. Only 11 percent of people of normal body weight had this telltale of Ad-36 infection.

Dhurandhar stresses, though, that infection is likely to be only of a bouquet of causes for obesity.

"In 10 years, people may be able to walk into a clinic and be told that their obesity is due to X cause, such as genes, the endocrine system or pathogens. That may have a more productive outcome than a blanket treatment right now, (which) is not very successful."

Neville Rigby, of the European Association for the Study of Obesity, says that such unconventional views usually get a good hearing among scientists, for no one claims to have a monopoly of wisdom when it comes to this fast-growing disease.

"It's a very complex story, it's not a single issue," said Rigby. "But the overarching question is how much we consume and how much we burn."

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