But research may bring some comfort. Bacteria, and not just gluttony or laziness, may be to blame. The bacteria can actually make genes generate fat.

Scientists have believed that microscopic organisms in the gut, microbiota, might play a crucial role in gaining weight but were never able to prove it.

Groundbreaking research by a Chinese scientist has revealed a precise link.

In 2004, microbiologist Jeffrey Gordon from Washington University School of Medicine in St. Louis, Missouri, and his colleagues first showed a general link between obesity and gut microbiota in mice.

Another leading researcher said that while a link was believed to exist, proving it was another matter.

"The list of diseases that they may play a role in is just growing and growing," Lita Proctor, director of the US National Institutes of Health's Human Microbiome Project in Bethesda, Maryland said.

"But the problem is that we're only able to look at associations and aren't yet able to conduct cause-and-effect studies."

A research team led by Zhao Liping, a professor of microbiology and associate dean at the School of Life Sciences and Biotechnology at Shanghai Jiao Tong University, pointed out the precise link between a particular kind of bacteria and obesity.

The paper was published on Dec 13 in the journal of the International Society for Microbial Ecology.

It showed that a pathogen, or infectious agent, isolated from the gut of an obese human induced obesity and insulin resistance in germ-free mice.

In the clinical study, researchers found an excessive growth of endotoxin-producing bacteria, accounting for 35 percent of the gut bacteria, in an obese patient whose initial weight was 175 kg.

After an intervention with specialized nutritional formula, the bacteria decreased to non-detectable amounts and the patient lost 51.4kg.

It provided the key piece of evidence.

The patient also recovered from hyperglycemia and hypertension.

The intestinal bacterium involved was Enterobacter cloacae.

"The endotoxin released by the bacterium can activate a gene that helps generate fat. And it also deactivates a gene that consumes fat," Zhao said at a news briefing on Tuesday in Shanghai.

Zhao's study on the connection between obesity and gut microbiota came from personal experience.

In 2004 when he read the findings about a connection between obesity and gut microbiota in mice, he wondered if such a link existed in humans.

Then he began a diet that combined whole grains and fermented foods, such as yams and bitter melon.

After two years he lost 20 kilograms.

"Intestinal bacteria play an indispensable role in the genesis and development of chronic diseases, such as obesity, diabetes and coronary heart disease. The study will help find how bacteria affect us," Zhao said.

According to the Ministry of Health, China has 120 million people classified as obese.

"Chinese obesity is catching up with Western countries," Qu Shen, an obesity expert from Shanghai No 10 People's Hospital said.

"There are many reasons for obesity, such as lack of physical activity, increased calorie intake, genes, environment and intestinal bacteria," Qu said.

"The new research provides a direction to fight obesity," he said. (China Daily)