For years we have been dieting rigorously after the year-end binges, afraid that when summer comes, the bulges we have accumulated will betray our gluttony. Now scientists say, it's a little bug that causes obesity. Wang Hongyi reports in Shanghai.

As the holiday season with its bountiful feasting arrives, millions of festive revelers are keeping an eye on their figures. But scientists have found that weight gain is not about too much Christmas turkey, hot chocolates or holiday lethargy, but some bacteria in your guts.

Chinese scientists recently discovered a type of intestinal bacteria that may be to blame for obesity.

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

"The endotoxin released by the bacterium can activate a gene that helps generate fat. And it also deactivates a gene that consumes fat," Zhao says.

Scientists have long believed that microscopic organisms in the gut, microbiota, may play a crucial role in weight gain, but they had never been able to prove it.

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

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," says Lita Proctor, director of the US National Institutes of Health's Human Microbiome Pro-ject in Bethesda, Maryland. "But the problem is that we're only able to look at associations and aren't yet able to conduct cause-and-effect studies."

Zhao's research team isolated the pathogen, or infectious agent, from the gut of an obese human and 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.

Based on this information, researchers intervened by feeding the patient a specialized nutritional formula to decrease the bacteria in his intestines to non-detectable amounts.

After 23 weeks, the patient lost 51.4 kg, with his Type-2 diabetes, fatty liver disease, hyperglycemia and hypertension having all but disappeared.

Researchers observed mice that were specifically bred gained weight when such intestinal bacteria were injected to these insulin resistant mice. It provided the key piece of evidence, and the bacterium involved was identified as Enterobacter cloacae.

The results were published in a paper on Dec 13 in the peer-reviewed Journal of the International Society for Microbial Ecology.

Zhao's study on the connection came from personal experience.

In 2004, when he read the findings on a connection between obesity and gut microbiota in mice, he wondered if such a link existed in humans. He began a diet that combined whole grains and fermented foods, such as yams and bitter melon, which are believed to change the growth of bacteria in the digestive system.

Zhao not only monitored his own weight loss but also the microbes in his guts.

With the diet combining probiotics with whole grains, he lost 20 kg in two years. His blood pressure, heart rate and cholesterol level also came down. The changes persuaded him to focus on the microbe's role in the transformation and he started research on mice that later expanded to humans.

In 2009, Zhao started his first clinical trial on humans in China at a time when obesity was rocketing and the incidence of diabetes had spiked from roughly 1 percent of Chinese adults in 1980s to nearly 10 percent.

A total of 123 clinically obese volunteers, with a body mass index (BMI) of at least 30, was recruited. They were involved in a nine-week tailored program that included probiotics foods, and were subjected to regular checkups and monitoring of gut microbiota and metabolic parameters.

The 93 participants who completed the trial showed a median weight loss of about 7 kg. Meanwhile, toxin-producing bacteria in their intestines decreased and beneficial bacteria increased.

Encouraged by these results, Zhao expanded trials to three other cities in China with a total of more than 1,000 patients.

"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 says.

The publication of his research has provided some potential implications for academics and researchers.

Qu Shen, a domestic obesity expert says, "There are many reasons for obesity, such as lack of physical activity, increased calorie intake, genes, environment and intestinal bacteria. The new research provides a new direction to fight obesity. Some new drugs maybe developed for treating obese patients who are affected by such bacteria."

"If obesity is caused by bacteria, it could be infectious and picked up from some unknown environmental factor, or a parent. It might not be behavioral after all," says Dr David Weinkove, lecturer in biological sciences at Durham University in the United Kingdom.

So far, Zhao's specialized nutrition diet is only for clinical trial, and not available to the public, but he suggests keeping a balanced diet in daily life.

Contact the writer at

wanghongyi@chinadaily.com.cn.