Friendly Microbes Could Make You Fat

ST. LOUIS _ New science shows that there’s a reason you can pack on a pound or two if you nibble a few holiday cookies while your skinny friend can snarf a whole plate and not gain an ounce.

Part of the reason is friendly bacteria in your gut. Some of these bacteria are too friendly, acting like over-indulgent grandparents who show their love with food.

“Forever people have wondered what it is that’s different between people who have a proclivity toward obesity and those who don’t,” said Margaret McFall-Ngai, a professor of medical microbiology and immunology at the University of Wisconsin-Madison.

A team of scientists led by Dr. Jeffrey I. Gordon at Washington University has discovered that obese people may get more calories from food than lean people do because they have a different mix of those friendly bacteria. In two studies appearing Thursday in the journal Nature, the researchers show that obesity is linked to the makeup of bacterial communities inside our intestines.

It’s impossible to overstate how important this contribution is, McFall-Ngai said.

The study of the relationship between people and their microbes is a breakthrough field of science that is opening up entirely new ways of looking at and treating obesity and other diseases.

For example, genetic surveys of the bacteria from more than a dozen unrelated people show that more than 4,000 types of bacteria can live in the human intestine. Each person seems to have a signature mix of species that stays constant over time, Ruth E. Ley, a post-doctoral researcher in Gordon’s lab found.

Those surveys are only a first estimate of the number and complexity of the organisms living within us, said Dr. Martin J. Blaser, chairman of medicine at New York University.

“The diversity in the human colon is unfathomable,” Blaser said. Our relationship with bacteria has evolved over a billion years, and it is no accident that we carry the organisms we do, he said.

The new research reinforces one of Blaser’s ideas, he said. He thinks that the obesity epidemic may be due to shifts in the types of microbes that live inside us.

“I think they are changing as a result of modern life, especially antibiotics,” Blaser said.

About 90 percent of the bacteria in the colon fall into two major groups, or divisions, called the Bacteroidetes and the Firmicutes. The Firmicutes extract more calories from food than the Bacteroidetes do.

Gordon and his colleagues found that obese mice and people had 50 percent fewer of the inefficient Bacteroidetes group _ than their lean counterparts.

The researchers tested the idea that bacteria are responsible for weight gain by taking bacteria from either lean or obese mice and transplanting them into mice raised to have no bacteria in their intestines. The so-called germ-free mice who got bacteria from obese mice gained twice as much fat as those who were recipients of bacteria from lean animals even though they ate the same amount of food.

Researcher Peter J. Turnbaugh had the unenviable job of examining the mice’s droppings to find out how many calories each group had used from their food. He discovered that the mice with the obese mix of bacteria extracted about 2 percent more calories from their food than those who were colonized with the lean mixture.

That may seem like a small difference, but the extra calories add up over time, Gordon said. Dieters typically consume 1,200 calories per day. If humans have a similar difference in calorie extraction, an obese person may get 24 more calories each day than a trim person who eats the same number of calories. Over the course of a year, 24 extra calories per day adds up to a pound of weight gained _ the average for people over age 25, Gordon said. And most people in the United States consume far more calories than that.

The researchers also studied 12 obese people who went to a weight-loss clinic at Washington University. They measured the proportion of the two groups of bacteria in the people’s guts as they followed either a low-fat or low-carb diet. As the people lost weight, the proportion of inefficient Bacteroidetes bacteria increased, suggesting that the amount of fat the host carries also influences which microbes prosper in the colon.

“This could be a vicious cycle,” Gordon said. As people get fatter, their intestinal bacteria change and pull more calories out of food causing further weight gain.

The “pioneering” and “cutting-edge” studies demonstrate that humans are ecosystems unto themselves, McFall-Ngai said. A delicate balance exists between humans and our microbes and helps determine how healthy we are, she said.

“It’s still early days to know what it all means,” McFall-Ngai said.

For instance, the scientists don’t know yet whether people prone to obesity start out with a different mix of bacteria, what causes the shift in the balance between lean and obese, or how bacteria measure the amount of fat their host carries. And they don’t know if giving more of the Bacteroidetes to people might act as a weight-loss treatment, or how to manipulate bacterial populations to control weight.

“It’s too premature to recommend any course of action yet,” Gordon said.

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