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 that appeared Thursday in the journal Nature, the researchers show that obesity is linked to the makeup of bacterial communities inside our intestines.

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.