Some get fat with a slice of bread, others eat an entire pig and nothing ...

For a long time, solely genetics were put behind these metabolic differences. But now scientists believe that bacteria living in your gut may have the last word. Some microbes are more powerful in breaking down nutrients in your guts in a digestible state.

Researchers at Washington University in St. Louis, Missouri had already proved that transferring these bacteria into lean mice makes them as slim as a pumpkin and that lean and obese mice have different microbial communities in their gut.

Now the same research team proved this difference exists in people as well, and that the gut bacterium is influenced by diets. A dozen obese volunteer subjects were on either a low-fat or a low-carbohydrate diet for a year. At regular intervals, their intestinal microbial flora - which compasses hundreds of species - was investigated by sequencing parts of a gene for ribosomal RNA that all species have.

At the beginning of the investigation, Firmicutes, a type of gram-positive bacteria, represented more than 90% of the bacterial flora present in the volunteers, while gram-negative Bacteroidetes species barely made up to 3%, while in normal weighted people Bacteroidetes formed some 30% of microbes.

At the end of the research, the subjects had lost 2% to 6% of their weight, the Firmicutes had dropped to 73%, and the Bacteroidetes increased to about 15%. Scientists do not know if Firmicutes was in excess before patients got obese, or if those bacteria multiplied as they were getting fatter.

In another study, the genomes of gut microbes in healthy mice and genetically obese mice were analyzed. Microbes from the obese mice had more genes for processing starches and complex sugars, being more efficient in producing calories rich sugars and fatty acids, which the gut easily absorbs.

When this type of microbes was transplanted to germ-free mice, they presented a 47% increase in body fat over 2 weeks. "In contrast, germ-free mice supplied with microbes from lean mice had only a 27% increase," said Jeffrey Gordon from the research team.

"The obesity epidemic cannot be explained in human genetic or even simple lifestyle change terms alone," said microbiologist Jeremy Nicholson from Imperial College London. "There has to be another dynamic factor, and that is the gut microbes."