After just one day of switching from a plant-based diet to a high-fat-and-sugar diet, mice with human intestinal bacteria developed bacteria associated with obesity in humans, and soon became grossly obese (Science Translational Medicine, November 11, 2009).
Dr. Jeffrey Gordon of Washington University in St Louis first showed that certain types of bacteria in the human intestinal tract can break down food more efficiently and help you absorb a greater percentage of calories from the food that you eat. He also showed that humans whose intestinal tracts are dominated by these bacteria tend to be overweight.
In this new study, Dr. Gordon created germ-free mice and fed them a low-fat, plant-rich diet. Then he fed them bacteria extracted from human stool and continued to feed them a low-fat, plant-based diet for one month. By sequencing the microbes' 16S rRNA gene, he showed that the intestinal bacteria in the mice were the same as those living in a healthy human's intestines.
One month later, he switched half the mice to a high-fat, high-sugar diet. After 24 hours, the intestines of the mice had increases in the obesity-causing bacteria, Firmicutes, and decreases in the obesity-preventing Bacteroidetes. The mice continued to grow fatter and fatter, even when switched back to the low-fat, plant-based diet.
What does this mean to you? When you eat a diet rich in refined carbohydrates (fruit juices, sugared drinks and foods made from flour and sugar) and fat (meat, fried foods, and fatty desserts), you develop intestines full of bacteria that thrive on these foods, break down these foods more efficiently, and then absorb far more calories from these foods. If you want your gut flora to help you maintain a healthful weight, you should eat primarily fruits, vegetables, whole grains, beans, seeds and nuts.