Fast foods, cookies and ice cream, sodas and energy drinks -- Americans eat a lot of processed foods high in fats and simple sugars. Not surprisingly, this type of diet can lead to weight gain and is one of the main culprits in the obesity epidemic in this country. But, is there more to this story? -The foods you eat serve as food for the community of microorganisms that inhabit your digestive tract. Those microbes have their own food "preferences," metabolizing different types of food molecules and releasing their byproducts, which your body may absorb. -Scientists have hypothesized that a high-fat, high sugar diet actually alters the composition of the microbial community that inhabits the beginning of the large intestine, which contributes to obesity. Because of the difficulties of carrying out experiments on humans, scientists have used mice as an animal model in which to test this hypothesis. Designing a controlled experiment: In one experiment, scientists raised mice in germ-free conditions, so the mice lacked intestinal microbes. The mice were fed a low-fat diet rich in the complex plant polysaccharides, such as cellulose, that are often called fiber. When the mice were 12 weeks old, the scientists transplanted the microbial community from the intestine of a single “donor” mouse into all of the germ-free mice. Then they divided the mice randomly into two groups and fed each group a different diet. Group 1 (the control group) continued to eat a low-fat, high-fiber diet. Group 2 (the experimental group) ate a high-fat, high-sugar diet. Identify the components of this experiment by dragging the labels into the appropriate bins. -Diet -Weight Gain -change in body fat -age in mice -final composition of the microbial community -initial composition of the microbial community (1) Factor being tested (independent variable) (2) Factors controlled (kept consistent) (3) Factors to be measured (dependent variables)