Question

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. Part A - 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 germn-free mice. Then they divided the rice 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. "Donor" mouse Group 1: Control group Low-fat, high-fiber diet High-fat, high-sugar diet Germ-free mice Group 2: Experimental group

Answer 1

Factors being tested (independent variable): diet

factors controlled : age of mice, initial composition of the microbial community

Factors being measured (dependent variable): body fat, weight gain, the final composition of the microbial community