An expanding waistline may have less to with what a person eats than what’s already inside, say microbiologists Jeffrey Gordon and Fredrik Backhed at the Washington University School of Medicine in Saint Louis. Variations in the population of bacteria living in the gut may explain why some people pack on extra pounds while others stay slim. Gordon and Backhed base their claim on a study of two groups of mice, one exposed to normal intestinal microbes and another raised in a germ-free bubble. The germ-free mice had 42 percent less body fat, even though they were fed one-third more calories....