This will make you feel healthier and better about yourself. Joined a gym but confused about what exercises to do to lose weight? If similar microbial changes have comparable effects in humans, it could have far-reaching implications for our diets. The very ideas of “nutritional value” and “calorie content” of food seem to vary based on the microbial population of the person eating it and, potentially, her immune status. A person’s own microbes—and those contained in any given food—would have to be considered as another component of the already flimsy calories-in, calories-out equation.