"Atwater [Wilbur Atwater] also didn't know that our guts are home to billions of bacteria—some mooch away nutrients, while others help break down food to release more nutrients. Gut microbiota varies from person to person, and diet plays a big factor in who lives in our intestines. Obesity and starvation, for example, both change the bacterial community and its ability to extract nutrients from food. Carmody's new research suggests microbial differences between mice who rely on diets of processed versus unprocessed food, too."
Why Calorie Labels Are Wrong | Mother Jones