The human gastrointestinal tract weighs about 2.5 pounds and burns about 12 kcal per hour, and that’s just at rest on an empty stomach. Digestion costs much more, about 10 percent of the daily calories consumed, or 250 to 300 kcal per day for the typical adult. It’s unclear how much of the energy burned by the gut is attributable to the trillions of bacteria toiling away in our microbiome. A recent study in mice by Sarah Bahr, John Kirby, and colleagues suggests the calories burned by the microbiome might account for as much as 16 percent of BMR in humans, which would mean that the resting
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