Is weight loss as simple as calories in, calories out? In the end, it’s your gut microbes and leftovers that make your calories count

Two foods may have the same number of calories, but other food factors and your microbes influence which calories your body uses more of. Martin Barraud/OJO Images via Getty Images

Is the adage “calories in, calories out” true? The short answer is yes, but the full story is more nuanced.

From the moment food touches your tongue to the time it leaves your body, your digestive system and gut microbiome work to extract its nutrients. Enzymes in your mouth, stomach and small intestine break down food for absorption, while microbes in your large intestine digest the leftovers.