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.
“Calories in, calories out” refers to the concept that weight change is determined by the balance between the calories you consume and calories you expend. This includes not only the number of calories you eat due to appetite and absorb via digestion, but also how well those absorbed calories are burned through metabolism.
Recent research indicates that a significant factor influencing people’s variable appetites, digestion and metabolism are biologically active leftover components of food, known as bioactives. These bioactives play a key role in regulating the body’s metabolic control centers: your brain’s appetite center, the hypothalamus; your gut’s digestive bioreactor, the microbiome; and your cells’ metabolic powerhouses, the mitochondria.
