To most people, calories are the North Star of nutrition: a rigid quantity assigned to each and every food that never wavers or changes. Two individuals who eat the exact same thing in the exact same amount will always absorb the exact same number of calories, right? Or will they?
“This is probably one of the more robust dietary myths that circulates,” Janice Dada, MPH, RDN, a certified intuitive eating counselor based in California, tells SELF. Contrary to what you’ve probably thought all your life, calorie absorption can vary depending on who’s chowing down. “Our understanding of calories and weight management is evolving as research reveals how digestion, gut microbiome, and food structure influence how much energy we actually absorb from the foods we eat,” Tannaz Eslamparast, RD, MSc, founder and CEO of Empowered Eating Nutrition & Wellness and a digestive health specialist, tells SELF. “This shifts the focus from just counting calories to considering individual biology and the types of foods we consume.”
So is everything we’ve ever been taught about nutrition labeling a lie? Here’s everything you need to know about how calorie absorption works, and why it can differ from person to person.
Why calorie absorption differs between people
Before we dive in, we first need to explain what the term “calorie” really means.
“The easiest way to think about the topic of calories and absorption is that a ‘calorie’ is the measurement of the energy your body gets from consuming and breaking down that food,” Maddie Pasquariello, MS, RDN, who runs the Brooklyn-based practice Nutrition With Maddie, tells SELF. “So if your body can’t digest something, it won’t gain any calories from that thing. The food just passes through with calories unabsorbed.” So in other words: Calories only “count” when whatever you eat has been fully broken down.
But people exhibit natural differences in digestion and metabolism that can affect this process, according to Eslamparast. “This includes variation in digestive enzyme levels, gut transit time (how quickly food moves through the digestive system), and gut microbiome composition,” she says. Below, we’ll explain some of these forces in more depth.
Gut microbiome balance
While around 90% of calories from food are absorbed in the small intestine, some of the remainder (namely, those endemic to the hard-to-digest stuff that survives unscathed, like fiber) are further absorbed by gut bacteria in the colon. “Although the differences here aren’t typically major, everyone’s gut microbiomes are naturally a bit different, and some are better than others at getting energy from the same meal,” Pasquariello says. This distinction is a result of the balance of microorganisms in your gut, according to Dada: For example, high concentrations of methane-producting microbes are associated with enhanced energy extraction from high-fiber diets. Resulting short-chain fatty acid compounds can contribute up to 10% of your daily calories.