Not everyone gets the same amount of calories from food. And here’s why.

For most people, calories are the north star of nutrition: a fixed amount assigned to each food that never changes or changes. Two people who eat the same thing in the same amount will always absorb the same number of calories, right? Or will they?

“This is perhaps one of the most powerful nutritional myths out there.” Janice Dada, MPH, RDNsays a California-based certified intuitive eating consultant. Contrary to what you’ve thought your whole life, calorie absorption can vary depending on who’s eating. “Our understanding of calorie and weight management is evolving as research reveals how digestion, Gut microbiomeThe structure of food affects how much energy we actually absorb from the foods we eat. Tanaz Islamparast, RD, MScfounder and CEO of Empowered Eating Nutrition & Wellness and a digestive health specialist, tells SELF. “This shifts the focus from simply counting calories to looking at individual biology and the types of foods we consume.”

So is everything we’ve learned about nutritional labels a lie? Here’s everything you need to know about how calorie absorption works, and why it can vary from person to person.

Why does calorie absorption differ between people?

Before we dive in, we first need to explain the true meaning of the term “calories.”

“The easiest way to think about calories and absorption is that a ‘calorie’ is a measure of the energy your body gets from consuming and breaking down that food.” Maddie Pasquareello, MS, RDNwho runs the Brooklyn-based Nutrition With Maddie practice, tells SELF. “If it’s your body I cannot If you digest something, you won’t gain any calories from that thing. Food passes through it without calories being absorbed. In other words: calories are only “counted” when everything you eat has been completely broken down.

But people show natural differences in digestion and metabolism that can affect this process, according to Islamparast. “This includes variation in levels of digestive enzymes, intestinal transit time (how quickly food moves through the digestive tract), and the composition of the gut microbiome,” she says. Below, we’ll explain some of these forces in more depth.

Microbiome balance in the intestine

While about 90% of calories from food are absorbed in the small intestine, some of the rest (i.e. those settling for hard-to-digest things that remain intact, e.g. Fiber) They are also absorbed by gut bacteria in the colon. “Although the differences here are not usually large, each person’s gut microbiome is naturally slightly different, and some are better than others at getting energy from the same meal,” Pasquareello says. This distinction is a result of the balance of microorganisms in your gut, according to Dadda: For example, high concentrations of methane-producing microbes are associated with enhanced energy extraction from Diets rich in fiber. The resulting SCFAs can contribute up to 10% of daily calories.