Not Everyone Gets the Same Amount of Calories From Food. Here’s Why

To most people, calorie absorption is 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, a certified intuitive eating counsellor based in California, tells us. 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, founder and CEO of Empowered Eating Nutrition & Wellness and a digestive health specialist says. “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 labelling 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, who runs the Brooklyn-based practice Nutrition With Maddie, explains. “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.

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 fibre) 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-producing microbes are associated with enhanced energy extraction from high-fibre diets. Resulting short-chain fatty acid compounds can contribute up to 10% of your daily calories.

Because of these differences in energy extraction efficiency, “the composition of the gut microbiome can result in more or less calories absorbed by two people from the same food,” Pasquariello says. Take a baked potato. Depending on the population of resistant-starch-reducing bacteria in someone’s microbiome, “one person might extract more energy from a baked potato than another,” Eslamplast says.

Digestion is largely mediated by proteins known as digestive enzymes in the small intestine. Most digestive enzymes are produced by the pancreas, but some originate in the mouth, stomach and small intestine itself. Enzymes are often targeted to specific nutrients. Lipase breaks down fats, amylase breaks down carbs and proteases and peptidases both break down proteins, to name a few. If digestive enzymes “are absent or not present in sufficient amounts, it can result in the body’s inability to break down sugars, fats, proteins and dairy products” (and thus the inability to extract the resident calories), Pasquariello says

Health conditions that can cause this issue include:

• Congenital sucrase-isomaltase deficiency, which prevents your small intestine from producing the enzymes sucrase and isomaltase, both necessary to break down certain sugars, like sucrose and maltose and certain products of starch digestion, like dextrin.
• Exocrine pancreatic insufficiency, which prevents your pancreas from producing enough of the enzymes needed to digest carbs, proteins and fats.
• Lactose intolerance, which prevents your small intestine from producing enough lactase, an enzyme needed to digest lactose, a sugar naturally found in dairy products.

Numerous other diseases and disorders can also mess with calorie extraction, though not through the digestive enzyme route. People who have celiac disease, Crohn’s disease or IBS “can all absorb nutrients differently or at a deficit, meaning someone might absorb fewer calories from the same meal as someone else without one of these conditions,” Pasquariello says; a phenomenon known as malabsorption. When a person with celiac disease eats gluten (a protein found in grains like wheat, barley and rye), for example, it triggers an immune reaction against tiny bumps called “villi” that line the small intestine. Villi help your body capture nutrients from food, so an attack on them reduces the absorptive surface and hurts this essential function. Crohn’s likewise affects the villi. Meanwhile, IBS can reduce GI transit time, triggering increased contractions that accelerate the movement of food so that it doesn’t make contact with the surface long enough for calories to absorb.

What this means for you

Overall, “this is not something I’d spend a lot of time worried about” (assuming you don’t have one of the medical conditions mentioned above), Pasquariello says. While it’s important to make sure you’re digesting food properly and address any concerns with an expert, you’ll be “much better off focusing on the overall nutrient density and quality of the foods in your diet than focusing on how every gram of food is or isn’t being absorbed compared to someone else.”

So practically speaking, this information doesn’t have too many applications. Conceptually, however? The implications are huge. Calories “are not always a fixed measure of energy for every person,” Eslamplast says. “Instead, calorie absorption is influenced by individual biology and food structure, supporting a more personalised approach to nutrition and weight management rather than focusing solely on calorie counting.”

If everyone extracts a slightly different number of calories from food, after all, then you don’t know exactly how many you’re actually consuming regardless of the number on the nutrition facts label. Far from representing a loss of control, however, this might free you up to eat intuitively rather than obsess over your stats.

“When we focus on numbers like weight [in kilograms] and calories, it leads us astray,” Dada says. “Whereas if we were to focus more on things that felt more intuitive, such as, ‘How does the food feel in my body? Am I paying attention to the different signals that my body is giving me about how much I’m eating?’”well, that’s when you’re on the right track.