BOSTON (AP) — The popular idea that weight loss simply comes down to burning more calories than you consume turns out to be a significant oversimplification.
Trendy diets rise and fall, but nearly all of them share the same basic premise: eat less, burn more, lose weight. In theory, that logic holds. Tracking calories can even be a helpful strategy. The trouble is, the numbers themselves are far less reliable than most people realize — and the math gets complicated fast.
A tangle of biological and dietary factors determines how — and whether — our bodies actually process the calories we take in. Experts say the type of food you eat is every bit as important as how much of it you consume.
“Different foods have very different effects on the brain, liver, fat cells, muscle function, pancreas and all organs related to metabolism and body weight,” said Dr. Dariush Mozaffarian, a cardiologist and director of the Food is Medicine Institute at Tufts University.
A calorie is defined as the unit of energy the body can extract from carbohydrates, proteins, and fats. That definition sounds clean and simple, but the issue of calorie accuracy recently made headlines when a lawsuit alleged that the maker of David protein bars had placed misleading labels on its products, overstating calorie and fat content. That lawsuit has since been withdrawn.
The claims were grounded in a method called bomb calorimetry, which determines calorie content by literally burning food and measuring the heat produced. This approach counts every possible calorie. But the human body is not a furnace — it doesn’t extract energy the same way. Most food labels, including those on the David bars, report only the calories the body can realistically absorb. Technically, neither figure is wrong, but for the purpose of dieting, only the latter is relevant.
“You could put sawdust into a bomb calorimeter and you would get basically 4 calories per gram,” said Dr. David Ludwig, an endocrinologist and researcher at Boston Children’s Hospital. “If you’re a termite, yes, you’ll get calories from it. But humans won’t.”
Even the calorie counts printed on standard nutrition labels — which reflect what the body is likely to metabolize — can be off by as much as 20% due to rounding rules. Beyond that, how digestible an ingredient is, whether it has been cooked, how heavily it was processed, variations in manufacturing, and even a person’s individual genetic makeup all play a role in how many calories the body actually takes in. And that’s just the beginning.
The body decides whether to burn or store calories based partly on energy demands — but food quality plays a major role too. High-glycemic foods like white bread, pasta, and sugar are quickly converted into usable energy and signal the body to store calories, according to Ludwig.
Foods containing resistant starches — found in some beans, whole grains, and seeds — are harder for the body to convert and don’t trigger that same fat-storage response. They’re also more difficult to digest, meaning fewer of their total calories are absorbed.
“Having a snack of 8 ounces of sugary beverage, 100 calories, should be better for your weight than 1 ounce of nuts at 200 calories, right?” Ludwig said. “That’s the opposite of what actually happens because those 100 calories, even if they’re fewer at that moment, they shift your body toward storing fat and leave you hungrier sooner.”
That hunger, of course, leads to eating more — and more calories.
Even ripeness and cooking methods factor into the equation. Calories from cooked foods are absorbed more readily than from raw versions of the same food, while unripe produce — like a green banana — yields fewer absorbed calories than a ripe one. A medium banana is listed at 105 calories, but how many of those your body actually uses depends on how ripe it is when you eat it.
Processing foods, even in simple ways, also changes caloric impact. “One classic example is that the calories in whole almonds are absorbed substantially less well than the calories in almond butter,” Ludwig said. “Just processing the almonds into almond butter causes a change in how much they will be absorbed.”
Ultra-processed foods bring yet another wrinkle. Diets heavy in these products have been shown to reduce the number of calories the body burns while at rest, Mozaffarian said — meaning a greater share of what you eat ends up being stored rather than burned.
Individual differences add another layer of complexity. Genetics influence how our bodies handle calories, said Dr. Fatima Cody Stanford, an obesity specialist at Harvard Medical School. Even something as routine as a poor night’s sleep can alter how the body processes food, meaning you might absorb more or fewer calories from the exact same meal on different days.
So what should people actually do? Ludwig said calorie counts can serve as a rough reference point, and many people benefit from having some structure to guide portion sizes and food choices. But the research points clearly toward prioritizing the quality of what you eat, not just the quantity. Experts recommend steering clear of ultra-processed foods — especially refined starches — and centering your diet on whole, minimally processed foods, with a strong emphasis on fiber-rich, plant-based options.
“We need to think about calories in a much more sophisticated fashion than the number on the package,” Ludwig said. “The number on the package can do more harm than good by misleading people into thinking that it’s simply an accounting problem.”
