The math involved in calculating the number of calories in your favorite drink involves more guesswork than you might think.
The Food and Drug Administration began requiring standard nutrition labeling — including the number of calories per serving — on most packaged foods in the 1990s. Obesity rates skyrocketed in the United States over the next few decades, prompting a 2016 change in the law to write calorie counts on nutrition labels in big, bold font.
How accurate are these calorie counts?
Popular protein bar brand David’s is currently fighting a lawsuit alleging that the company’s bars contain almost twice as many calories as the labels say, based on independent testing. The company’s founder said the lawsuit used an incorrect test method to measure calories, and that the fat substitute the company uses in protein bars does not have as many calories as real fat (about 9 calories per gram).
Nutritional scientists say there is a big difference in how many nutrients and calories each person absorbs during digestion.
That’s why the FDA allows up to a 20% difference when counting calories.
For example, if a frozen meal is labeled as having 500 calories, it could have 600 calories and still fit, says Lindsay Moyer, a registered dietitian at the Center for Science in the Public Interest.
Dr. Zhaoping Li, head of clinical nutrition at UCLA, said the amount of energy we get from food is affected by many factors, such as “how well your stomach digests it, how well it gets into your small intestine, and what energy can be harvested after the microbiome works on it in the large intestine.”
Together, these variables can cause a discrepancy between how many calories a food contains and how many are listed on the nutrition label – even if the company has made a good effort to count them.
“It’s speculation,” said Li.
Or as Marion Nestle, professor emeritus of nutrition, food studies and public health at New York University, puts it: “Calories are the eye candy.”
How do you measure calories in food?
The most accurate way to measure energy and calories in food is called a bomb calorimeter, Li said. This process works on the idea that when one substance releases heat – in this case, food – another, in this case whoever is digesting that food, is drawn in as energy.
To make this calculation, food scientists seal a piece of food in a pressurized, oxygen-filled metal container—a “bomb.” That container is sealed with an insulated box filled with water.
Scientists set food on fire and let it burn. Then, they take the temperature of the water. They use an equation to tell them how much energy – and thus calories – are in that piece of food, based on how much the burning piece of food raises the temperature of the surrounding water.
But the bomb’s calorimeter calculations can be flawed, Nestle said.
“Not all nutrients are absorbed, and the only thing that counts as calories is what is absorbed through the intestinal wall – fiber calories, for example, don’t count,” she said.
Nestle said it is common for food manufacturers to estimate the number of calories in packaged foods using the Atwater factor. This calculation uses a fixed number of calories per gram for macronutrients, mainly fats, carbohydrates and proteins.
Each gram of carbohydrates – which includes sugar – contains about 4 calories. The same is true for proteins. Fat contains about 9 calories per gram. Food scientists can get a basic estimate of the number of calories in a food based on this calculation.
“And that’s close enough,” Nestle said. “You have to be comfortable with measurements.”
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Some companies, especially restaurants that want to calculate the calorie values of menu items, may use the US Department of Agriculture’s website that maintains calorie estimates for individual foods, such as a roma tomato or a slice of commercially prepared white bread. They may also use a direct database accounting system that takes into account restaurant suppliers, says CSPI’s Moyer.
However, this involves a bit of guesswork.
“Let’s say they get ingredients from a supplier. That supplier may have tested them, but they may have been cooked or processed in a way that changes the nutritional value of the ingredient,” Moyer said.
Then there is the human factor. Restaurants that prepare meals in-house probably have a greater variety of calorie counts than, say, a protein bar.
“If someone makes your sandwich at a place that does subs, you may see them take a slab of meat that has been weighed, so these calorie counts are more accurate. But if they take a squeeze bottle of sauce and walk away for a while, that will be different,” said Moyer. “I wouldn’t treat calorie labels on the menu the same way as pre-packaged food like a bar.”
Fewer calories from fiber
Certain types of food are hard to break down, meaning the body actually absorbs fewer calories from them than the label says.
For example, accurate calorie counting is more difficult for plant foods, which have cell walls that can be felt for teeth to penetrate, since chewing is the first stage of digestion, Moyer said.
“People often absorb fewer calories from these foods than what is listed on the label,” she said.
Almonds are another good example, he said. People absorb more calories from almond butter than from whole almonds because “you’re basically chewing it.”
Several studies conducted by the US Department of Agriculture found that whole almonds, cashews, lentils and chickpeas all have fewer calories when processed than the labels say. Walnuts, rich in protein and omega-3 fatty acids, have 21% fewer calories than commonly believed, according to USDA estimates.
Insoluble fiber is another nutrient that may not provide any calories, as the body does not have the enzymes needed to digest its chemical bonds – meaning it passes through the digestive system without being broken down into energy. Whole grains, nuts, seeds and the skins of fruits and vegetables contain insoluble fiber: Although they contain calories from sugar, fat and protein, the calories from insoluble fiber are not absorbed.
“Typically, fiber is a carbohydrate that we can’t fully break down and digest, so fiber has few, if any, digestible calories,” Moyer said.
The large intestine can extract some, but not all, of the calories from another type of fiber, called soluble fiber. Oats, beans, lentils, apples and avocados are all high in soluble fiber. Soluble fiber can also prevent the digestive system from absorbing other fats and cholesterol.
So, should you rely on calorie counting?
There is limited research measuring the accuracy of packaged food labels, and the FDA does not test every product on the market.
“It’s kind of ‘buyer beware’ when the facts on a food label sound too good to be true,” Moyer said. “Compare similar products; if you’re looking at a peanut butter line and you see one that’s 120 calories per serving when they all say 180 calories per serving, that would raise a red flag for me.”
Paying attention to the calorie count on nutrition labels is a good place to start, he said, but no company can say that it counts calories down to single digits, for example, that a slice of frozen pizza has 352 calories.
“People mean well, and the calculator software will spit out a number with an accurate calorie count, but that’s a joke. Nobody knows about that range,” Moyer said.
