The math it took to calculate the number of calories in your favorite snack involved a lot more guesstimation than you may think.<\/p>\n
The Food and Drug Administration began requiring standardized nutrition labeling<\/a> \u2014 including the number of calories per serving \u2014 on most packaged foods in 1990. Obesity rates skyrocketed in the United States over the next few decades, spurring a 2016 change to the rule to list calorie counts <\/a>on nutrition labels in a large, bold font.<\/p>\n Just how accurate are these calorie counts?<\/p>\n The popular protein bar brand David is currently fighting a lawsuit<\/a> alleging the company\u2019s bars contain nearly twice as many calories as the labels claim, based on independent testing. The company\u2019s founder said the lawsuit used an incorrect testing method to measure calories, and that the fat substitute the company uses in the protein bars does not contain as many calories as true fat does (about 9 calories per gram).<\/p>\n Nutrition scientists say there\u2019s wide variation in how many nutrients and calories each person absorbs during digestion.<\/p>\n That\u2019s why the FDA allows a discrepancy of up to 20% when counting calories<\/a>.<\/p>\n For example, if a frozen meal is labeled as having 500 calories, it could have as many as 600 calories and still be compliant, said Lindsay Moyer, a registered dietitian at the Center for Science in the Public Interest.<\/p>\n Dr. Zhaoping Li, chief of the clinical nutrition division at UCLA, said the amount of energy we get from food is affected by many factors, such as \u201chow well your stomach digests it, how much is absorbed in your small intestine, and what energy can be harvested after the microbiome works on it on the large intestine.\u201d<\/p>\n Together, these variables can cause inconsistencies between how many calories a food actually contains and how many are listed on that food\u2019s nutrition label \u2014 even if a company has made an earnest attempt at calculating them.<\/p>\n \u201cIt\u2019s guesswork,\u201d Li said.<\/p>\n Or as Marion Nestle, a professor emerita of nutrition, food studies and public health at New York University, puts it: \u201cCalories are kind of eye-rolling.\u201d<\/p>\n How do you measure calories in food?<\/p>\n The most accurate method to measure energy and calories in food is called a bomb calorimeter, Li said. The process works on the theory that when one thing releases heat \u2014 in this case, food \u2014 it\u2019s absorbed by another, in this case whoever is digesting that food, as energy.<\/p>\n To make this calculation, food scientists seal a piece of food in a pressurized, oxygen-filled steel container \u2014 the \u201cbomb.\u201d That container is encased in an insulated box filled with water.<\/p>\n The scientists light the food on fire and let it burn completely. Then, they take the temperature of the water. They use an equation to tell them how much energy \u2014 and therefore calories \u2014 are in that piece of food, based on how much the burning piece of food raised the temperature of the water surrounding it.<\/p>\n But the bomb calorimeter\u2019s calculations can be flawed, Nestle said.<\/p>\n \u201cNot all the components of foods are absorbed, and the only thing that counts for calories is what gets absorbed across the intestinal wall \u2014 fiber calories, for example, don\u2019t count,\u201d she said.<\/p>\n Nestle said it\u2019s much more common for food manufacturers to estimate the number of calories in packaged food using an Atwater factor. This mathematical equation uses a fixed number of calories per gram for macronutrients, specifically fat, carbohydrates and protein.<\/p>\n Every gram of carbohydrates \u2014 which include sugar \u2014 contains about 4 calories. The same is true for protein. 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 equation.<\/p>\n \u201cAnd that is close enough,\u201d Nestle said. \u201cYou have to be comfortable with approximations.\u201d<\/p>\n Latest news on diet and nutrition <\/p>\n Some companies, particularly restaurants looking to calculate calorie counts for menu items, may use a U.S. Department of Agriculture database<\/a> that houses calorie estimates for individual foods, such as a roma tomato or a commercially prepared slice of white bread. They may also use a more specific database calculation program that takes into account the restaurant\u2019s suppliers, CSPI\u2019s Moyer, said.<\/p>\n Still, this involves a bit of guesswork.<\/p>\n \u201cLet\u2019s say they get ingredients from a supplier. That supplier may have already done an analysis, but then it may be cooked or processed in a way that changes the nutrients of the ingredient,\u201d Moyer said.<\/p>\n Then there\u2019s the human factor. Restaurants that prepare food in-house likely have more variation in calorie counts than, say, a protein bar.<\/p>\n \u201cIf someone makes your sandwich at a place that makes subs, you might see them grab a preweighed slab of meat, so these calorie counts could be pretty accurate. But then if they take a squeeze bottle of sauce and just go for a while, that will vary,\u201d Moyer said. \u201cI wouldn\u2019t treat calorie labels on a menu the same as a prepackaged food such as a bar.\u201d<\/p>\n Few calories from fiber<\/p>\n Certain kinds of food are harder to break down, meaning the body actually absorbs fewer calories from them than a label may state.<\/p>\n For example, accurate calorie counts are particularly difficult to calculate for plant foods, which have cell walls that can be heard for teeth to break through, since chewing is the first phase of digestion, Moyer said.<\/p>\n \u201cPeople typically absorb slightly fewer calories for these foods than what the label calculates,\u201d she said.<\/p>\n Almonds are another good example, she said. People absorb more calories from almond butter than from whole almonds because \u201cit\u2019s basically chewed for you.\u201d<\/p>\n Several studies conducted by the U.S. Department of Agriculture have found that whole almonds<\/a>, cashews<\/a>, lentils and chickpeas<\/a> all have fewer calories once digested than what labels estimate. Walnuts, which are rich in protein and omega-3 fatty acids, have 21% fewer calories than typically believed, according to USDA estimates<\/a>.<\/p>\n Insoluble fiber is another nutrient that may not contribute any calories at all, since the body doesn\u2019t have the enzymes necessary to digest its chemical bonds \u2014 meaning it passes through the digestive system without getting broken down into energy. Whole grains, nuts, seeds and the skins of fruits and vegetables contain insoluble fiber: While they do have calories from sugars, fats and proteins, the calories that come from insoluble fiber aren\u2019t absorbed.<\/p>\n \u201cGenerally, fiber is a carbohydrate that we can\u2019t fully break down and digest, so fiber has few, if any, digestible calories,\u201d Moyer said.<\/p>\n