How Do They Figure Out The Amount Of Calories In Food?

What IS a calorie,anyway?

How Do They Figure Out The Amount Of Calories In Food?

Mariam Sharia

Taco Bell

What IS a calorie, anyway?

Welcome to the Glad You Asked series, a shame-free zone where we tackle topics you’re too embarrassed to ask even your BFF about. Don’t worry, we gotchu.

“Calorie.” Dreadful word. You know how many a bag of Cool Ranch Doritos contains. You know you should be ingesting 2000–2500 of them daily (depending on a lot of factors, like your age, weight, etc.). And you know the dismally small amount you burn running a mile (like, 100, give or take. Is running worth the effort? Who’s to say?)

But what is a calorie?

A calorie is a unit of measurement for energy, like a watt is to power or a gram to mass. One calorie is the amount of energy required to heat one kilogram of water 1° Celsius at sea level. In the United States, nutritionists substitute the capital-C “Calorie” for what is technically a “kilocalorie” (1 kilocalorie = 1000 calories).

Originally, scientists determined calorie counts by placing a food item into a sealed, insulated metal container surrounded by water — called a bomb calorimeter — and quite literally bombing it. Once the food was completely burned, the resulting rise in water temperature indicated the amount of heat (energy) emitted—hence, “calories burned.”

While this method was undoubtedly pretty fun, it was also time-consuming and ultimately unhelpful. It provided the amount of true calories accurately enough but failed to take into consideration that not all food is digested the same way, and some components aren’t metabolized at all. Fiber, for instance, is lost in the form of waste.

Today’s food labels use the Atwater system, which was developed in the late 19th century and calculates total calorie count by adding energy-providing components (carbs, protein, alcohol) and subtracting indigestible components (i.e., the aforementioned fiber).


Wilbur O. Atwater’s conversion tables distinguished the practical energy values of hundreds of different foods. Although he worked over a century ago, his research helped bring to fruition the USDA’s Food Composition Database — a collection of figures down to the calorie (and ingredient, and mineral, and vitamin) of any food item, branded or otherwise, from raw broccoli to Sriracha to Tofurkey.

Many companies send their food samples to any number of private food labs in order to determine caloric count. McDonald’s, for example, sent its Happy Meal salads in for independent testing despite the presence of several “fast-food salad” entries in the USDA database.

For others, it’s far simpler. Since the FDA requires companies to print accurate data on food labels but doesn’t care how they gather this information, many (permissibly!) guesstimate their calculations.

Like most restaurants, for example

They’ll often determine a menu item’s total calories by adding together the amount of calories in its components (per the USDA database) and dividing by the number of servings. A quick grilled cheese example:

2 slices of white bread: 154 calories

2 slices of cheddar cheese: 226 calories

1 tbsp of salted butter: 102 calories

Total calories = 482 calories

And that doesn’t even include the tomato salad! | Flickr

Since the caloric counts of the “basic,” unbranded items are averages (for instance, the 102 calories in a tablespoon of salted butter is the average of Land O’Lakes, Kerrygold, Breakstone’s, etc.) the precision here is a bit comme ci, comme ça, as our Brie-loving friends would say.

For a further sprinkling of shiftiness, consider how a 2011 Tufts University study found that 20% of food items from a 42-restaurant sample actually had 100 more calories than advertised. The majority of these “errors” were made on the “diet” side of the menu.

Are cookbooks shifty, too?

A responsible recipe developer or cookbook publisher knows that determining an accurate calorie count means taking into consideration when food changes states.

Brooke Lark/Unsplash

As recipe guru and dietitian Jessica Cox points out, there’s a difference between the raw calorie count and cooked calorie count of any given food. It changes depending on method of cooking. She gives a pork roast as an example:

A fatty 3-pound pork roast will get smaller when all its fat drips off in the oven. To figure out the caloric count of the final product, Cox would consult the USDA for the amount of calories in a 3-pound raw roast, then stack that against the final weight of the roast once cooked, determining the number of portions that are actually available.

Since our hypothetical roast has cooked down to 2.5 pounds from its former 3, there are fewer available portions (13 rather than 16). However, each 3-ounce serving of our cooked pork has more calories than its original, raw state.

This is because our roast lost more water than fat during its transformation into dinner, meaning the fat that remained became more concentrated. Quite simply, water weight is lost and density increases.

What does this mean for me?

Throw in variations in cooking methods (don’t fry it if you’re on a diet), food processes (even chopping and grinding affect calorie measurements), and the fact that most testers only count food calories “available for consumption” (so, not the leftover marinade in the bowl) and it’s easy to see how calorie counts — whether on food labels, in recipe books, or at restaurants — are unreliable at best and totally pointless at worst.

And those aren’t even the only factors to consider. To really hone in on an accurate calorie count, you’d have to ask: Has the food evolved to survive digestion? Has it been microwaved? How do the billions of bacteria in our body aid or handicap our digestion? It’s also important to note that no two people will get the same number of calories out of the same food.

We produce different enzymes, come from different heritages (i.e., Russians have longer large intestines than Poles, meaning they get more calories out of the same foods) and harbor a variety of different bacteria suited for breaking down nutrients of one sort and not another.

No standard exists, so it’s kind of a free-for-all — like guessing how many jellybeans are in the jar and winning the prize because nobody knows how wrong you actually are.

So, maybe The Dreaded Calorie isn’t really the enemy after all. Maybe it’s our desk jobs, lack of exercise, and complete disregard of “suggested serving size” that is at fault for our obesity epidemic. Maybe, instead of merely adding together the numbers on food labels — numbers originally drawn from a 19th-century scientific process and based largely on guesstimation — we start thinking of meals as fuel.

Just some food for thought.