The next time you check out the calorie count label on a candy bar or on the menu at a restaurant, remember to take it with a grain of salt.

Chipotle has long billed itself as a healthy fast-casual alternative to fast food chains. But earlier this week the New York Times published a breakdown of exactly how many calories an average Chipotle meal contained — and it turns out that there are a lot of hidden calories in that steak burrito with extra cheese and guac. Over 25 percent of Chipotle meals on GrubHub (an online food delivery service) contained over 1,350 calories – well over half the recommended daily caloric intake. The average Chipotle meal totals to about 1,070 calories.

For all the hubbub about the Chipotle’s caloric horror – it’s worth asking: how do calorie counts come about? And how accurate are they?

The calorie count you see on a packaged food label is actually just an indirect estimation made using a 100-year-old system called the "Atwater system." In the 19^th century, American chemist Wilbur Olin Atwater burned samples of food and measured the amount of energy from the heat they produced. Each unit of energy was measured as a kilocalorie which has since been simplified to "calorie."

Atwater estimated an average calorie count for protein (4Kcal/g), carbohydrates (4Kcal/g), and fat (9Kcal/g) and to this day, food manufacturers use those numbers to estimate the caloric content in any given food product. For example, if a Chipotle soft flour tortilla had 46g of carbohydrates, 7g of protein, and 10g of fat, then the label (or in this case, Chipotle’s online nutrition calculator) would say that the flour tortilla contained 302 calories. We tried this and found that Chipotle underestimated by 2 calories, but we’ll let them slide for that.

What the Atwater system doesn’t account for is the drastic differences between processed foods and unprocessed foods. People get more calories from eating processed foods than unprocessed foods because unprocessed foods take more energy to break down and digest. In a study conducted in 2003, researchers in Japan fed rats two different kinds of chow — solid pellets and puffed pellets. The puffed pellets were more processed while the solid pellets were standard fare for lab rats. The rats ate the same number of calories in pellets and exercised the same amount as each other but the rats who ate the puffed pellets ended up gaining 30 percent more body fat than their counterparts eating solid pellets.

In other instances, calorie counts are an overestimation because we don’t digest all the food that we eat. A study published in 2012 in the American Journal of Clinical Nutrition found that because we don’t completely digest all the fat in almonds, the actual calorie count of a one-ounce serving of almonds is actually 129 calories — 32 percent less than their original calorie estimation according to the Atwater system.

More calorie confusion abounds when restaurants come into the picture. Studies from Tufts University have shown that some fast food restaurants underestimate the calorie count of their products by as much as 18 percent and others overestimate the calorie count of their products by about 10 percent. Researchers speculate that some of the calorie inaccuracy comes from differences in portion sizes. 

So how many calories are actually in that Chipotle burrito? We can’t know for sure – but it is still probably safe to say that it’s a lot.