Today, nearly every packaged food you can find includes a calorie count on the label. These numbers describe how much energy your body gets from a particular bar of candy or box of cereal. And while the effectiveness of counting calories has been debated, for many, it’s a helpful weight loss tool. But can you really count on calorie labels?

The calories listed on labels come straight from the manufacturers — and are regulated by the FDA . But the agency allows for a 20 percent margin of error. Because of this, the caloric content is often higher than labeled, yet still within FDA limits. A 2013 study on the food label accuracy of snack foods found that their average caloric content was more than 4 percent higher than the calories listed. The researchers suggest this was because the foods contained more carbohydrates than listed.

Beyond that, the counts on labels can be inaccurate for other reasons, too. The idea of calorie counts comes from 19th century American chemist Wilbur Olin Atwater , who created the current system by calculating the average number of calories in a single gram of protein, fat and carbohydrate. Yet different foods are digested in different ways, which has an impact on the calories you extract. For example, a series of studies by Agricultural Research Services found that people absorb fewer calories from nuts like almonds, walnuts and pistachios — which can be harder to digest completely — than the labels indicate.

“We’ve all begun to see that managing weight loss is not just about calories in and calories out; food quality is very important,” says former dietician Ann Albright, currently director of the Division of Diabetes Translation at the Centers for Disease Control and Prevention. “Calories are important to be aware of, but they’re insufficient when it comes to guiding your nutritional behavior and practices.”

Read More:

When Dieting, Should We Be Fasting or Grazing?

The Biggest Factor Behind Obesity May Be One We Don't Want to Hear

Does Drinking Caffeine Help Stop Hunger Cravings?