Nuts Don’t Have as Many Calories As You Thought

Tree nuts just got a little bit leaner. Research over the last few years reveals that tree nuts — including almonds, cashews, walnuts, and pistachios — have fewer calories than previously estimated. How is that even possible? We dove into the science to find out.

Calories are most often calculated using “Atwater factors,” developed by the eponymous chemist around the turn of the 20th century. Wilbur Atwater fed subjects simple diets to deduce how much each macronutrient — protein, carbohydrate, and fat — is typically digested. He reasoned that we take in 4 calories for every gram of protein eaten, 4 calories per gram of carbohydrate, and 9 calories per gram of fat. Ever since, calories are usually estimated by multiplying the number of grams of each macronutrient by either 4 or 9 and adding them up.

But in the last few years, David Baer and Janet Novotny, researchers at the United States Department of Agriculture, discovered this math doesn’t hold for tree nuts. By giving subjects the same base diet with or without the addition of tree nuts, the researchers found nuts are not as well digested as the Atwater factors assume. Some of the nuts’ protein, fat and carbohydrates are pooped out, meaning those calories come back out too. By the new calculation, one ounce of almonds is only 129 calories, 19% fewer than the 168 calories determined using the Atwater method. An ounce of cashews is 137 calories rather than 163 (a 16% drop). Pistachios’ and walnuts’ calories dropped 5 and 21%, respectively.

The discrepancy is probably due to our bodies inability to break down the cell walls of nuts. “Until we break open that cell wall the things inside the cell aren’t available for digestion,” says Baer. The less the nut is processed the greater the difference in calories: roasting makes nuts more brittle and slightly easier to digest, while nut butters are so processed there’s no difference between Atwater’s and the newer calculation. Even between people there is a little bit of variation in calories extracted, probably due to chewing differences. 

This means that nutrition labels for nuts and nut products overestimate calories, which may deter people from eating them. “There’s fairly well-established research across the board on the health benefits of nuts,” says Stephanie Csaszar, a Registered Dietitian at the snack company KIND. “But calories are still something that’s often checked.” Because almonds and cashews are predominant ingredients in its bars, KIND is updating its labels to show ten to thirty fewer calories per bar. But it’s hard to know how every company counts calories; the FDA allows companies to choose between approved methods, including Atwater’s and the newer one.

Even with fewer calories, nuts remain nutritional powerhouses. “These are nutrient dense foods, and there’s still a lot of literature that shows health benefits,” from heart to brain function, says Baer. So your favorite on-the-go snack still packs the same mean punch. “Tree nuts haven’t changed,” says Baer. “We’re just doing a better job measuring the calories.”