Philip J. Hilt in The New York Times:
The method most commonly used to assess the number of calories in foods is flawed, overestimating the energy provided to the body by proteins, nuts and foods high in fiber by as much as 25 percent, some nutrition experts say.
…An adult aiming to take in 2,000 calories a day on a low-carbohydrate, high-protein diet may actually be consuming several hundred calories less, he and other experts said. Calorie estimates for junk foods, particularly processed carbohydrates, are more accurate. The current calorie-counting system was created in the late 1800s by Wilbur Atwater, a scientist at the Department of Agriculture, and has been modified somewhat over the past 100 years. Researchers place a portion of food in a device called a calorimeter and burn it to see how much energy it contains. The heat is absorbed by water; one calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. When experts talk about calories, however, they usually mean kilocalories; one kilocalorie equals 1,000 calories. Those are the amounts you see on food labels.
…Almonds are routinely listed as having about 160 calories a serving, when the real figure is about 120 calories, said Karen Lapsley, the chief scientist at the California Almond Board. Some manufacturers are considering making the change on their labels.