By Sarah Zhang, Mother Jones
To understand the modern calorie counting system, we have to go back to the late 19th century, when Wilbur Atwater sat in his Connecticut lab and burned a bunch of food. Then he weighed some human feces and urine. The burning told him the amount of energy in the food; the human waste told him the amount excreted. Subtract the two, and the Atwater system was born: four calories per gram of protein, nine for fat, four for sugars. Check out the label on a supermarket salad in 2013, and the caloric value you see is largely based off of Atwater — even though scientists are increasingly suspicious the number is wrong.
Debate over the science of calories comes at the same time that health-conscious legislators are requiring restaurants to show calorie counts on their menus. Following the lead of New York City and California, Obamacare has a provision to make calorie labeling mandatory at chain restaurants across the country. For that to make a difference to our health, however, we have to understand what a calorie really means. At the meeting of the American Association for the Advancement of Science in February, a panel of scientists presented research that is just beginning to tease out the nuances of the calorie. Whether food is cooked, the energy it takes to break down tough-to-digest foods, and gut bacteria activity all affect how much energy humans actually get from eating.