America’s century-old system for counting calories comes from chemist Wilbur Atwater. In 1887, he began to research the energy we get from eating by measuring the stored energy in foods and subtracting the amount left in people’s bodily excretions.
Atwater’s research has since been boiled down to the 4-9-4 rule: Each gram of protein, fat, and carbohydrate provides, respectively, 4, 9, and 4 calories of energy. The United States Department of Agriculture (USDA) has used these figures for decades, tweaking them only to account for different qualities—such as the digestibility—of specific foodstuffs.
But in the past few years, nutritionists have clamored for a reappraisal. For one thing, they say the present system ignores the difference between raw and cooked food. Harvard University researchers assert, based on mouse studies, that processed food is easier for the body to absorb, so it provides more calories. That goes for baked or blended food. Even a handful of chopped peanuts gives you more energy than whole ones.
In 2011, USDA researchers, with a grant from the nut industry, reported that the caloric value of pistachios had been overstated by 5 percent. In 2012, they found almonds were overstated by 32 percent, or 40 calories per serving. So you might not want to take nutrition labels at face value.
This article was originally published in the November 2015 issue of Popular Science