It’s a household word but still a mystery to many.
Ask ten people what a calorie is and at least nine will tell you ‘It’s the stuff in food that makes me fat.’ Calories are one of the most commonly counted things on the planet, but how many people know what they’re really counting?
The calorie is a unit of heat energy.
It was originally developed as a way to measure the efficiency of fuel burned in steam engines. When scientists turned their attention to humans, they borrowed the concept of the calorie as a way to quantify food as fuel for the human engine. In theory, the amount of heat that can be provided by any particular bit of food is the same whether it’s burned in a steam engine or a human body. More edible calories mean more energy for work, like coal in a human stove.
To measure the energy in various foods, early 20th century nutritionists burned small amounts of each inside a bomb calorimeter—a lab tool that surrounds a food-filled capsule with water. They assigned caloric values by calculating the different amounts of heat given off by different foods—one calorie for each one degree increase in the temperature of the surrounding water. These calculations are what we still use today; the calorie count on a box of Honey Nut Cheerios is calculated in 100 year-old Atwater units.
A calorie is a calorie is a calorie?
Scientists are just now teasing out the nuances of the calorie. Advances in understanding were presented at this year’s annual meeting of the American Association for the Advancement of Science and it’s clear that all calories are not created equal.
Raw and unprocessed foods have fewer calories than we thought; or at least fewer than we are able to digest. The more a food is handled the more calories it gives up in digestion, and it can mean a swing of 30 or 40%. Most foods keep the calories contained inside their cell walls, so you have to do something to rupture the walls. The chopping, mixing, and heating of cooking might be enough to crack open the cells for some foods, but if you really want all the calories, you just need to eat factory-processed foods.
We’re also learning more about the body’s mechanism during digestion. Digestive tracts and their microbes are determined by genetics and cultural factors so you see big variations, like people of Russian descent with five more feet of intestines than the rest of us, and Japanese citizens with marine bacterium in their gut that help digest sushi. The old Atwater bomb calorimeter can’t even come close to figuring calories for these populations.
We understand enough to know that traditional calorie counts don’t apply to every food and every body.
Ironically, this understanding comes just as the federal government is getting ready to launch a nationwide requirement for posted calorie counts in restaurants. The labeling, based on out-dated Atwater units, might not be accurate, but for now it’s the best method we have for quantifying calorie values, and one worth paying attention to as a defense against obesity.
In 2013, these were the most-searched calorie terms on Google: