We’re thinking about calories all wrong.
We all know what calories are. We see the calorie labels on food and know that we put calories in our bodies when we put food in our mouths. And we all know that we burn calories when we exercise. Our Apples Watches and FitBits tell us so.
You may also know the “calories in, calories out” principle. This principle says that we gain weight when the calories we put in our bodies (eating) are greater than the calories we use (exercise). And conversely, we lose weight when the calories in are less than the calories out.
But it’s a little more complicated than that. There’s only one way to put calories into our bodies — by eating. But it turns out there are lots of ways that calories get used and leave our bodies. And exercise is one of the smallest and least significant of all these ways. (This is why it’s incredibly difficult to exercise your way out of a poor diet.) All of our bodily functions require a lot of energy and burn lots of calories — thinking, pumping blood, regulating body temperature, building and repairing muscles, and a host of other activities.
One of those bodily functions that requires energy and burns calories is digesting food. That’s right — it takes calories to break down and digest food. Our bodies require calories to extract new calories from food. And this activity requires more energy and calories than exercise. Simply put, digesting food is a significant portion of our calories out.
This is where the concept of gross-calories versus net-calories comes into play. Gross-calories are simply the number of calories you put in your mouth. It’s the calorie count on the wrappers. It’s what we’re used to thinking about.
Net-calories are the number of calories you put in your mouth minus the number of calories you body uses to digest that food. This is how many calories are actually available for your body to use after extracting them from food. Because net-calories measures how many calories are actually available to your body, they’re a more useful measurement.
Now here’s why this is important. Different foods require significantly different amounts of energy to digest. This means that two foods that have the same number of gross-calories will have different net-calories if they require different amounts of energy to digest.
Let’s take an example. Imagine an apple and a handful of candy. Say both have 100 calories. Under a gross-calorie framework, there’s no caloric difference between the two foods. But it may only take 5 calories to break down the candy and access the 100 calories while the body has to work a lot harder and expend 25 calories to break down the fibrous apple and access the 100 calories. So the net-calories from the apple are 75 (100–25) while the net-calories from the candy are 95 (100–5). Same gross-calories, but very different net-calories
This may make intuitive sense, but there’s actual research backing it up too. Researchers have found that processed foods (candy, in our example) require far less energy to digest and that unprocessed food (apples, in our example) require a lot more energy to digest. The result is that processed food and unprocessed food that have the same number of gross-calories can have significantly different net-calories.
While the net-calorie difference between the apple and the candy may seem small, the small difference adds up throughout the day and over the course of days and weeks and months and years. Consistently eating a diet high in processed foods will yield a significantly higher net-calorie count than a comparable diet of unprocessed foods, even though the two have the same number of gross-calories. This in turn can lead to significantly different health and body composition outcomes over time.