Can You Trust Your Calorie Counter?

Whether it's the number of calories on the nutrition facts label or the readout on your Fitbit, chances are those calories counts are wildly inaccurate. So, what's the point of paying attention to calories?  Nutrition Diva explains.

Monica Reinagel, MS, LD/N, CNS,
Episode #520
Donut and calorie counter

Nutrition Diva listener James writes: “I was wondering if you could devote an episode to calories: how they're measured, how we process them, what a bomb calorimeter is, and all the problems with counting calories.”

What Do Calories Measure? 

A calorie is a unit of measure, like an inch or a kilogram. Only instead of measuring length or weight, a calorie measures energy. Technically, a calorie is the amount of energy it takes to raise one gram of water by one degree Celsius.

Calories can be measured by something called a bomb calorimeter. You might have even built a crude version of a bomb calorimeter in science class. In broad strokes, you submerge a chamber in a bucket of water and put a thermometer in the water. Inside the submerged chamber, you set something on fire. The heat generated by the combustion raises the temperature of the water in the bucket, which you can measure with the thermometer. You can then calculate the amount of energy or calories were in the thing you set on fire. 

If we habitually take in more energy than we use, we gain weight. So, we use calories as a guideline to determine how much food energy a person needs.

We used to use bomb calorimeters to calculate how many calories a given food contains. These days, however, it’s more common to estimate the number of calories based on how much protein, fat, and carbohydrate a food contains.  (These amounts can be determined through chemical analysis.)

When we’re using the word "calorie" in relation to food, by the way, we’re actually referring kilocalories.  Sometimes you’ll see calories abbreviated as kcal and that’s what that refers to. When we say that a food that contains 60 calories, it technically contains 60,000 calories. But all those zeros would would be a pain to deal with so we just lop them off. This drives phycisists and chemists absolutely nuts.

Why Do Calories Matter? 

Just like in that bomb calorimeter, our bodies combust food to release its stored heat or energy. We then either use that energy to power our biological processes or, if we have more energy than we need, we store it for future use. If we habitually take in more energy than we use, we gain weight. So, we use calories as a guideline to determine how much food energy a person needs. We need enough to fuel growth and maintain function, but not so much that we start storing a lot of fat.

There are calculators that can estimate your daily calorie needs, taking into account your age, sex, height, and activity level. And then there are databases and labels to tell you how many calories are in various foods.  However, as listener James suggests, there are a few problems with this system...


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