Food and calories: How do we measure them?

Some time ago a part of our Hungry Pumpkin team was on a business trip and found themselves eating at the hotel’s buffet. The food was marked with many labels, such as “vegan”, “rich in fiber”, “rich in protein” and “low fat”. Among these others, our observant teammate noticed that some foods were labeled “low calorie” while others had a label reading “high energy“. Even though these two labels are speaking about the same thing (calories and energy), they used a sneaky way to express it for the foods rich in calories, well knowing that the word calories has negative connotations.

But, what is a calorie really?

In physics, a calorie is a unit of measurement for energy. Just like with any other measurement, you can have a low value for it or a high value. Someone who weighs 100 kg is heavier than another person weighing 70 kg, and likewise, a food with 500 calories contains more energy than food containing 100 calories. With this in mind though, calories are not actually the official unit for measuring and indicating energy anymore; that honor goes to the Joule (J). The calorie is an older unit, though it is still used in some fields today, for example in nutrition.

One calorie (1 cal) is defined as the amount of energy that is required in order to heat one gram of water to one degree celsius. Physicists have chosen to adopt the joule because it applies better to a standard unit system. This particular measurement is defined as the amount of energy needed to move a 1-N (one newton) object one meter (this is approximately equivalent to moving 100g one meter). If we convert calories into joules, we find that 1 cal = 4.2 J.

Unfortunately, here we also encounter an issue with food labeling. The unit used in nutrition normally is in fact a kilocalorie. Just as with other measurements, kilo means that we have multiplied the unit by a thousand. A kilogram is 1000 grams, and a kilocalorie is 1000 calories. This sort of notation makes sense for larger units as it saves us a whole lot of zeros at the end of the number. But as you may have noticed, the packaging sometimes is written using calories (cal) instead of kilocalories (kcal). Now, this would be ok except or the fact that nutrition labels are usually referring to kilocalories while referring to them as calories (in other words, an error which leads to inaccuracy of a thousand-fold proportion). For example, the United States writes Cal (with a capital C) to indicate kilocalories rather than using the proper symbols (possibly a result of continued use of the imperial measurement system rather than metric).

In our blog we stick with the technically correct notation, so calorie (cal) refers to a single caloric unit and kilocalorie (kcal) refers to a thousand calories. If in doubt when reading the nutrition labels on food, look at the numbers. If it’s 10-100 per serving then they’re bound to be talking about kilocalories. On the other hand, if the number is more than 10000 then you can be sure they’re referring to actual individual calories.

Measuring calories: incineration.

To look at how calories are measured in food, let’s take peanuts as an example. Like we said before, one calorie (1 cal) is the amount of energy needed to heat one gram of water by one degree Celsius. Now, before we get into how it’s done, we need to give you the obligatory “don’t try this at home!” speech. The experiment we are going to describe was carried out by qualified researchers with appropriate equipment and took all necessary safety measures into account. So please don’t try this yourself unless you meet those conditions too.

Now, the experiment: we took a single unsalted peanut and put it on a needle before setting it above a flame to burn. We placed a 15 ml tube of water above the burning peanut and waited for our poor little nutty friend to completely burn. At the same time, the water began to boil, meaning that it had been warmed from room temperature to boiling temperature. You can also see the procedure in the video here.

How are calories measured in food.

The water heated to just below 100 degrees celsius, which means the 15 ml of water was heated up about 80 degrees (from around 20 to around 100). This means that in burning the peanut we released 15 x 80 = 1200 cal. This means that our little peanut contained at least 1200 cal. Many of you may know that the average person needs 2000 calories per day, so this result may surprise you; this, however, is where our caution about units comes in. The measurement from the peanut is in calories (cal) and the dietary requirement advice is actually in kilocalories (kcal). This means the peanut contained only 1.2 kcal of energy, a tiny portion of your daily requirement.

This measurement, however, is not entirely accurate; not only did the peanut’s energy heat the water but also a lot of it went into heating the surrounding air and dissipated. This is why scientists working with foods to determine calorie content use a more complicated set up including a device known as a bomb calorimeter. It works according to the same principle but ensures that this experimental error is accounted for. The food that they want to measure is first dried up and ground. It is then placed in the bomb calorimeter, which is a steel container surrounded by a water bath, all of which is surrounded by thermal insulation. The chamber is filled with pure oxygen (enough to allow the entire food sample to bun) and the food is ignited. The resulting energy release is very fast and turbulent, actually creating a small explosion. This heat is then absorbed by the surrounding water, which is measured with a thermometer; the temperature rise in the water is used to calculate the amount of energy in the food. The scientists then make additional corrections, for example taking into account the non-digestibility of fibers. With their more precise methods, scientists have measured that 1 g of peanut contains 5.67 kcal. So in comparing that to our number (1.2 kcal per 0.64 g of peanut, or 1.88 kcal per gram), we see that a lot of heat was lost into the air and thus could not be measured.

More energy than a stick of dynamite!

In burning the peanut, the energy trapped in the chemical bonds of the molecules is released as heat. This is actually quite similar to what happens in our bodies, except that inside of us the energy is put to use rather than just dissipating into the air.

To put this on display a bit, it’s time to do some calculations. One kcal is equal to the work needed to raise up an adult male by two floors. You can figure this out by remembering that 1000 cal = 4200 J, where one joule is the energy required to move 100 g of weight by one meter. One kilocalorie is thus enough to raise 70 kg by 6 meters. Or, to put it in a more practical context, one peanut contains the amount of energy needed to walk 10 stories. Now that we know this, the statement that two pieces of chocolate contain more energy than a stick of dynamite begins to make sense. These two seemingly innocent little pieces contain about 200 kcal, which equates to about 840 kJ! That is almost a megajoule or the amount of energy needed to raise an adult person one kilometer. That being said, this doesn’t take into account losses of energy; in reality, your body cannot convert all energy from food, instead of using only about 20%.

So no need to go crazy counting your calories. Though it is important not to consume too many calories, a healthy relationship with food is also important. Calorie counting can become an obsessive endeavor for some and lead to behaviors associated with eating disorders and disordered eating. If you think your caloric intake may be too high (or low!) don’t be afraid to reach out to a dietician who will be in the best position to help you figure out what (and how much) your body needs.

Additional reading