We talk about how many calories a meal has. But what are calories, exactly?
We are used to measuring food energy in calories. When we digest our food, this is the maximum amount of energy our body can extract from it - to keep warm and active. We talk about minimum daily calorie intake - or maximum daily intake if you are on a diet.
It is often stated that the minimum daily calorie intake for women is 1,200 calories and 1,800 calories for men. But of course these numbers depend on size, weight, age, activity and many other factors.
There are several formulas floating around that calculate your minimum daily intake based on some or more of these factors. The most commonly used are Mifflin-St Jeor formula, the Katch-McArdle formula and the Harris-Benedict formula.
Check your own situation at Calorie Calculator - Daily Caloric Needs (freedieting.com).
But what is a calorie?
Historically a calorie is defined as the amount of energy required to increase the temperature of 1 kilogram of water from 0 degrees Celsius to 1 degree Celsius.
For all practical purposes, we could say that a calorie is the amount of energy required to increase the temperature of a liter of water by one degree Celsius. This simplification stands up at the temperatures we humans live in. A kilogram of water is a liter of water at 0 degrees Celsius. And the difference of warming up from 0 to 1 degree Celsius and warming up from 20 to 21 degrees Celsius may be ignored.
What about Joules?
In the International System, the unit of energy is Joule (J). 1 calorie equals about 4.19 J.
1 Joule = 1 Newton * meter which equals 1 kilogram * meter2 / second2. What is there not to love about the International System?
There is one big confusing aspect in this story. That is, that dietary calories are really kilocalories, or 1,000 calories. So, the above daily intake for women and men is really 1,200 or 1,800 kilocalories or kcal. Often in the literature you will read energy expressed in Calories (note the capital C). This is an obscure and unnecessarily confusing way to express in kcal.
These days, food energy is generally expressed in both kiloJoule (kJ) and kilocalorie (kcal). kJ is now the preferred unit of energy.
If you want to be aware of your energy intake, you probably already read the labels on the products you buy. You may have noticed that the energy values are expressed in kJ/kcal.
Calculating your actual daily intake is easy with packaged products. For example, unprepared risotto rice contains about 1,500 kJ or 350 kcal of energy per 100 grams.
Small, medium and large eggs contain 66, 82 and 107 kcal respectively. Source: Calories in Egg 🔸 | Nutrition Facts and Calorie Counter (your-calories.com).
Now, food consists of fats, carbs, protein et cetera.
Given that, roughly,
- 1 gram of Protein = 4 kcal
- 1 gram of Carbohydrate = 4 kcal
- 1 gram of Fat = 9 kcal
- 1 gram of Alcohol = 7 kcal
it is easy to see that foods with higher fat percentages automatically will contain more kcal than leaner products.
Trivia Question: how many Calories in a glass of dry white wine?
Assuming that dry white wine does not contain any sugars and that the alcohol content is 12% by volume, a glass of wine (125 ml) contains 15 ml of alcohol. The specific weight of alcohol is slightly lower than that of water (about 800 grams per liter), so 15 ml of alcohol equals 12 ml of alcohol equals 15 grams of alcohol. Which means that a glass of dry white wine contains about 84 kcal!