How to calculate the calories of food?

Do you know how important it is to measure the calories of what we eat? Eating what we need is key to a healthy diet. See why.

Last update: June 23, 2022

We all need energy to do any work; not only physical, but also biological. This energy is obtained through what we eat and we measure it in calories. Then, calculating the calories in food will allow us to ensure adequate energy intake.

The calories we eat with food are necessary for several issues:

  • Physical activity.
  • Heartbeat.
  • Nervous system function.
  • Tissue repair.
  • Reproduction.

Likewise, if we want to lose weight or gain it, we need to know the different methods that allow us to calculate the calories in food. Keep reading and you will see how easy it is.



What are calories?

The calorie is a unit of measurement of the amount of energy that food and certain substances provide. In chemistry, it is also defined as the amount of heat required to raise the temperature of one gram of pure water by 1 degree Celsius and at the pressure of 1 atmosphere.

In the field of nutrition, the calorie and the kilocalorie can be used interchangeably. However, although they are related, in terms of energy measurement they are not the same, since 1 kilocalorie is equal to 1000 calories.

For example, when we say that 100 grams of cooked ham provide 213 calories, we are actually talking about 213 kilocalories and 213,000 calories. But talking about calories with the corresponding mathematical units would scare anyone.

All the calories we need are found in food and that is why we can establish the energy or caloric value of a food. It is defined as the amount of energy produced by macronutrients when oxidized and metabolized.

For this reason, food is a potential reserve of energy expressed as a kilocalorie (kcal) or calorie (cal). In other international systems, kilojoule or joule measurements are also used.

The equivalent of 1 calorie is 4.19 joules.

What are the sources of calories in food and what data do we use to calculate their contribution?

Not all nutrients in food are capable of generating calories. Only the macronutrients – fats, proteins and carbohydrates – produce chemical energy when oxidized.

Alcohol, although not considered part of food, also produces energy, but without transporting nutrients. That’s why we talk about empty calories then.

Alcohol is a substance that provides empty calories.

How much does each nutrient provide?

Scientist Wilbur Atwater calculated the energy input of each macronutrient by measuring the rise in temperature of the water surrounding a special bomb calorimeter. These values ​​have been used to calculate calories produced by food for more than a century.

Based on the Atwater method we obtain the following:

  • 1 gram of fat produces 9 kcal.
  • 1 gram of carbohydrates or proteins provide 4 kcal.
  • 1 gram of alcohol or ethanol produces 7 kcal.

The main caloric sources in the diet are carbohydrates and fats, since proteins must be used for other purposes and not to be oxidized as a fuel source.

In extreme cases of protein consumption for energy production, our muscle mass would be consumed, hair and nails would not grow and the skin would become flaccid, among other deteriorations.

How many calories do we need to eat with food?

The number of calories we need per day is known as overall energy expenditure. It represents the sum of several factors that vary in each one:

  • Resting energy expenditure: It is the largest part of the daily energy expenditure, which varies between 50 to 75% of the total. It is obtained from the basal energy expenditure to maintain organic functions, which depends on age, gender, menstrual cycle, pregnancy or lactation status, among others.
  • Thermic effect of food: is the caloric expenditure that occurs during digestion, metabolism and utilization of nutrients. This factor requires 5-10% of all daily calories.
  • Physical activity: It considerably increases the caloric requirement per day. For this reason, it is the main ally of hypocaloric diets to lose weight. The cost ranges from 15 to 30%.


How to calculate the calories contained in food?

Food manufacturers that have to report calories in food They use the traditional bomb calorimeter method. Directly evaluating the heat production of a food and, therefore, its energy contribution.

But not all of us have calorimeters in our homes. That is why we must use other indirect methods.

1. Determining total calories from the Atwater method

Most of the food is sold packaged and almost all of them have nutritional information on their labels. There appears the content of calories and nutrients per serving or per 100 grams of the product.

You can check the calories expressed on the label and if you want to be more sure, apply the foundation of the Atwater method. For this you must review the amount of carbohydrates, protein and fat that the portion or 100 grams provides.

For example, the container of whole milk powder reports that 100 grams contains 25.8 grams of protein, 26.7 grams of fat, and 38 grams of carbohydrates.

The step-by-step procedure would be as follows:

  1. Multiply each macronutrient by its caloric equivalent. The grams of protein and carbohydrates are multiplied by 4 kcal and fat by 9 kcal. For the example milk protein powder it would be 25.8 grams per 4 kcal; for fat 26.7 grams per 9 kcal and for carbohydrates 38 grams per 4 kcal.
  2. By multiplying we obtain that 100 grams of milk provide 103 kcal from protein, 240 kcal from fat and 152 kcal from carbohydrates.
  3. Now add up each individual count. 103+240+152=495 kcal. This amount should match the caloric intake listed on the label.
  4. Take into account the size of the portion that you are going to use. The amount of macronutrients and calories will depend on this. For example, if instead of 100 grams of milk you need to use 120 grams, the macronutrient values ​​will be different.
  5. Apply a rule of three. Knowing that 100 grams of milk contains 25.8 grams of protein, multiply 120 x 25.8 and then divide by 100. You do the same for fats and carbohydrates.
  6. Repeat steps 1 to 3. With the new macronutrient values, now find the caloric intake of the 120 gram serving you will consume.

2. Use the most popular food composition guides

As an alternative to the information shown on the label, there are guides that document the nutritional value of the most common foods. Each country analyzes those of its own gastronomy and those most used in the region.

A reliable guide, for example, is that of the Handbook of the Nutritional Value of Food in Common Units and the Nutritive Values ​​of Foods from the USDA.

In this type of guide you have to specify the size of the portion: ounces, grams or cups. In addition, the foods are listed in alphabetical order or grouped into sections: fruits, cereals, meats, among others.

In the case of any homemade preparation that does not appear, you must calculate the calories provided by each main and secondary ingredient, such as oil or ground bread, for example. With pencil and paper you can apply the Atwater method.

Another detail to consider in case of using these guides is select the foods that most resemble the one of interest. If it is breast with skin or without skin, its caloric value is different, for example.

The other point is the variety, especially in the case of vegetables and fruits, cheeses, nuts, meats and fish. Some packages also have subgroups, such as low or high fat, canned in oil, tomato or water, whole or refined products.

When extracting the information of the amount of carbohydrates, make sure to take into account the available ones. The fiber must be subtracted from the total carbohydrates, since as it is not digested, it does not provide calories.

We get a quick look at the calories per serving on the labels, which we can then check with other methods.

3. Use a virtual calculator

There are virtual calorie calculators to have the nutritional information of food. The United States Department of Agriculture contains information on more than 6,000 foods. There they record the caloric intake, macronutrients and micronutrients.

On the other hand, the calculator WebMD it only offers the number of calories and the serving size. There are others that provide information on fresh foods and meals prepared in restaurants.

4. Download some app to your phone

To make the process a little easier, some apps allow you to calculate the calories of food. Even count the calories of what you eat a day to know if you are within the energy requirement you need.

Other mobile applications modify and adjust calories according to the level of physical activity. While there are also those that allow you to make adjustments to increase or decrease calories and even distribute nutrients.

It is very easy to use these applications. You just have to enter age, sex, weight, height, goal and the foods you eat during the day.

How accurate are these methods for calculating food calories?

Although the calorimeter gives reliable results, there is a wide variety of foods in each group and the metabolism of each nutrient is individualized. The Atwater method does not take into account the process of digestion and metabolization of each nutrient. This makes caloric calculations somewhat imprecise.

In this sense, a group of experts determined the calories provided by pistachios using a biological method of metabolizable energy through intake, absorption and excretion. They showed that these nuts offer 5% fewer calories than the Atwater method reports.

That is why We shouldn’t base our diet and food choices solely on calories from a calculator. Metabolic individualities will determine the net amount of calories and nutrients that the body will use.

Therefore, any diet that we carry out must go through the expert hands of a nutrition professional. Including its evolution over time.

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