Why a calorie IS NOT “just a calorie”


Everyone who has ever tried to count the calories in order to lose some weight knows how messy and difficult that can be. Things can go pretty well during the morning when counting the calories of two fried eggs and a slice of bread, but as soon as you start to prepare a dinner or want to eat outside, things get pretty fussy. Even a simple meal like a tomato soup can become difficult when you realize your soup is cooked with a homemade pasta.

What is a calorie?

By definition, a calorie is the amount of heat needed to raise the temperature of one gram of water one degree Celsius (likewise amount of energy needed to raise the temperature of 1 kilogram of water 1 degree Celsius is one kilocalorie). Sometimes scientists use Joules to describe the amount of potential energy in the food we eat (1 cal = 4.19 Joules).

How the calories in foods are calculated?

Because of different regulations in different parts of the world, food manufacturers are allowed to use different methods to estimate the number of calories in food. In the European Union, the energy conversion factors are specified in the Codex Alimentarius, while the USA legislation allows any one of five methods to calculate the energy content of foods. The most popular method of estimating calories is the “4-4-9” method developed by professor W.O. Atwater. The agricultural chemist Wilbur O. Atwater in order to determinate the energy content of food burned food like potato or corn in a device called “bomb calorimeter”. A bomb calorimeter measures a rise in water temperature when a sample of food in a sealed vessel is combusted. This rise in the water temperature allows scientists to calculate calorie (energy) content of food.


But this method is not frequently used today. Instead, companies usually calculate the calories on the labels with the Atwater General Factor System even though the system is over 100 years old.

The Atwater General Factor System

The system uses a single energy value (factor) for each of the main nutrients (protein, fat, and carbohydrate), regardless of the food in which it is found. The energy values/factors are estimated as follows: one gram of protein contains 4 kcal, one gram of carbohydrate contains 4 kcal, one gram of fat contains 9 kcal, one gram of alcohol contains 7 kcal. The factors are calculated based on the average heats of combustion of each nutrient and corrected for losses in food digestion, absorption, and urinary excretion.

The total caloric value of certain food is calculated by adding up the calories provided by all nutrients.

The extensive general factor system

The commonly eaten foods contain a mixture of fermentable and non-fermentable fiber. Non-fermentable fiber does not provide any energy and it is assumed that in the commonly eaten food 70 percent of dietary fiber is fermentable. In order to separate total carbohydrate into available carbohydrate and fiber the extensive general factor system was created. With this system, the dietary fiber component of food is usually subtracted from the total carbohydrate before calculating the total amount of calories because some type of fiber can not be digested.

The Atwater specific factor system

After 50 years of research in 1955, the Atwater specific factor system was introduced. This system derives different factors for proteins, fats and carbohydrates, depending on the foods in which they are found. It’s been discovered that energy absorption from protein varies. Typical absorption from animal protein is higher than the general Atwater factor- 4.36 kcal/g of protein in eggs — and lower from most vegetables — generally 2.44 kcal/g of protein. Similar, for total carbohydrate the absorption factor may vary from 2.70 kcal/g in lemon and lime juices to 4.16 kcal/g in polished rice.

The net metabolizable energy (NME) factors.

All three of the systems discussed above have considered the ME (metabolizable energy), the amount of energy measured as heat after complete combustion of food in a bomb calorimeter and corrected for losses in digestion, absorption, energy lost from the body surface and urinary excretion.

Metabolizable energy values were modified further to account for the energy that is lost as heat and obligatory thermogenesis, i.e. energy that would not be available for the production of ATP (adenosine triphosphate) in the body. This resulted in the net metabolizable energy (NME) factors. So the ME describes the food energy available for heat production and body gains. NME deducts obligatory thermogenesis from ME in an attempt to indicate the food energy that is converted to ATP energy within the body. The NME system retains a general factor approach, i.e. a single factor for each nutrient. The NME factors are

  • protein 3.2 kcal/gram
  • fat 8.9 kcal/gram
  • carbohydrates (monosaccharides) 3.8 kcal/gram
  • fermentable dietary fibre 1.9 kcal/gram
  • Non-fermentable dietary fiber 0 kcal/gram
  • alcohol 6.3 kcal/gram

Notice the difference of the NME factor for protein 3.2 kcal/g versus the Atwater general factor 4.0 kcal/g. That presents a 24 percent decrease in energy from protein. Also, notice the lower NME values for dietary fiber and alcohol. It is assumed the energy loss through the heat of dietary fiber fermentation is higher than participated with the Atwater general factor system and lower NME value of alcohol is due to thermogenesis following alcohol consumption.

The mix-up factor”

Which of the above mentioned food energy conversion factors will be used to estimate calories in foods depends on the national regulations. Sometimes the energy content of foods may be calculated in a different way within a single database. For example, Codex Alimentarius uses the Atwater general factors with additional factors for alcohol and organic acids. So the accuracy of reported calories per serving that appear on food labels is in general very questionable.

Other factors involved

Besides inaccuracy in the estimation of food calorie count, there are other factors involved in the nutrient and energy estimation of food.

1.If you Google the calorie content of tomato (100 g), you will find 100 grams of tomato has 18 calories, 0.2 g of fat, 3.9 g of carbs and 0.9g of protein.plate-403597_640But you should know the fact that the calorie counts in databases are averages. Research shows that the true calorie content of food is often significantly higher or lower. For example, soil and growing conditions influence the nutrient and energy content of the above mentioned tomato. The same applies to milk/eggs/meat: the nutrient/energy content of the food depends on what the animal ate and how it lived.As mentioned before, we don’t absorb all the calories/nutrients we consume with our food. Foods are not completely digested and absorbed. Some of the food energy is lost in the faeces. Likewise, compounds derived from the incomplete catabolism of protein are lost in the urine.

2.Some food has fewer calories than previously anticipated. For example, the study published in the British Journal of Nutrition in 2012 showed pistachios contain less energy than that calculated from the Atwater general factors.

3.Cooking and preparation change the amount of energy and the nutrient in foods. Cooking usually makes more of the calories available for absorption, and food labels don’t always reflect that.


4.Each man digests and absorbs food differently. Many factors determine nutrients’ digestion and absorption in the body. One of them is the gut flora: the composition of the microbiota has been shown to differ between lean and obese individuals. Other factors that may affect the individual calorie absorption are hormones, genes, sleep, the amount of brown fat tissues, etc.

5.Even if you decide to count the calories every day, it is good to know that studies proved people are generally quite bad at estimating caloric intake. Research showed even trained nutritionists can underestimate calories in food by an average of 30 percent.


Should we ignore the calorie count that appears on food labels??

But despite all facts, awareness of the calorie count in foods has its uses. I think the information listed on a label can give you a good idea of food you are about to eat. Although counting calories for weight loss is not an exact science, the general principle still applies for most of us. If you take in more calories than you expand, you will gain weight. Likewise, if you take in fewer calories than you expend, you will lose weight. It is important to find balance. Knowing exactly in number how many calories you ate isn’t a “must”. Watching for portions size and adopting healthy eating habits can lead you to your goal.


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