Many experts still believe that “one calorie is one calorie.” If we apply this idea to weight control, we have to only be concerned with the number of calories in our diet, regardless of what different foods we eat or how we eat them. Of course, the human body is not that simple, and many researchers have warned against this kind of thinking.

In explaining this, I thought I should make a distinction between reactions that occur "inside" the body (after absorption) and those that occur “outside" the body (before absorption) (*1).
In this article, I would like to consider the Atwater coefficient, which is the basis for calorie labels on food products, as an issue that occurs outside the body. 

By feeding volunteers various foods and calculating the difference in the heat of combustion between the food and the excreta, he approximated the calories absorbed by his volunteers. It is said that Atwater also took into account the dietary fiber which we can’t digest (*2), and proteins, some of which are excreted in the urine as urea after being absorbed. 

More than one-hundred-twenty years after this experiment, these "Atwater coefficient" are still the basis for caloric calculations for all foods[1].

( *2: It is now known that dietary fiber produces some energy through fermentation and breakdown by  the bacteria in the large intestine[2].)

We ingest food and then break down complex food molecules into simple structures such as glucose and amino acids by various digestive enzymes. After that, we get an energy source by absorbing them. Naturally, this is a completely different process compared to burning food in the laboratory. 

   
According to Professor Rob Dunn (North Carolina State University), every calorie count on food labels is based on estimates or approximations, and are not an accurate reflection.

Recent research has revealed that how many calories we get from a given food depends on a variety of factors, including which species we eat, how we cook food, which bacteria are in our gut, and how much energy we use to digest different foods (diet-induced heat production).[4]
   

A study by Janet A. Novotny at the U.S. Department of Agriculture (2012) found that when people eat almonds, they take in just 129 kcal per serving, not the 170 kcal listed on the label. It is beginning to be proven that nuts such as peanuts, almonds, and walnuts have a more robust cellular structure than other foods with similar levels of energy sources, and that their cell walls limit digestion. It is possible that the Atwater coefficient overestimates the digestibility of nuts.[5]
    

Furthermore, some have pointed out that industrial food processing not only exposes food to high temperatures and pressures, but also softens food by adding air, to make it even easier to get more calories.

Corn, for instance, which is considered indigestible, is made into potage, and raw peanuts are roasted and processed into peanut butter. These processes must have dramatically increased the amount of energy available to the body. In other words, not all pork dishes are the same. The energy used for digestion and the absorption rate differ when roasting a chunk of pork versus making it into pâté.
   

Some research has shown that the energy required for digestion is not the same. This is called “diet-induced thermogenesis,” and it requires a great deal of energy to convert proteins into amino acids, fats into fatty acids, and carbohydrates into glucose. It is said that when proteins are broken down into amino acids, they require much more energy to digest than fats, because enzymes must unravel their tightly wound bonds[6].

It also differs between whole grains and refined wheat. A 2010 study found that people who ate 600-or 800-kcal portions of whole-wheat bread with sunflower seeds, kernels of grain, and cheddar cheese, expended twice as much energy to digest that food as those who ate the same amount of white bread and "processed cheese products.” Consequently, those who ate whole-wheat bread substantially obtained ten percent fewer calories, they said[7].

Most babies have lactase, an enzyme necessary to break down lactose sugar in milk, but it is said that most adults don’t produce this enzyme.

It has also been found that when starches such as rice and spaghetti are left to cool after being cooked, some of these starches crystallize into structures that digestive enzymes cannot easily break down.

What’s more, some microbes are present only in specific ethnic groups. Some Japanese, for example, have a microbe in their intestines which is suitable for breaking down seaweed. It has been found that these intestinal bacterium stole the seaweed-digesting genes from a marine bacterium that lingered in raw seaweed [4].
    

The general Atwater coefficients were calculated based on the average daily diet of Americans at the time. Digestibility for carbohydrates, fats, and proteins were assumed to be 97, 95, and 92 percent respectively, and after adjusting a little for this, protein and carbohydrates were set at 4kcal/g, fats at 9kcal/g, and alcohol at 7kcal/g[8]. Although metabolizable energy values vary slightly for proteins, depending on whether they are vegetable or animal protein, and for carbohydrates, depending on whether they are sugar or starch, the coefficients were derived by a system of an average.

On the other hand, specific Atwater coefficients are also allowed, which divides food into several groups and applies a representative coefficient of that group to the entire group.

It is said that the U.S. Food and Drug Administration (FDA) allows a total of five variations on the theme including these, and some point out that depending on the method chosen by the food company, there may be variation on the calorie labels[9]. When such uncertainties add up, the daily caloric intake could vary widely.
    

Prof. Dunn mentions as follow:

(1) It is possible to modify the Atwater system for every food group, as in the almond example. However, this would require a challenge to reexamine the amount of nutrients retained in the excreta for every food. 

(2) However, even if we completely revised calorie counts, they would never be accurate because how many calories we extract from food depends on a complex interaction between food, the human body, and its many microbes. In particular, the process of digestion is so complex that it is probably impossible to derive an accurate formula for calorie calculation that will suit everyone.

(3) Instead, we should think more carefully about the energy we get from food in the context of human biology. Processed foods are easily digested in the stomach and intestines, and thus provide a lot of energy for very little work. On the other hand, vegetables, nuts, and whole grains require more sweat to digest, offer far more vitamins and nutrients than processed foods, and keep our gut bacteria happy[4].
    

I believe that researchers and nutritionists at that time, including Atwater, were committed to ensuring that people could have an adequate amount of nutrition, and the calorie counting system they created had great merit. But I suspect it has been misperceived by some and is now causing problems with those who are overweight.
The reason why the issue of being overweight remains unresolved indefinitely is because many people are too fixated on the caloric value of foods.

Let's say you eat, as in the example in the section 2-[III], 400kcal of a meal: whole-wheat bread, nuts, and grilled chicken. Assuming that, after taking into account the energy required for digestion, you obtained ten percent fewer calories (360kcal), the argument that "wouldn't it be the same if you ate 360 kcal worth of white bread and chicken terrine?" is complete nonsense.

I I have been explaining throughout this blog that the fundamental difference between obese people and lean people can be explained by the difference in set-point weight, and that having a higher set-point weight is related to an increase in absorption efficiency, which is induced by intestinal starvation.  In addition, since one of the key factors causing intestinal starvation is how fast you digest food, both digestion and absorption ability are extremely important in my theory.

      
Nevertheless, if we believe that only numbers based on averages of subjects are all we have, we ignore them. As Prof. Dunn mentions, I don’t believe that the complexity of digestion and absorption for a diverse population can be described by a system using an average.

(Please read the following article for other issues in "calorie counting.")

There is No Meaning in Simply Calculating Calories You Consume

Those foods contain far more vitamins and minerals than processed foods, and their fiber content keeps our gut bacteria in good condition, gives us moderate satiety, prevents rapid absorption of glucose, and provides many other health benefits. Depending on how you eat them, it should be possible to lose weight without worrying about caloric intake.