Not all calories are the same

Geschreven door Nathan Albers

Geschatte leestijd: 4 minutenA lot of calories are counted. It is the most well-known method for assessing the impact of food on weight. However, you may wonder how much the number of calories actually says about the amount of energy you consume.

Calories
Thermogenesis by Food
Paying Tribute to Microbes
Processed Food vs. Unprocessed Food
Personal Differences
Not All Calories Are the Same

Calories

The concept of calories was originally devised to indicate how much work capacity certain foods provided. For fats, it was estimated that they yielded 9 (kilo) calories per gram, and 4 kcal for each gram of carbohydrates and proteins. Additionally, fibers are sometimes separately calculated at 2 kcal per gram. These are figures known to many, but unfortunately, they can be misleading.

There’s a reason we list “of which sugars” on packaging under carbohydrates. A useful distinction that says a little more about the extent and manner in which the body absorbs energy from them. However, the carrier of those nutrients, the type of food in which they are packaged, also affects the amount of energy yielded. Today, I wrote about nuts and weight, and the fact that not all energy from almonds and pistachios, for example, is absorbed by the body. The number of calories they contain, as indicated on packaging, is higher than the amount you actually absorb [1]. This difference was almost 25%. And there are more exceptions like this. The calculations for the amount of energy that food yields based on the mentioned macronutrients are averages of different types of food.

Furthermore, within one type of food, the absorbed amount of energy can vary depending on the method of preparation. Cooking vegetables, for example, breaks down the cell walls, making the energy more readily available and absorbed. This also applies, for example, to the starch in sweet potatoes [2]. Hard-boiled potatoes therefore provide fewer calories than soft-boiled ones. However, in some vegetables, the cell walls remain intact, allowing them to withstand digestion to a greater extent. In that case, fewer calories are absorbed.

Thermogenesis by Food

Diet-induced thermogenesis is the energy lost when digesting food due to the heat generated in the process. This can be seen as the relative effort the body makes to process food and an indication of the actual amount of energy the food yields.

Protein (but also alcohol) cause a higher release of heat than carbohydrates and fats [3]. Therefore, the same amount of protein effectively yields less energy (because more is lost) than fats. Despite the fact that they both yield 4 kcal per gram. Protein also provides a greater feeling of satiety, but here I will mainly focus on the energy provided.

Paying Tribute to Microbes

Then there is the way different healthy foods are processed, or rather where they are processed. Most food is ‘available’ to the extent that it is already completely absorbed in the small intestine. More complex foods like fibers, but also almonds and cassava, travel to the large intestine where they are processed by microbes that convert them into fatty acids (and thus available energy). However, these microbes also use energy for this process. Energy that you don’t receive, but would expect based on the number of kcal per gram.

In addition, the use of the immune system also consumes energy, depending on possible pathogens in the type of food. This also involves higher thermogenesis and therefore energy consumption [4]. However, it is not clear how much energy is involved in this. Let alone that this is taken into account in food tables.

Processed Food vs. Unprocessed Food

Also influencing is the difference between processed and unprocessed food. In a 2010 study, people were given 600kcal or 800kcal portions of whole grain bread and cheddar or the same amount (in kcal) of white bread and processed cheese [5]. The unprocessed food required twice as much energy to process. In this way, whole grain bread provided 10% fewer calories.

Personal Differences

Then there are of course the personal differences in how the body deals with nutrients. For example, did you know there can be significant differences in the length of the intestines of different people [6]? In a 2002 study of 100 men and 100 women (post-mortem), this varied from 5 meters to over 13 meters! Moreover, there was a correlation with weight. Not very surprising given the above explanation of the role of the intestines. Longer intestines provide more opportunities for nutrient absorption.

Also, the microbes in those intestines can differ. For example, some Japanese population groups have microbes that have acquired genes from sea bacteria. These genes help them break down seaweed. This may mean that the same portion of sushi yields more calories for them than for the “average” Dutch person, for example.