Many people check the calorie content on food labels to manage their energy intake and prevent weight gain. Calorie counts, however, are often inaccurate. The calories on a label can differ considerably from the energy you actually absorb. How does that happen?
The Atwater factors
Food provides energy and building blocks for our body. Digestive enzymes break down complex nutrients into sugars, amino acids and fatty acids, which travel through the bloodstream to our cells. There, the energy from chemical bonds is used to keep the body running.
In the 19th century, Wilbur O. Atwater developed a system to calculate the average calories of fat, protein and carbohydrates. Since then, food calories have been expressed in kilocalories (kcal):
Fats: ~9 kcal/gram
Proteins: ~4 kcal/gram
Carbohydrates: ~4 kcal/gram
Fibre: ~2 kcal/gram, because it is harder to digest
But new studies show that this approach is far too simplistic.
Atwater took into account energy loss through stool, but not what happens to food in your stomach and intestines. An example: the heat of combustion of 1 gram of protein is 4 kcal, but of the proteins we eat only part is converted into calories. We also reuse part of it to make other proteins, such as muscles, enzymes, and so on. In other words: if we eat 20 grams of protein, no 80 (20 x 4) kcal are generated, but only part of it. Exactly how much depends on many factors and even differs from one individual to another. As a result, the calories on labels are often overestimated.
Factors that influence actual calorie absorption
To accurately calculate the total calories someone obtains from a particular food, a large number of factors must be taken into account. We will go through the most important ones.
Digestibility of food
The cell walls of plants influence how much energy we extract from them. Cooking breaks down cells, making calories more available. Harder seeds and nuts are less digestible, so our body absorbs less energy.
Example: 100 grams of almonds contains 170 kcal on paper, but in practice people absorb on average only 129 kcal. A difference of 25%.
Preparation method of food
The way food is prepared has an enormous influence on the energy absorbed. Cooking, grinding or fermenting makes nutrients easier to digest. Research with mice and humans shows that cooked or processed food often provides more calories than raw food. Processed foods are absorbed faster and increase calorie intake with less effort from the body.
Individual metabolism
Everyone has a unique metabolism, influenced by genes, gut bacteria and enzymes. This explains why two people process the same meal differently. Even if two people eat the same sweet potato or the same piece of meat, prepared in the same way, they do not get the same number of calories from it. People differ in almost every characteristic, including inconspicuous traits such as the length of the intestines. The longer the small intestine and large intestine (colon), the more nutrients can be absorbed. People also differ in the specific enzymes they produce. Some people do not produce the enzyme lactase, which is needed to break down lactose sugars in milk. The result is that one person drinks a calorie-rich latte and converts the lactose into energy, while the other develops diarrhoea.
The role of the microbiome
Your intestines are home to trillions of micro-organisms that determine how many calories you absorb from food. Some bacteria help to absorb calories more efficiently, while others limit this.
Slim people often have a different gut flora than people who are overweight, with the latter more often dominated by Firmicutes. Those bacteria make food more efficiently digestible, so that more nutrients end up in the bloodstream and excess energy is stored as fat. Moreover, some microbes occur only in specific populations; for instance, some Japanese people have a gut bacterium that can break down seaweed better.
Modern diets, rich in easily digestible processed foods, can reduce the populations of these beneficial bacteria. This makes it harder to make use of calories from fibre-rich, tough food such as celery.
The type of food
The type of food also influences how much you eat. A 2019 study in Cell Metabolism showed that people who ate ultra-processed food consumed on average 500 calories more per day than when they ate unprocessed food – even though both meals contained the same number of calories and comparable nutrients.
Variation in raw materials
Even if a product is made according to the same recipe, the calories of the ingredients can vary due to factors such as ripeness and time of harvest. Manufacturers therefore use average values, which is entirely understandable. Legally, margins are permitted: for fat content, for example, a deviation of 20% on 10–40 grams per 100 grams. So usually count on a margin of 10% for the calorie content; if it says 200 kcal/100 grams, then in reality this can lie between 180 and 220 kcal.
Conclusions
The calorie content on labels usually gives a distorted picture of the energy the body absorbs.
Calorie counting is never completely accurate, due to the complex interaction between nutrition, digestion and microbes.
Focus on the quality of your food instead of calories alone: unprocessed vegetables, nuts and whole grains provide energy in a healthy way, while ultra-processed food gives calories quickly with little nutritional value.
Healthy choices and a varied diet support a healthy weight and an efficient use of energy.
Bron: Dr.ir. Eric De Maerteleire — auteur: Sofie Van Rossom, gezondheidsjournalist