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Energy - or what exactly are "calories"?

Our body consumes energy all day long - for every movement and for all life-sustaining functions such as breathing, the cardiovascular system or metabolism. It draws this energy from the food we eat every day. Our food consists primarily of three groups of nutrients: Carbohydrates, proteins (proteins) and fats. They are also called macronutrients because they are present in large quantities in the diet.

Another energy source is alcohol. Dietary fiber can also provide energy, but its contribution is very small. The ingested nutrients are broken down in the digestive tract, transported to the body cells and broken down there. Within this process, the required energy is released.[1]

Calories and kilojoules as a unit of measurement for energy

Some of us count calories meticulously. Others roll their eyes. Calories are not only on everyone's lips, they are also declared on many food packages, and we encounter them in diet guides and on YouTube. The term is so familiar to most of us that hardly anyone wonders what it means.

"Calorie" is an obsolete unit of measurement, but it has become colloquially accepted. It is misleadingly an abbreviation for kilocalories (kcal). Basically, a calorie describes the energy needed to heat 1 gram of water to 1 degree Celsius. More precisely, from 14.5 to 15.5 degrees Celsius.[2]

The energy in food is also called calorific value and is measured in the units kilocalories (kcal) or kilojoules (kJ). One kilocalorie is equal to 4.184 kilojoules and can be converted into each other: 1 kcal is equal to about 4.2 kJ, 10 kJ is equal to about 2.4 kcal.[3]

The energy requirement consists of the resting energy consumption and the energy consumption for physical activity. The amount of energy that a person converts during the day in a state of complete rest is referred to as the basal metabolic rate. A person's basal metabolic rate depends largely on his or her age, physical and mental activity, and physical condition. The organ with the highest basal metabolic rate is the human brain.

How much energy do we need?

The largest part of the energy supply is required for resting energy consumption during normal stress. This includes routine work of the body, such as breathing, cardiac activity, metabolism and maintaining a constant body temperature.

The nutrients in our food have the following energy content:[4]

• Protein 4 kcal/g
• Carbohydrates 4 kcal/g
• Fat 9 kcal/g

How much food and calories we need depends on various criteria: Age, gender, body weight, the body size, body muscle mass, occupation and leisure activities - each of us is different. Hormone balance also has an effect on the amount of energy needed.

So how much energy we should consume through food depends entirely on who we are and what we do. Nevertheless, there are guideline values for our daily calorie requirements. These are based on the assumption that the average person spends most of their working day sitting down and their free time doing rather passive activities. The more physically active we are, the higher the energy requirement.

It is internationally recognized to express energy expenditure, the so-called PAL value (PAL = physical activity level), as a multiple of the resting energy expenditure. Depending on the degree of muscular activity in work and leisure, the PAL value can range from 1.2 to 2.4. For working adults with a predominantly sedentary job and rather less physical activity during leisure time, the PAL value averages 1.4 to 1.5.[5]

Guideline values for energy intake per day for adults:[6]

However, these are calculated average values that do not have to be adhered to exactly to the calorie. Even minor deviations from the norm, such as less physical activity or being overweight, change the individual energy consumption.

For adults, the DGE uses a mean body mass index (BMI) of 22, which corresponds to a daily energy intake of 2,300 kcal for men and 1,800 kcal for women aged 25 to 50 years with low physical activity.[7]

Calorie is not equal to calorie

And quantity does not equal quality: just because we eat the recommended amount of food does not mean that we are full. If our diet contains too few nutrients, we will still feel hungry.

It makes a difference whether we eat gummy bears or wholemeal bread for the same number of calories, for example. The reason: sugar is absorbed into the blood much faster than carbohydrates from wholemeal bread. As a result, the body releases more insulin - which in turn transports the sugar from the blood into the body cells particularly quickly.

Our body stores excess sugar: large amounts of sugar mean large amounts of insulin. A rapid rise in blood sugar is followed by a rapid drop in blood sugar, which in turn means that we get hungry again. With wholemeal bread, on the other hand, the starch is broken down slowly and gradually transported into the blood. The blood sugar level remains constant for longer and we don't get hungry again so quickly. The risk of consuming too many calories is therefore lower.

Anyone who approaches the topic of nutrition and calories today often avoids fat without knowing that fat fulfills important functions in our body. Fat is a carrier of flavor and taste and of the fat-soluble vitamins A, D, E and K. So going without is not an option: rather, calorie intake depends on the right amount of nutrients, vitamins and fluid.[8]

How many calories are in which food?

Calorie tables can provide an orientation as to which food supplies how much energy. However, the information is not exact - also because foods are not always the same. Their composition changes, for example in the case of fruit and vegetables, depending on the variety, weather, soil conditions or degree of ripeness.[9]

Foods with a high water content, such as fruits and vegetables, tend to provide less energy, so they have a low energy density. This means that they have less energy per gram than other foods. Foods that are very high in fiber also usually contain comparatively few calories: These include, for example, whole-grain bread or brown rice, or legumes such as peas, beans or lentils. The energy density of foods can also be influenced. If a food is dried, like a raisin, then the energy density increases. 100 grams of grapes have about 70 kcal, while their dried version, raisins, have about 300 kcal per 100 grams.[10]

Very fatty foods, on the other hand, usually contain a lot of calories. In particular, foods with a combination of fat and sugar, for example confectionery such as chocolate or cakes, have a high energy density. This means that these foods contain a lot of calories in relation to a fixed quantity. 

Foods that are very starchy and contain little fiber can also have a high energy density, such as husked rice or white bread. In principle, very water-rich foods have a low energy density. Foods high in fiber and protein, such as whole grains, legumes, and lean meats, have a medium energy density.[11]

[1] Nutrients - building blocks for your health. German Nutrition Society, 6th edition, 2022

[2] Pschyrembel Online | Calorie

[3] Nutrients - building blocks for your health. Deutsche Gesellschaft für Ernährung e.V., 6th edition, 2022

[4] German Nutrition Society - DGE e.V. (2015): Energy. www.dge.de (last call: 17.12.2022)

[5] Nutrients - building blocks for your health. Deutsche Gesellschaft für Ernährung e.V., 6th edition, 2022

[6] Nutrients - building blocks for your health. Deutsche Gesellschaft für Ernährung e.V., 6th edition, 2022

[7] This is how many calories humans need (aerzteblatt.de)

[8] Fat | HDL | LDL | Cholesterol | Blood lipids | Lipid metabolism disorders - ernaehrung.de

[9] Urs Schaden. The Diet Doc. Find your perfect weight loss strategy! Riva Publishers (28 December 2019)

[10] I. Elmadfa, E. Muskat, D. Fritzsche, A. Meyer. The big GU nutritional value calorie table. Gräfe und Unzer Verlag GmbH; 3rd edition (August 3, 2021).

[11] I. Elmadfa, E. Muskat, D. Fritzsche, A. Meyer. The big GU nutritional value calorie table. Gräfe und Unzer Verlag GmbH; 3rd edition (August 3, 2021).