Nutritional value and energy


The nutritional value of a food or drink item determines how healthy it is and what its components are. The value is determined by the amount and proportion of nutrients present. It is important that the food you eat provides the nutrients your body requires, as the body itself cannot make most of the nutrients. Nutrients can roughly be divided into two categories: those that provide energy (e.g. proteins, carbohydrates, fats and alcohol) and those that don't however are essential for normal body functioning (e.g. vitamins and minerals).

The impact of calories on our body

No life without energy! Energy can be defined as the capacity of a physical system (such as our body) to perform work. The harder we work and the heavier our workload, the more energy we need. Nevertheless our body needs most of its energy in the form of calories simply in order to function properly. Even when we are asleep, our body uses up to 25 percent of its total energy consumption, since it must continually work to maintain our body temperature at 98.6° Fahrenheit, and to maintain the function of our heart, lungs, gastrointestinal system and brain.

The amount of daily energy we need at rest is called the basal metabolic rate (BMR). BMR depends on a number of general factors such as weight, height, age and sex, as well as on more individual factors such as levels of stress, hormonal balance and illness. Research has shown that men have a slightly higher BMR than women. This is basically due to the fact that women have higher body fat percentages and less muscle mass. As fat is less active than muscle and therefore burns less energy, women have a lower BMR. The same applies to people who are overweight. Their bodies consume relatively less energy than those of people with low or normal body weight. The BMR of an average man that weighs 165 lbs is somewhere between 1500 and 1800 kilocalories, whereas the BMR of an average woman that weighs 140 lbs is between 1300 and 1500 kilocalories per day.

Digestion has a major impact on basal metabolic rate. When we eat, we activate digestion and increase the basal metabolic rate. When our body digests fat, our BMR is increased by approximately 4%. Digestion of carbohydrates increases our BMR with 6% and that of proteins with up to 25 to 30%! The latter can be explained by the complex structure of proteins. Because they are complex molecules, the body takes much longer to break them down.

If we follow a balanced diet with a normal proportion between proteins, carbohydrates and fats, our digestion will increase the BMR by approximately 10%.

Another 10% of energy is lost in feces because some substances are not completely digested when our body expels them. We can differentiate between internal energy – the energy needed for normal body functioning – and external energy: the energy we consume for all other activities such as work, play, household activities, sports, …

A lot of research has been done in the area of energy consumption during physical activities. With the help of a so-called calorimeter you can easily measure the amount of calories you burn with a specific activity. For example: an average man of 165 lbs burns 95 kilocalories an hour while sitting or standing, whereas an average woman of 145 lbs burns only 75 kilocalories with the same activity. The same man burns approximately 150 kilocalories an hour getting dressed and 480 kilocalories walking stairs, whereas our average woman respectively burns 100 and 420 kilocalories an hour with those activities.

Kilocalories and kilojoules

All processes in our bodies, along with all organs and systems, need fuel to enable them to function. The fuel our body uses comes from the food and drink we consume. Carbohydrates, fats and proteins are the most important nutritive substances that deliver energy. They contain chemically bound energy that is released during respiration. However, not all ‘fuel’ delivers the same amount of energy. Using a calorimeter (a device to determine the calorific value) in a lab setup, burning carbohydrates releases 4,2 kilocalories of energy. Proteins and fat respectively burn 5,6 and 9,4 kilocalories. Our body breaks down nutrients during digestion. The amount of energy (or calorific value) that is being released during digestion, is expressed in kilojoules (kJ)* or kilocalories (kcal). Please find below the calorific value for different nutrients:

  • One gram of carbohydrates delivers on average 4 kcal (17 kJoule) of energy
  • One gram of proteins delivers on average 4 kcal (17 kJoule) of energy
  • One gram of fat delivers on average 9 kcal (37 kJoule) of energy
  • One gram of alcohol delivers on average 7 kcal (29 kJoule) of energy
  • One gram of fibers delivers on average 2 kcal (8 kJoule) of energy

*) Although the use of kilojoules to express energy is recommended by international convention since 1978, the concept of ‘calories’ is so widespread that most people prefer to use that term. To convert joules into calories, you have to take into account that 1 kilocalorie equals 4,184 kilojoules.