Calorie Values and Energy Intake From Foods

In this article, we show you how energy is provided to the body and why a balanced energy intake is important.

Calorific Values of Nutrients

In principle, the intake of food has two functions: to provide energy and to make (bio-) chemical building blocks available.

Focussing on energy production, calories (cal), and the nowadays mainly used joules (J) are the relevant measurement units. Technically, a calorie is the amount of heat that is needed to raise the temperature of one gram of water from e.g. 14,5°C to 15,5°C (normal pressure at sea level).

1000 cal or 1 kcal is roughly the amount of energy that is burned by a sleeping 75kg person per minute. One calorie corresponds approximately to 4.184 joules depending on the reference value. The thermal energy, which a nutrient contains is called calorific value.

Besides the basic physical calorific value, the physiological calorific value is particularly relevant for the energy intake of an organism. This makes the physiological calorific value an appropriate tool for the measurement of the metabolic energy supply.

The most important physiological calorific values of nutrients are:

• 38,9 kJ/g fats (9,29 kcal/g)
• 17,2 kJ/g proteins (4,11 kcal/g) (physical calorific value: 23 kJ/g)
• 17,2 kJ/g carbohydrates (4,11 kcal/g)
• 29,7 kJ/g ethanol (alcohol) (7,09 kcal/g)

Are All Calories Equal?

In a normal diet, the provided energy from a calorie is always the same and all carbohydrates, fats, and proteins are converted into energy by our body at a defined rate.

To avoid too many entirely different systems to gain energy from nutrients (fats, carbohydrates, proteins, alcohol, etc.) the cells in the human body use mainly glucose and/or fat to generate energy.

Following a meal, a part of the glucose is directly obtained by cells from the bloodstream, a part is linked into long glycogen chains that are stored in the hepatocytes and muscle cells, and the remaining part is converted into fat and is embedded in dedicated fat storage cells. It is commonly believed that it is not possible for the human body to convert fat back into glucose.

Metabolic Rate

The energy turnover of an organism can essentially be divided into basal metabolic rate and working metabolic rate:

The basal metabolic rate is defined as the amount of energy the body needs when resting completely with an empty stomach at an indifferent temperature in the morning (the definition of the resting metabolic rate is similar but less rigid).

In comparison, the working metabolic rate is defined as the amount of energy that the organism consumes per day for additional services (physical activity) and includes the basal metabolic rate. Factors such as gender, age, weight, body size, muscle mass, and health condition influence all types of metabolic rate.

Daily Energy Intake

The standard values for daily energy intake vary from country to country. Therefore, guideline values for the daily energy supply can be different.

D-A-CH reference values that were published in 2015 by the German Society for Nutrition (DGE) e.g. are about 2700 kcal/day for men (25 to 51 years) and 2100 kcal/day for women (25 to 51 years) regarding a moderate physical activity level.

Generally speaking, as long as the energy intake is greater than the energy expenditure weight is gained – if energy intake and expenditure are nearly equal – the weight is not changed. And if the energy intake is exceeded by the expenditure, people lose weight.

In addition, there are some major points that facilitate maintaining a normal or healthy weight. Exercise expends calories and can result in increased muscle mass that is able to consume energy even while resting.

Without exercise, muscle cells are replaced by fat cells that need less energy than muscle cells. Therefore, the basal metabolic rate often decreases over a lifetime and if energy uptake is not adapted people begin to gain weight.

Author: Dr. Markus Burkard