Chat with GPT: Is the calorific value of food really measured by burning it?

The calorific value of food indicates how much energy our body can obtain from it - usually expressed in kilocalories (kcal) or kilojoules (kJ). But how is this value actually determined?

Disclaimer

In this blog series, I ask ChatGPT on a variety of topics, from world politics to everyday issues, to get concise summaries and answers. The answers provided by ChatGPT are generated from large datasets, which may include biases, for example from Western perspectives. While the model strives for accuracy, it can sometimes produce incorrect or misleading information. I do not take responsibility for the validity of the content. Though I aim to ensure that the answers reflect common sense, I encourage you to cross-check facts and verify information through trusted sources. You are also welcome to reach out to me with suggestions for improvement.

ChatGPT’s answer

Historical: Combustion in the bomb calorimeter

In the past, the calorific value was actually measured by burning the food, using a so-called bomb calorimeter:

• The food is completely dried (dried powder).
• It is burnt in pure oxygen in a closed container (“bomb”).
• The resulting heat heats a defined amount of water.
• The increase in temperature of the water is used to calculate the energy released in kilojoules (kJ).

Today: Calculation based on macronutrients

Nowadays, the calorific value is no longer measured experimentally, but calculated - on the basis of standardized conversion factors, the so-called Atwater factors.

Standard values:

Macronutrient	Kilocalories (kcal/g)	Kilojoules (kJ/g)
Fat	9.0	37
Protein	4.0	17
Carbohydrates	4.0	17
Fiber	2.0	8
Alcohol (ethanol)	7.0	29

Physiological vs. physical calorific value

The calorific value on packaging is the physiological calorific value, i.e. the amount of energy that the body can actually utilize. In the case of proteins, for example, part of the nitrogen is lost (via urea). Dietary fiber provides only partial energy through bacterial fermentation in the large intestine. The maximum energy measured in the calorimeter, on the other hand, is the physical calorific value, i.e. the total amount of energy with complete oxidation.