Essential Knowledge for Weight Loss: Comprehensive Guide to Energy Measurement and Calorie Fundamentals

Energy Calculation: What You Must Understand When Seriously Dieting

 

Just as a moving car requires fuel to provide power, all human life activities require energy to sustain them. Therefore, without energy, there would be no life, no you, and no me.

 

The process of energy supply works roughly like this: after we eat, the food essentially undergoes a slow combustion within our stomachs, releasing energy. This energy enables us to maintain body temperature and perform normal physiological functions. This process is not dissimilar to burning wood in a stove.

 

Energy is measured in calories, a term derived from the English word ‘calorie’. It refers to the amount of heat required to raise the temperature of one gram of water by one degree Celsius at one atmosphere of pressure. However, the calorie is an extremely small unit; even a single grape can provide thousands of calories. Therefore, for practical calculation, scientists adopted the metric unit of 1,000 calories to represent energy, termed kilocalories or large calories. Note that many confuse calories with kilocalories. In reality, the calories displayed on treadmills refer to kilocalories, and colloquial references to calories typically denote kilocalories.

 

It is worth reiterating here:

Kilocalories and large calories are essentially the same thing, with 1 kilocalorie equalling 1000 calories. Hence, calories are also referred to as small calories. In China, the more commonly used unit of energy is the kilojoule. As we learnt in secondary school physics, the conversion between kilojoules and kilocalories is approximately 1 kilocalorie ≈ 4.2 kilojoules. The values for energy and heat in this book are identical.

 

Energy itself is not a nutrient; it is released when carbohydrates, fats, and proteins from our food are broken down and metabolised within the body. Hence, these three macronutrients are also termed energy-yielding nutrients. Among them, fat has the highest energy density per gram, yielding 9 kilocalories. Protein and carbohydrates yield 4 kilocalories per gram each.

 

One might wonder how these values are measured. The principle is straightforward: as mentioned earlier, food combustion can be likened to burning firewood. To determine a food's energy content, we similarly burn it. The instrument used to measure food calories is called a ‘calorimeter’. This device ignites the food within a sealed chamber using an electric spark. The temperature rise of the surrounding water caused by the food's combustion is then measured, thereby determining its caloric value.

 

Of course, the values mentioned above for the three major nutrients represent averages. Not all fats contain 9 kcal per gram. For instance, fats in meat and eggs are slightly higher, at approximately 9.03 kcal per gram, while fats in plants are lower, around 8.37 kcal per gram. Nine kilocalories is indeed an average value. A similar situation applies to carbohydrates and proteins. For instance, among carbohydrates, glucose actually has a lower calorific value than starch.

 

Furthermore, these figures represent the net calorific value provided by food within the body. When burned externally, foods yield higher heat outputs. For instance, protein burns at an average of 5.56 kcal/g in a calorimeter, yet when ingested, it provides only an average of 4 kcal/g. This discrepancy arises because the oxidation of food within the body inevitably incurs partial energy loss.