Delving into Calories
Behind the phenomenon of "losing 1kg" lies a grand process of energy conversion woven by physics and chemistry. The food we eat contains highly concentrated energy.
1. Concrete examples of the vast energy contained in food
Let's confirm just how large the energy held by food is using concrete figures. For example, one slice of bread (about 160 kcal). Converting this into the physical energy unit of "joules," it is equivalent to about 670,000 J (joules).
This is comparable to the energy required to lift a 60kg object from the ground to a height of over 1,100m in one go.
Alternatively, it is equivalent to the energy required to keep a 100W light bulb lit for about 2 hours.
As for 1kg of human fat (about 7,200 kcal), it stores a vast amount of energy, enough to accelerate a 1-ton object to 1,200 km/h, or to raise the temperature of 7.2 tons of water by 1 degree.
2. The source of life obtained from molecular "bond energy"
The source of this energy lies in the difference in "bond energy" that holds molecules together.
The carbohydrates and fats we ingest are molecules in a high-energy state where carbon (C) and hydrogen (H) are complexly bonded. The human body takes in oxygen (O2), breaks down these molecules over time, and ultimately converts them into carbon dioxide (CO2) and water (H2O), which are extremely stable (low bond energy) states.
Overview of the chemical reaction: Carbohydrates + Oxygen = Carbon Dioxide + Water + Released Energy
The difference in bond energy between the "before reaction (food)" and "after reaction (carbon dioxide and water)" in this process is the true nature of the energy used to sustain life, drive muscles, and power thought. Humans sustain life activities through the difference (surplus energy) generated by rearranging molecules into more stable forms.
3. Hess's Law: The equivalence of combustion experiments and metabolism
The process by which the human body breaks down food and extracts energy is extremely complex. However, an absolute principle also covered in high school chemistry, "Hess's Law (Law of Constant Heat Summation)," supports this calculation.
Hess's Law: If the starting state (food and oxygen) and ending state (carbon dioxide and water) of a reaction are the same, the total sum of heat exchanged is constant, regardless of the path taken.
Therefore, whether oxidized in the body over a long period using enzymes or burned in an experimental apparatus in a short time, the total amount of energy released is the same. This law physically guarantees that the results of external combustion experiments can be applied to nutritional calculations.
4. Physical measurement principle: Bomb calorimeter
The measurement device based on this law is the "bomb calorimeter."
Complete combustion: A dried sample is placed in a steel sealed container (bomb) and burned completely using an electric spark under high-pressure oxygen.
Energy recovery: The total energy released is measured as the temperature rise of the water surrounding the container.
Calculation formula: Q = mcΔt (Q: heat quantity, m: mass of water, c: specific heat of water, Δt: temperature change)
Using this physical formula, the amount of energy held by a specific food can be strictly defined. The reaction formula for when glucose is oxidized is as follows.
C6H12O6 + 6O2 = 6CO2 + 6H2O + 2808kJ
The value "2808kJ" shown on the right side is the total amount of energy released when the molecules transition to a stable state.
5. Correction to metabolic energy: Atwater factors
Unlike complete combustion in a laboratory, the human body cannot absorb and utilize all of the ingested energy. Based on experiments, the scientist Atwater defined factors for calculating the actually usable "metabolic energy." This is the basis for the "4-9-4" used in diet management.
Protein (4 kcal/g): Because nitrogen cannot be completely oxidized in the body and is excreted as urea which retains energy, the value is lower than the heat of combustion.
Fat (9 kcal/g): It has a high digestion and absorption rate of about 95% and is a very high-density energy source.
Carbohydrates (4 kcal/g): It has an extremely high digestion and absorption rate of about 98%, making it an efficient energy source.
6. Contribution of dietary fiber and the complexity of calculation
Dietary fiber was once considered "0 kcal" because it is not digested, but it has now been clarified that through fermentation by bacteria in the large intestine, a portion is absorbed as short-chain fatty acids. Therefore, in modern standards, it is included in calculations as having an energy value of about 0 to 2 kcal/g depending on the type. If this is ignored, errors will occur in energy calculations for meals containing high-fiber foods.
7. Fluctuations due to individual differences and environmental factors
Hess's Law is strict, but energy conversion efficiency in living organisms is affected by the following factors.
Differences in gut microbiota: Even if the same food is ingested, the efficiency of energy extraction varies depending on the individual's gut environment.
Fluctuations in metabolic rate: Energy expenditure efficiency changes due to environmental temperature, individual basal metabolism, or the influence of metabolic regulatory factors.
Condition of the digestive system: If the condition of digestion and absorption is poor, the calculated energy will not be fully taken into the body.
Conclusion
The goal of "losing 1kg" is, physically speaking, the very process of energy conversion where "fuel (fat) stored in the body is oxidized and that energy is released to the outside as heat or work."
The temperature change of water observed by primitive humans and the energy amounts in modern nutritional management are connected by the same physical laws. Food is fuel for sustaining life, and its combustion efficiency is determined by the complex interaction between physical laws and biological phenomena.
If you want to study heat and temperature again, click here!