The unit, source, and determinant of daily human heat energy requirements

**(II) Thermal Energy**

To sustain life and perform necessary labor, humans must obtain energy from their diet daily to meet the body's needs. Generally, healthy adults maintain a dynamic balance between energy intake and expenditure from their diet; otherwise, it can lead to either emaciation or obesity, both of which are detrimental to health. So, how much energy does each person need daily? Where does dietary energy come from?

1. Units and meanings of thermal energy

In nutrition, the unit for expressing heat energy was formerly the kilocalorie (kcal), a unit of heat energy from physics. One kcal is the amount of heat energy required to raise the temperature of 1000 grams of water from 15°C to 16°C. Its full name is kilogram-calorie, abbreviated as kilocalorie. Now, the internationally accepted unit is the kilojoule (kJ). The conversion relationship between these two units is as follows:

1 kilojoule = 0.239 kilocalories; 1 kilocalorie = 4.184 kilojoules.

2. Three major factors determining the amount of heat energy supplied to the human body

The energy required by the human body every day can be divided into three parts: the energy required to maintain basal metabolism, the energy consumed in various activities (including physical and mental labor), and the energy consumed by the special dynamic action of food.

(1) Energy required to maintain basal metabolism: Basal metabolism refers to the energy metabolism required to maintain basic life activities in a suitable temperature environment (18℃～25℃) when the human body is awake, resting, and quiet 12 to 18 hours after a meal. In other words, it is the amount of heat energy required to maintain normal body temperature, respiration, heartbeat, secretion, etc.

(2) Energy required for labor: The energy required for labor accounts for the majority of the body's total energy requirement. The energy consumed by labor is related to the intensity of labor, the duration of labor, and the level of skill in the work. The greater the intensity of labor, the longer the duration of labor, and the less skilled the worker, the more energy is consumed.

(3) Energy consumed by the specific dynamic action of food: Ingesting any food increases the body's energy metabolism at rest. This is because the ingested nutrients have a specific stimulating effect on metabolism, increasing energy expenditure. This increase in energy metabolism caused by food intake is called the specific dynamic action of food. This effect only increases the body's energy expenditure, not its energy source. Various foods and nutrients exhibit specific dynamic actions, with protein being the strongest, accounting for 20% of its own energy supply, carbohydrates about 5%–6%, and fats about 4%–5%. When consuming a typical mixed diet, the additional energy expenditure caused by the specific dynamic action of food is approximately 10% of the daily basal metabolic rate.

3. Methods for determining the human body's energy supply

The following methods can be used to determine how much energy each person needs to provide each day:

(1) Simple estimation method: This is the simplest method, which estimates based on each person's daily living and working conditions. For example, the estimated energy supply for an average-sized adult based on their living and working conditions is as follows:

Light physical labor: work that mainly involves stillness and minimal movement, with some recreational activities allowed in spare time. Examples include indoor office work, assembling and repairing household appliances, sales clerk work, laboratory operations, and lecturing. The recommended energy intake for men is 10,878 kJ (2,600 kcal); for women, it is 10,041 kJ (2,400 kcal).

Moderate physical labor: driving motor vehicles, operating machinery, electrical installation, metal processing and cutting, etc. The required energy supply for men is 12,552 kJ (3,000 kcal); for women, it is 11,715 kJ (2,800 kcal).

Heavy physical labor: such as non-mechanized agricultural labor, steelmaking, dancing, sports, loading and unloading of vehicles and ships, logging, quarrying, stone breaking, etc. The required energy supply for men is 14226 kJ (3400 kcal); for women it is 13389 kJ (3200 kcal).

Under the same living and working conditions, people's energy requirements vary depending on their age and body type. On a per-unit-weight basis, children and adolescents in their rapid growth and development require relatively more energy than adults, while this energy requirement decreases with age. Generally, the energy requirement for adults is based on an age of 20-30, decreasing with age. The recommended values ​​from the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) are: 5% reduction for those aged 40-49, 10% reduction for those aged 50-59, 20% reduction for those aged 60-69, and 30% reduction for those aged 70 and above.

(2) Daily observation method: This involves observing a person's daily activities (also called self-observation) and recording the time required for each activity. Then, the time required for the same action is added together to calculate the time (in minutes) required for various activities within 24 hours. The total energy consumption of each action within 24 hours is then added together with the basal metabolic rate to obtain the total energy consumption for the day. After adjusting for body surface area, 10% of the energy consumed by the special dynamic action of food is added to obtain the total energy consumption for the day.

4. Sources and distribution of human body heat energy

(1) Thermogenic substances: The heat energy required by the human body comes from carbohydrates, fats and proteins in food. These three nutrients that can generate heat energy are collectively called thermogenic substances or heat-generating nutrients.

The number of calories released by the oxidation of each gram of pyrogen in the body is called the caloric value or energy number. It is estimated that 1 gram of carbohydrates and protein each release 17 kilojoules (4 kilocalories) of energy after oxidation, while 1 gram of fat releases 38 kilojoules (9 kilocalories). Knowing the mass of pyrogens ingested from food allows calculation of the total energy intake. For example, if a person ingests 500 grams of carbohydrates, 60 grams of fat, and 100 grams of protein per day, the energy intake from their diet is: 500 grams of carbohydrates × 17 kilojoules (4 kilocalories) = 8500 kilojoules (2000 kilocalories), 60 grams of fat × 38 kilojoules (9 kilocalories) = 2280 kilojoules (540 kilocalories), and 100 grams of protein × 17 kilojoules (4 kilocalories) = 1700 kilojoules (400 kilocalories), totaling 12480 kilojoules (2940 kilocalories).

(2) Distribution of Energy Sources: Carbohydrates, proteins, and fats are three energy-producing nutrients that release energy after oxidation in the body, and they can be interconverted during metabolism, but they cannot completely replace each other. Therefore, there should be an appropriate distribution ratio in the diet. According to the dietary habits of the Chinese people, carbohydrate intake should account for 60% to 70% of total energy, fat should account for 15% to 25%, and protein should account for 10% to 15%. Eating only vegetarian food, only meat, or only high-protein diets is not in line with nutritional principles.

**(III) Nutritional Value of Commonly Used Foods**

1. Grains

Grains include rice, wheat, corn, buckwheat, sorghum, etc. In the diet of Chinese residents, about 80% of the calories and about 50% of the protein come from grains, while a considerable proportion of B vitamins and minerals are also provided by grains.

Grains have a relatively low fat content, about 2%. They are highest in carbohydrates, averaging around 70%, with rice and wheat flour containing even higher levels (over 75%), while other grains range from 67% to 70%. The carbohydrates in grains have a high bioavailability, exceeding 90%, making them the most economical source of energy for the human body. They contain approximately 1.5% inorganic salts, primarily phosphorus and calcium. In addition, they also contain significant amounts of magnesium and iron.

Because grains mainly contain water-soluble vitamins, which are mostly concentrated in the germ and outer layer of the grain, they should be washed and rubbed sparingly during cooking to prevent significant vitamin loss. The longer the soaking time and the more times the rice is rinsed, the greater the vitamin loss.