One of the greatest differences between trees and human beings is the fact that trees do not move about, while human beings do. Men travel, carry loads, dig in the ground, swim, and do many things that require effort.
Probably if the wind did not blow, a tree would almost never move. It uses food only for growth. Its roots often move earth, and may even split rocks, but these things happen as a result of growing rather than of moving. Men, on the other hand, must have food not only to grow but to give them energy so that they may move about. As food gives energy it also gives heat.
Activity in the cells.
The cells of the body are always at work. When food is digested in the intestines, it is taken up by the blood and carried to all parts of the body. At the same time the blood carries oxygen which is taken in through the lungs by breathing. When food and oxygen come together in the cells, a slow burning, known as oxidation, takes place. The process is called oxidation, because oxygen unites with the food to bring about the burning. During the burning the food is changed into heat and energy. Of course there is no real fire in the cells, but the results are the same as if there were. Waste materials are formed during oxidation, just as ashes are formed by a fire in a stove or furnace. The heat which is formed is used to keep the body warm. The energy is used for doing work. In other words, it is the force that enables you to do such things as write, throw a ball, use a hammer, or even think.
Oxygen from the lungs.
The breath is so important that early people often looked upon it as a spirit or ghost. They knew that some change took place when a person died and said that his breath left his body. This explains how the term “breath of life” came to be used. Breath, of course, is only air that enters the lungs and comes out again. The early people were right in thinking that breath is necessary to sustain life, but they did not know just why it is needed.
Modern science has cleared up all the mystery about breathing. The lungs contain many tiny blood vessels separated by very thin membranes from the air taken in by breathing. Every particle of blood in the body passes through the tiny blood vessels of the lungs and thus comes close to the air. Some of the oxygen from the air passes through the thin membranes into the blood as it moves along. At the same time some of the waste materials from the blood pass through the thin membranes into the air. In other words, the blood as it passes through the lungs takes up oxygen and gives off waste materials. The chief waste material given off is known as carbon dioxide. The blood carries the oxygen from the lungs to the cells, where it is used in the oxidation of food. In return, the cells give off carbon dioxide.
Gaining heat and energy from exercise.
Exercise in some form is needed for the production of heat and energy. It is much like a policeman who keeps the crowds moving when there is a jam in traffic. He does not give anybody strength, but he makes everybody move. Likewise, exercise keeps the muscles active, improves digestion, and causes every part of the body to work better.
Lack of energy should not be confused with fatigue. A person may feel tired or listless even though he has not been working or playing. Perhaps what such a person needs, if he is well, is exercise. He feels tired because his blood is not moving fast enough to carry away the waste materials from the cells… Exercise will speed up the circulation of his blood, increase perspiration, and cause him to take more oxygen into his lungs. All this helps to move waste materials from his body and gives him a muscle tone which he did not have before. In other words, exercise helps him to build up energy in his body.
Gaining heat and energy from foods.
The actual .materials that are burned in the cells to produce heat and energy come from foods. Sunlight, air, and exercise never in themselves produce heat and energy. All that they can do is to help keep the cells active so that heat and energy will be formed. Not all foods, though, are energy-giving foods. Some merely help the body to grow.
People who are engaged in light work sometimes think it is unnecessary to eat energy-giving foods. They are mistaken, however, in their idea. Certain cells, such as those of the heart, stomach, and lungs, are active all the time and become sluggish if they are not fed properly. Of course the more active a person is, the more energy-giving foods he needs because more cells are at work. Some people, though, eat more energy-giving foods than they need and thus make the cells work harder than necessary. Usually a person can tell by his feelings whether he is getting sufficient energy from his food.
The temperature of the body.
The bodies of animals show little change in temperature. Birds, for example, have a temperature of about 102 degrees Fahrenheit, but fish usually stay about as warm as the water in which they live. The temperature of the human body remains at about 98.6 degrees Fahrenheit, whether the surrounding air is cold or warm. The body usually makes enough heat to permit a person to live in a great variety of places, whether near the poles or near the equator.. provided he wears the right amount of clothing. It acts somewhat as if it had a thermostat to keep the temperature even. The heat, of course, is produced by the oxidation that takes place in the cells. In order to produce the heat, however, the body must depend upon good foods and plenty of oxygen.
Measuring the heat produced by foods.
Some foods produce more heat than do others. Scientists have found a way to measure the amount of heat various foods produce. They have also found a way to measure the amount of heat that must be produced to keep the body at an even temperature of 98.6 degrees. Thus it is possible to discover just what foods should be eaten and how much of each kind should be eaten.
When water is placed over a fire, some of the heat goes into the water and raises its temperature. If none of the heat is wasted, the rise in temperature shows how much heat was produced. In the same way, foods may be burned to show how much heat they produce. If wheat, for example, is burned so that it heats water, the amount of heat which the wheat produces may be measured by noting the change in the temperature of the water. In measuring the heat that foods produce, scientists use a certain quantity of water, which they weigh in kilograms. A kilogram is a little over two pounds. The heat itself is measured in calories. A calorie is the amount of heat needed to raise the temperature of one kilogram of water one degree on a Centigrade thermometer. A Centigrade thermometer is one that has the freezing mark at 0 degrees and the boiling point at 100 degrees.
Records show that fats produce more heat than any other foods. A pound of fat, for instance, produces 4,082 calories, and a pound of carbohydrates only 1,815 calories. It is clear, therefore, why Eskimos eat such large quantities of fats. If they ate sugar alone, they would probably freeze to death. An ordinary meal at the dinner table should contain calories about as follows: lean beef, 250; baked potato, 245; bread, 230; eggs, 179; bacon, 188; butter, 169; milk, 123; sugar, 81.