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No record exists to tell us who first began to think about why we eat or about various effects of foods, but the ancient Greek philosophers and doctors commented along these lines. Socrates said, the purpose of food is to replace the water lost through the skin and the loss of heat from the body. Hipocrates, the “father of medicine”, thought that growing bodies should have more heat than those of older people and so require more food, that neither overeating nor fasting is a good idea.

Lavoisier thought that the combustion¹ that produces body heat should occur in the lungs, but later several chemists could show that the oxygen combines with the red corpuscles of the blood and is carried to the tissues in all parts of the body to supply energy where it is needed.

Understanding of the composition of foods and what happens to them after they are eaten had to wait for several discoveries in chemistry. What happens to food from the time it is eaten until it is oxidized to produce heat and mechanical energy, could not be learned until the chemical nature of foods was discovered. And the foods appear to be very complex organic substances.

Foods are composed of organic substances far too complex to be understood with the state of chemistry as it was in the first half of the nineteenth century. Modern work on digestion and nutrition began about a century ago.

Although scientists had attempted to learn why we eat, they could learn very little. We might sum up their accomplishments by saying they learned that all animals inhale oxygen, combine it with the food to produce carbon dioxide, heat, and the energy with which they could move about. The chemistry of that time could not tell more.

Now we know that we should eat to have energy. When the nutritionist uses the word “energy” he means the capacity to do work. To him “work” is movement. All work requires energy and work involves motion; therefore, the more a person moves about, the more energy he requires. Even when a man is asleep he is still partly in motion because his heart, lungs and most of the other organs are working.

1- combustion - сгорание

Text № 2 «SUGAR AND STARCH»

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Among common carbohydrates are sugar and starch. Although these substances differ widely from one another in properties and constitution, they show a very definite point of resemblance. They are all composed of carbon, hydrogen and oxygen, and the hydrogen and oxygen are always in the sane ratio to one another as in water, i.e.¹, two of hydrogen to one of oxygen. The name, therefore, of carbohydrates was given because these compounds seemed to be built up of carbon and water in different proportions. Thus glucose, as sugar, has a formula C₆H₁₂O₆ – which might be represented as 6C+6H₂O. It may be pointed out that this last method of representation is only referred to for the purpose of showing how the name arose.

There is hardly a plant that does not contain either sugar or starch or cellulose or even all the three of them. The sugar and starches are among our most common foods, and the celluloses though not useful as a food, are found the main constituents of wood, paper, cotton and other fibres or fibrous materials.

Our ordinary everyday life leads us to think that there is only one sugar, viz²., that we use as a sweetening agent for tea. In fact there are many sugars, they are glucose (so called dextrose or grape-sugar), fructose (also called levulose) and galactose.

Glucose or grape-sugar is found in large quantities in grapes. When these are dried in the sun to form resins, the glucose in the juice separates out as hard brown nodules³. It is frequently found mixed with fructose in the juice of fruits, in the roots and leaves of plants and in honey. It can also be obtained from cane sugar and starch. Glucose is soluble in about its own weight of water and is not so sweet as ordinary cane sugar.

It readily ferments with yeast and yields principally alcohol and carbon dioxide.

Fructose occurs with glucose in the juice of sweet fruits and in honey. It is more soluble in water than glucose, and is about as sweet as the latter. It ferments with yeast but not as rapidly as glucose.

1- id est – то есть

2- viz.=videlicet – а именно

3- nodules – (зд.) масса=комочки

 

Text № 3 «ENERGY REQUIREMENTS»

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No matter how quiet a person may be, his body is constantly at work. Certain processes and activities always go on as long as life continues. The blood must circulate and the heart must beat. Breathing must take place and elimination of body wastes must be carried on. All these require energy, which must be supplied by the food taken into the body. Carbohydrates, fats and proteins, when oxidized or burned in the body, give energy. However, protein is not widely recommended or used as a chief dietary source of energy. Foods classed as “protein-rich” should be the source of 10 to 15 per cent of the total energy requirement. Fats should furnish 20 to 30 per cent of the total energy. This amount is well utilized by the body and also provides the desired quality to the diet. The remaining 55 to 70 per cent of the energy requirement should be met by “carbohydrate-rich” foods, such as cereals, bread, potatoes, other vegetables and fruits and sugar.

The energy of fuel value of a food is measured in terms of calories. A calorie, indicated by cal., is the amount of heat required to raise the temperature of one kilogram of water by one decree Centigrade. A gram of carbohydrate has an energy or fuel value of four calories; a gram of fat nine calories; a gram of protein four calories. The daily requirement of a woman may range from 2,000 to 2,600 calories or more and the daily requirement of a man, from 2,500 to 3,700 calories or more.

The energy need depends chiefly upon the activity, age and size, but also upon climate and season. A tall person requires more calories than a short one of the same weight, engaging in similar activity. A cold climate tends to make the energy requirements higher than a warm one under similar conditions.

 

Text № 4 «VEGETABLE PROTEINS»

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Plants can build up from nitrates and salts of ammonia complex nitrogenous substances called proteins. A typical protein is eggalbumin, the white of an egg. It is colourless, thick, sticky¹, fluid², soluble in water. When put into boiling water, it coagulates, i.e., it becomes a solid mass and is no longer soluble in water. If the white of an egg be exposed to the air, it decomposes into a great number of substances. If heated with dilute acid, it breaks down into ammonia carbon dioxide, and a number of complex substances. Proteins are built up principally from the five elements: carbon, hydrogen, oxygen, nitrogen and sulphur.

The proteins found in the vegetable kingdom can be divided into seven main classes. We shall consider here only the glutenins, the prolamins and the globulins. The glutenins to which class wheat glutenin belongs are proteins which are not soluble in neutral salt solutions or in alcohol.

The prolamins are proteins soluble in alcohol. They were once called gliadins. To this class belongs gliadin from wheat or rye, hordein from barley, and zein from maize. The globulins are proteins insoluble in water but soluble in solutions of common salt. They are almost all coagulated by heating their solutions. It has long been known that, if a ball of dough (made from flour and water) be washed out, a compact mass of rubber-like material is left. This is gluten³. On treating with alcohol, it can be separated into two substances – glutenin and gliadin.

1. sticky – вязкий

2. fluid – текучий

3. gluten - клейковина