Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Doctrine of Viscera (Splanchnology)
Digestive System
Food Products and Nutrients

Nutrients are essential for the body's growth, The formation of new Cells to replace worn-out and dead ones, and the replenishment of energy required for vital Functions and reproduction. The total amount of food ingested, along with the absorbed nutrients and energy, must match the sum of substances and energy expended on the formation of new Tissues as well as those eliminated from the body.

In the form it enters the body, food cannot be absorbed into the Blood AND Lymph, nor can it be directly utilized to perform various vital functions. To be assimilated by the Digestive System, food must undergo mechanical and chemical Processing. Food is chewed in the Oral Cavity and mixed in The Stomach and Small Intestine with digestive juices, whose Enzymes break down nutrients into simpler components. Digested down to Amino Acids, Monosaccharides, and emulsified fats, these nutrients are absorbed and assimilated by the body. Water, minerals (salts, and Vitamins are assimilated in their natural state. The mechanical and chemical processing of food and its transformation into substances that the body can assimilate is called Digestion.

All chemical compounds used by the body as building blocks and Energy Sources (Proteins, fats, and CARBOHYDRATES) are called nutrients.

A person must regularly receive an adequate amount of nutrients (proteins, fats, and carbohydrates) from food, along with necessary water, mineral salts, and vitamins.

Proteins consist of hydrogen, oxygen, carbon, nitrogen, sulfur, phosphorus, and other elements. In the stomach and small intestine, dietary proteins are broken down into Amino Acids and their constituents, which are absorbed and used to synthesize human-specific proteins. Of the 20 amino acids required by humans, nine are essential, meaning they cannot be synthesized within The Human Body: valine, Histidine, isoleucine, leucine, Lysine, Methionine, Threonine, Tryptophan, and phenylalanine. These listed amino acids must be obtained through diet. Without these Essential Amino Acids, the synthesis of proteins vital to the human body is disrupted. Proteins containing a complete set of amino acids, including essential ones, are called biologically complete proteins. The most valuable sources are milk, meat, fish, and eggs. Plant-based proteins (from corn, wheat, barley, etc.) are considered incomplete because they lack a full Complement of amino acids necessary for the synthesis of human proteins.

Carbohydrates, which contain hydrogen, oxygen, and carbon, are utilized by the body as energy sources and for the formation of Cell membranes. Complex carbohydrates, known as Polysaccharides, enter the body through plant foods such as vegetables, fruits, starch, and other products. During digestion, polysaccharides are broken down into water-soluble Disaccharides and monosaccharides. Monosaccharides (such as glucose and fructose) are absorbed into the bloodstream and transported to Organs and tissues.

Fats serve as a source of energy and can be stored in the body as reserve Materials. They are a component of all cells, tissues, and organs, and also serve as rich energy reserves because, during starvation, fats are converted into energy-yielding carbohydrates. Fats are complex structures composed of carbon, oxygen, and hydrogen. During digestion, fats are broken down into their constituent parts—glycerol and Fatty acids (oleic, palmitic, stearic)—which occur in fats in various combinations and ratios. Fats can also be synthesized within the body from carbohydrates and protein breakdown products. Some fatty acids cannot be produced by the body; these are oleic, arachidonic, linoleic, and linolenic acids, which are found in vegetable oils.

Minerals also enter the body with food and water in the form of various salts containing calcium, phosphorus, potassium, sodium, sulfur, chlorine, iron, magnesium, and iodine. Many other elements are present in food in trace amounts and are therefore called Trace Elements. A growing body requires more mineral salts than an adult body, as they are involved in Bone tissue formation, organ growth, and are components of blood Hemoglobin, gastric juice, Hormones, cell membranes, and nerve synapses.

Water, which accounts for up to 65% of an adult's total body weight, is an essential component of tissue fluid, blood, and the body's internal environments. Vitamins, which are complex Organic compounds, are also present in food in small quantities. Vitamins are essential for metabolic processes, participate in all biochemical reactions, and influence the GROWTH AND DEVELOPMENT of the human body and its organs. The absence or deficiency of vitamins in the diet leads to severe disorders known as avitaminoses.

Food also contains dietary fiber, which is Cellulose found in plant cells. Dietary fiber is not broken down by enzymes, but it has The ability to retain water. This is crucial for digestion because swollen dietary fiber stretches the walls of the Large Intestine, stimulating peristalsis and the movement of digestive masses toward the rectum. The required food intake and the qualitative composition of nutrients (proteins, fats, carbohydrates, minerals, and vitamins) depend on age, gender, body weight, and physical activity.

The amount of energy consumed by the body—Energy Expenditure—is measured in calories (or joules). One calorie is defined as the amount of energy required to raise the Temperature of water by 1 °C (1 calorie equals 4.2 joules — J). In the body, The oxidation of 1 g of proteins yields 4.1 kilocalories — kcal, the oxidation of 1 g of carbohydrates yields 4.1 kcal, and the oxidation of 1 g of fats yields 9.3 kcal. Data on the Energy Requirements of workers engaged in various types of labor are presented in Table 9.

Class="center">Table 9 Daily energy requirement for individuals in various labor categories

To meet the body's daily nutritional needs during light work, the diet should contain at least 80–100 g of protein, and during heavy physical exertion, from 120 to 160 g. For children, taking into account their growth and energy expenditure, the amount of protein per 1 kg of body weight should be higher than for an adult. The total daily intake of animal and vegetable fats in the diet should be at least 50 g. The daily requirement for carbohydrates is 400–500 g.