Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter VI. BIOCHEMISTRY OF PHYSIOLOGICAL FUNCTIONS AND SPECIALIZED TISSUES

CHAPTER 26. HUMAN NUTRITION BIOCHEMISTRY. I. NUTRIENT COMPONENTS. DIGESTION OF NUTRIENTS

26.2. HUMAN BODY REQUIREMENTS FOR NUTRIENT COMPOUNDS

Nutrient compounds entering The Human Body with food must meet the aforementioned Energy Requirements and (in the case of Proteins) cover the daily expenditure on structural and catalytic elements of the body.

Carbohydrades

CARBOHYDRATES are the main source of energy in Human Nutrition. The daily requirement of a healthy human body for carbohydrates averages 450-500 g. Taking into account the ENERGY VALUE OF carbohydrates (Table 26.1), this amount of carbohydrates corresponds to approximately 70 (65-75)% of the daily dietary caloric requirements.

Since carbohydrates are the primary energy source in nutrition, the daily requirement for them is directly proportional to human physical activity, amounting for adults (according to F.F. Boyechko, L.O. Boyechko, 1993) to:

- for mental work — 430 g;

- for physical labor:

light — 490 g,

moderate — 560 g,

heavy — 630 g.

The main source of carbohydrates for the human body is plant starch (400-450 g/day). Grains of wheat, rye, rice, and corn contain 60-80% starch, while potato tubers contain 15-20%. Sources of dietary sugar, which consists mainly of sucrose, are sugar beet and sugar cane. The recommended daily allowance of dietary sugar for an adult is about 50 g, although actual consumption rates range from 20 to 100 g/day or more, depending on the dietary habits and socio-economic status of the population.

Dietary fibers consist of Polysaccharides — components of Plant Cell Walls and Extracellular matrix that enter the human body with fruits and vegetables and are not broken down by the Enzymes of the gastrointestinal tract. Dietary fibers include Cellulose, hemicellulose, lignins, Gums, Pectins, and pentosans.

In ruminants, the cellulose of dietary fibers is broken down by microorganism enzymes and serves as the main source of metabolic energy. The human body does not contain the enzymes necessary to digest these substances; however, dietary fibers play a significant positive role in the Physiology of Digestion. They stimulate the MOTOR FUNCTION OF the intestine, promote Water retention in the colon and The formation of fecal masses, adsorb excess Cholesterol and Bile acids, endogenous and exogenous toxic substances, limit the postprandial increase in alimentary Blood glucose, which is of therapeutic importance in the nutrition of patients with atherosclerosis and Diabetes Mellitus.

An important factor in rational nutrition is the balance of individual types of carbohydrates in the daily diet, which should constitute (on average, relative to the total mass of dietary carbohydrates): polysaccharides (starch) — 75%, sugar (beet, cane) — 20%, pectic substances — 3%, fiber — 2%.

Under a normal diet, about 75% of the carbohydrates entering the body are oxidized to final products — carbon dioxide and water, which constitutes the main source of ATP formation, 20-25% are converted into fats, and 2-5% are stored as Glycogen. Excessive carbohydrate consumption exceeding the immediate energy requirements of the body is accompanied by the activation of Lipogenesis, i.e., The conversion of excess glucose and fructose via acetyl-CoA into Fatty acids, triglycerides, and cholesterol. For every 100 g of excess carbohydrates, especially those entering the body in the form of sucrose — the main component of dietary sugar — about 30 g of fats are synthesized.

Exceeding physiological requirements for simple carbohydrates, in particular dietary sugar, is a risk factor for The Development of diseases such as atherosclerosis, Hypertension, diabetes mellitus, and obesity. At the same time, sugar consumption (mostly in the form of beet and cane sugars, consisting predominantly of sucrose) in economically developed countries is constantly increasing. In this regard, The Use of high-intensity sweeteners is gaining increasing importance: saccharin, aspartame (L-aspartyl-L-phenylalanine methyl ester), and the plant protein monellin. The sweetness (in conventional units) of natural sugars and sugar substitutes relates as: sucrose : glucose : fructose : lactose : saccharin : aspartame : monellin = 1.0 : 0.5 : 1.7 : 0.2 : 400 : 180 : 2,000.

Lipids (Fats)

Neutral fats (fats) are the second most important energy source in the human diet, obtained mainly from animal food products. The requirement for dietary fats averages 60-90 g/day, which provides about 30 (from 25 to 35)% of the daily calorie needs for an adult.

Under normal mixed nutrition (animal and plant foods), neutral fats (triacylglycerols) make up the largest portion (more than 90%) of the total amount of lipids contained in food products. With food, a person also receives several grams of Complex Lipids and about 0.5 g of cholesterol (as part of animal lipids).

In addition to animal fats, which are primarily of energetic value, a person should consume 20-25 g of vegetable fats, including 2-6 g of essential polyunsaturated fatty acids — linoleic and linolenic (A.A. Pokrovsky, 1974), which serve as precursors in the synthesis of biologically active Eicosanoids. Essential Fatty Acids are found in significant quantities in plant-derived fats, as well as in fish and poultry; the content of these compounds in other meat and dairy products is significantly lower. Adequate consumption of fats with a high content of polyunsaturated fatty acids is currently viewed as an important prerequisite for the Prevention of atherosclerosis, CORONARY Heart DISEASE, and cerebrovascular disorders in the population of industrialized countries. The average fatty acid COMPOSITION OF THE most common dietary fats is presented in Table 26.3:

Table 26.3. Content of saturated and Unsaturated fatty acids (% of total content) in common animal and vegetable fats

Proteins

Proteins are nutrients that primarily serve a plastic (structural) function for the human body; the Amino Acids they contain provide the building blocks for cellular and tissue proteins within the body's own biostructures (Fig. 26.1). Under normal nutritional conditions, the energy value of proteins is low; when carbohydrate and fat intake is sufficient, the non-nitrogenous residues of Amino acids are utilized to a minimal extent within the general metabolic fuel pool.

Fig. 26.1. Diagram illustrating the utilization of dietary proteins in the human body.

To determine the human body's protein requirement, METABOLISM/2.html">THE CONCEPT OF nitrogen balance is used. This is defined as a state in which The amount of nitrogen entering the body (primarily as protein components in food) equals the amount of nitrogen excreted (mainly as urea and ammonium salts). Under conditions of physiological rest and no physical activity, nitrogen balance is achieved with an intake of approximately 23 g of protein. However, given the varying Biological value of Proteins in a mixed diet under real-life conditions, a person's actual protein requirements are significantly higher.

The actual daily protein requirement depends on age, sex, and the biological value of the proteins, averaging (according to R. Berkow [Ed.]: The Merck Manual of Diagnosis and Therapy, 1992):

- for men:

aged 19–24 years — 58 g;

aged 25–50 years — 63 g;

- for women:

aged 19–24 years — 46 g;

aged 25–50 years — 50 g.

The biological value of proteins depends on the following factors:

a) the Amino Acid Composition of the dietary proteins;

b) the ability of the human body, particularly the enzyme systems of the digestive tract, to assimilate specific proteins.

The amino acid composition of dietary protein is a determining factor in the human body's ability to assimilate it. Among the twenty L-amino acids found in natural proteins, the human enzyme systems are capable of synthesizing de novo from other intermediates in sufficient quantities only Eight amino acids, namely: Alanine, aspartic acid, asparagine, glutamic acid, glutamine, Proline, Glycine, and Serine.

The remaining amino acids are not synthesized by the human enzyme systems and are known as Essential Amino Acids, which include: valine, leucine, isoleucine, Threonine, Methionine, phenylalanine, Tryptophan, and Lysine. Two amino acids (Arginine and Histidine) are synthesized in insufficient quantities in the body and are termed semi-essential (conditionally essential) amino acids. The human body depends on a constant supply of these Ten amino acids through dietary proteins—if even a single essential amino acid is missing, Protein Synthesis halts. The recommended daily requirements for essential amino acids in young adult males are as follows (A. Lehninger, 1985):

Valine — 1.50 g

Leucine — 2.02 g

Isoleucine — 1.30 g

Threonine — 0.91 g

Methionine — 2.02 g

Phenylalanine — 2.02 g

Tryptophan — 0.46 g

Lysine — 1.50 g.

In addition to amino acid composition, the biological value of proteins also depends on their digestibility by gastrointestinal enzymes, which yield free amino acids utilized by Cells. Compared to plant proteins, animal proteins contain a higher proportion of essential Amino Acids and are more readily hydrolyzed by gastric and intestinal proteases (with the exception of certain structural proteins).

If we take the biological value of a hypothetical protein that is fully assimilated by the human body as 100, the biological value of the total proteins in some of the most common foods can be ranked as follows:

reference protein (100) > human milk proteins (95) > beef proteins (93) > chicken egg proteins (87) > cow's milk proteins (81) > polished rice proteins (63) > corn proteins (36) > white bread proteins (30).

Balanced Diet

The aforementioned dietary requirements for a healthy human body are prerequisites for a balanced diet. However, people's actual dietary patterns largely depend on social conditions, particularly the economic status of the population, as well as ethno-national and religious factors. The aforementioned high percentage of carbohydrates in the daily diet (about 70%, and up to 90% of caloric intake in some countries) corresponds to the dietary habits of approximately 80% of the world's population, who consume a predominantly plant-based diet. In the diet of populations in economically developed countries, the proportion of carbohydrates is significantly lower (about 45% of daily caloric intake), with high-protein meat and dairy products taking up a larger share of the diet.

According to recommendations by the U.S. National Academy of Sciences, the optimal ratio of individual nutrients contributing to the daily caloric intake of the diet should be: carbohydrates — 58% (starch — 48%, sugar — 10%), fats — 30% (including 10% each of saturated, monounsaturated, and polyunsaturated fats), proteins — 12%. The Biochemical Composition of the most common foods is presented in Table 25.4.

Table 26.4. Average nutrient content in common foods (% of product mass)

A balanced intake of Vitamins and essential minerals is also a necessary condition for a healthy diet, which will be discussed in Chapter 27. From this perspective, an exceptionally important food product, especially for the growing body, is milk, which contains a significant amount of nutrients essential for the metabolism of vitamins and Trace Elements.