Classification of nutrition types
Approaches to menu planning depend on the person's goals and health status. Four basic directions are distinguished:
- Rational nutrition. Intended for practically healthy individuals. It is built on scientific principles taking into account gender, age, climate, and occupation. It increases resistance to the external environment and ensures normal development.
- Preventive nutrition. Designed for healthy individuals working in unfavorable occupational conditions. It relies on the protective action of specific foods (fermented dairy products, vitamins), which increases resistance to chemical and physical factors.
- Premptive nutrition. Prescribed to healthy individuals from risk groups. It utilizes the nonspecific action of food to inhibit the development of non-communicable pathologies (atherosclerosis, diabetes, hypertension). In practice, it is used quite rarely.
- Medical (dietary) nutrition. The diet of a sick person prescribed by a physician in the form of numbered hospital diets (tables) or medical diets. It is strictly adapted to the stage of a specific disease and possesses a pharmacotherapeutic and rehabilitative effect, helping to restore organ functions.
Main principles of diet planning
Adequate nutrition is based on strict physiological laws:
- Energy balance. Daily caloric intake must cover actual energy expenditure, which depends on body weight, basal metabolic rate, and physical activity.
- Nutrient balance. Proteins, fats, and carbohydrates must be supplied in proportions that provide the maximum beneficial effect.
- Enzyme matching. The chemical composition of food must correspond to the capabilities of the digestive systems, otherwise assimilation will be impaired.
- Fractional and rhythmic intake. Four meals a day is optimal. Gastrointestinal function is subject to circadian rhythms, so eating at the same time serves as a conditioned stimulus for juice production. Calorie distribution: breakfast — 20–25%, second breakfast — 10–15%, lunch — 40–45%, dinner — 20–25%.
- Safety. Absence of mycotoxins, pathogens, pesticides, and food additives above permissible limits.
- Freshness and variety. Only freshly prepared dishes have maximum biological value. Long-term storage and preservation destroy vitamins.
- Sufficient satiety. Plant-based food provides a quick effect due to the volume of the food bolus, but leaves the stomach quickly. Animal fats, conversely, delay gastric emptying, creating satiety for 4–5 hours.
Physiology of digestion: bioavailability and appetite
Bioavailability is the percentage of nutrients absorbed in the intestine relative to the ingested amount. A number of factors influence this process:
- Origin of products. Plant fiber reduces overall bioavailability, whereas animal-derived products are digested almost completely.
- Chemical stimulants. Vegetables and dishes rich in extractive substances (meat broths) have a powerful secretagogue effect. Fatty foods, conversely, inhibit secretion. Milk and bread cause a weak response.
- Culinary processing. Pureed, minced, and liquid consistencies facilitate the work of enzymes.
- Bad habits. Smoking before or after a meal blocks gastric secretion. Alcohol initially sharply stimulates juice secretion, but with regular use, it deprives the stomach of the ability to produce juice independently.
The sequence of dishes is extremely important. A classic lunch begins with an appetizer or soup (stimulating appetite with extractive substances), continues with the main course (the main source of energy for the prepared stomach), and concludes with dessert. If you eat dessert first, digestion will be impaired.
Taste analyzer and energy expenditure
Taste qualities are not just a whim. Consuming tasty food triggers reflex stimulation of neural centers. Russian physiologists have proven that when food is tasty, endogenous (own) proteins are actively secreted into the intestinal lumen. They break down into amino acids and mix with dietary ones, creating an ideal amino acid balance. Consuming "healthy" but bland and tasteless food (e.g., via a tube) does not cause such a response, which rapidly leads to nutrient deficiency.
To calculate the volume of nutrition, it is necessary to know a person's daily energy expenditure, which is divided into:
- Unregulated expenditure. This is the basal metabolic rate (BMR) — the energy required to maintain life at complete rest (in the morning, lying down, on an empty stomach in a comfortable climate), as well as the specific dynamic action of food.
- Regulated expenditure. Energy expended on physical or mental work.
Briefly: answers to topical questions
What numbered diets (Pevzner tables) exist and for what pathologies are they prescribed?
The provided sources indicate Pevzner's therapeutic tables No. 5 and 5a. In the acute phase, tables No. 5 and 5a are prescribed; daily caloric intake is reduced relative to the age norm, and meals are fractional — 5–6 times a day in small portions. In the remission phase, the patient continues to follow diet No. 5. Other numbered diets and specific pathologies are not indicated in the provided sources.
What is the physiological significance of the specific dynamic action of food (SDAF) and what are its values for proteins, fats, and carbohydrates?
The physiological significance of the specific dynamic action of food lies in the increase in the body's energy expenditure associated with food intake. This process is caused by the chemical impact of digestive products on assimilation and the reflex influence of the act of eating.
Values for nutrients:
- Proteins — cause the maximum effect, energy expenditure increases by 30–40% (duration 7–8 h).
- Carbohydrates — energy expenditure increases by 4–6% (duration 2–3 h).
- Fats — cause an insignificant increase in energy expenditure.
With a mixed diet, metabolism increases by 10–12%.
Which enzymes provide the hydrolysis of proteins, fats, and carbohydrates at different stages of digestion?
Nutrient hydrolysis at different stages of digestion is provided by specific enzymes, while enzymes for fat breakdown are not specified in the sources.
- Carbohydrates — salivary $\alpha$-amylase acts in the oral cavity. It is inactivated in the stomach. In the small intestine, pancreatic $\alpha$-amylase and brush border complexes operate: glycoamylase, sucrase-isomaltase, and $\beta$-glycosidase.
- Proteins — are broken down by endopeptidases (pepsin in the stomach; trypsin, chymotrypsin, and elastase of pancreatic juice) and exopeptidases (aminopeptidase, carboxypeptidases A and B).
To remember I.P. Pavlov's ideal physiological sequence of lunch, use the rule "Starter — Fuel — Indulgence": appetizer/soup triggers juices (starter), the main course provides a mass of nutrients (fuel), and dessert delivers gustatory pleasure (indulgence).
Class and exam questions
Why is it inadvisable to constantly eat infrequently but in large portions?
This will lead to an overload of the digestive apparatus. The mass of food will exceed the enzymatic capacity of the body, due to which nutrient hydrolysis will be incomplete, and the food simply will not be properly assimilated.
Why is starting work without breakfast considered harmful?
The absence of food after a long nocturnal fast leads to a sharp decrease in performance and disrupts established circadian rhythms of gastric juice secretion.
Can a healthy person abruptly switch from a meat-based diet to a vegetarian one?
Physiologically, this is undesirable. The digestive tract adapts the composition of gastric juice to specific foods. A sharp change in dietary patterns will cause a temporary digestive disorder and reduced bioavailability.
What else to study on this topic
- Organization of control over food additives and mycotoxins
- Specific dynamic action of food (SDA)
- Pathogenesis of nutrition-dependent (alimentary) diseases
- Circadian rhythms and their impact on digestive biochemistry
- The effect of extractive substances on gastric secretion
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