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Basics of Nutrition

For medical students2 min readUpdated 2026-10-10

Nutrition is the process of acquiring nutrients from the environment to meet the metabolic demands of the organism. It ensures tissue renewal, growth, the maintenance of basal metabolism, and the coverage of human energy expenditure.

Protein:Fat:Carb Ratio (by mass)1 : 2 : 4

TEF of ProteinsIncreases energy expenditure by 30–40%

TEF of CarbohydratesIncreases energy expenditure by 4–6%

Meal DistributionBreakfast 30%, lunch 40–50%, dinner 10–20%

Thermic Effect of Food (TEF)

Any food intake is accompanied by an increase in energy expenditure, known as the thermic effect of food (specific dynamic action). This process is triggered by afferent signals from oral and gastric receptors directed to the hunger center. Interestingly, heat production increases even during sham feeding, when food does not enter the stomach.

Different nutrients elicit varying degrees of response:

Proteins produce the most potent effect. They increase energy expenditure by 30–40%. This process begins 1.5–2 hours later and lasts up to 7–8 hours.
Carbohydrates lead to a more modest increase of 4–6%, with the effect lasting 2–3 hours.
Fats increase energy expenditure only negligibly.

Balanced Nutrition

The concept of a balanced diet is based on three main pillars:

Physiological compliance: the composition of ingested food must strictly match the body's current demands.
Energy equilibrium: the caloric content of the diet (adjusted for assimilation rates) must equal total energy expenditure. The energy expenditure formula includes basal metabolism, the thermic effect of food, and the cost of physical activity.
Macronutrient balance: proteins, fats, and carbohydrates should relate by mass as 1:2:4, and by caloric value as 15%, 30%, and 55%, respectively.

Theory of Adequate Nutrition

In addition to the classical approach, the theory of adequate nutrition (trophotropic theory) was developed, shifting the focus to the fine mechanisms of the digestive tract.

Its key provisions:

Food must replenish the molecular composition of the gastrointestinal enzyme systems.
Normal progression of both luminal and membrane (contact) digestion must be ensured.
Absorbed nutrients must strictly correspond to the body's current requirements (adequate resorption).
Great importance is attached to normal intestinal microflora.
This theory explains why, in genetic or acquired enzyme defects (enzymopathies), familiar foods become harmful. For example, milk ceases to be adequate food in lactase deficiency.

Components of a Complete Diet

Simply counting calories is not enough to build a proper diet. The quality of nutrients matters:

Proteins must contain an optimal ratio of dispensable and indispensable (essential) amino acids.
The fat profile requires fatty acids with varying degrees of saturation.
The carbohydrate portion must be diverse and necessarily include ballast substances (dietary fibers, such as pectin and cellulose).
Micronutrients (vitamins and minerals) are regulated taking into account age, sex, energy expenditure, season, and climate.

Mnemonic

Remember the P:F:C mass ratio using powers of two: 2^0 (1) : 2^1 (2) : 2^2 (4).

Frequently asked questions

What substances are classified as ballast carbohydrates (dietary fibers)?

Ballast carbohydrates (dietary fibers) include cellulose, hemicellulose, and pectic substances.

Sources also note that bacterial enzymes of the intestinal microflora break down cellulose, hemicellulose, and pectins that are not digested in the small intestine.

By what percentage do fats increase energy expenditure via the thermic effect of food?

The exact percentage increase in energy expenditure for fats is not specified in the sources: fats cause a negligible increase in energy expenditure due to the thermic effect of food.

For comparison: proteins increase energy expenditure by 30–40%, and carbohydrates by 4–6%.

What are the physiological functions of ballast substances (dietary fibers) in the gastrointestinal tract?

In the gastrointestinal tract, ballast substances enhance motor function and can also act as demineralizing factors.

Key effects of dietary fibers:

Enhanced motility — positive action associated with increased gastrointestinal motor function, used to prevent constipation.
Demineralizing effect — ballast substances can reduce the absorption of minerals.
Digestive function of microflora — bacterial enzymes in the colon break down fibers not digested in the small intestine.

What components make up the body's total energy expenditure?

Total energy expenditure consists of basal metabolism, the thermic effect of food, and the energy spent on physical work.

Which nutrient stimulates postprandial heat production the most?

Proteins cause the maximum increase in energy expenditure, raising it by 30–40%.

What role do bacteria play in adequate nutrition theory?

In the trophotropic theory of adequate nutrition, the intestinal microbiota is viewed as a crucial participant in digestion, without which normal nutrient assimilation is impossible.

Go deeper

Functions of nutrition (plastic and energetic)
Contributions to the science of nutrition
Specialized nutrition for specific groups
Principles of dietary allowance standardization