Human energy metabolism

“The human body is a ‘machine’ that can release chemical energy bound in the ‘fuel’ of food; this ‘fuel’ is carbohydrates, fats, proteins and alcohol” (WHO).

The preferential use of any of the listed sources has different characteristics in terms of the magnitude of energy exchange and associated metabolic shifts.

Features of various metabolic sources of food energy supply

Indicators	Glucose	Palmitate	Protein
Heat release, kcal:
Per 1 mole oxidized	673	2398	475
Per 1 g oxidized	3.74	9.30	5.40
Oxygen consumption:
Moth	66.0	23.0	5.1
L	134	515	114
Carbon dioxide production:
Moth	66.0	16.0	4.1
L	134	358	92
ATP production, moles:	36	129	23
Cost of ATP products:
Hell	18.7	18.3	20.7
V/d	3.72	3.99	4.96
S/d	3.72	2.77	4.00
Respiratory quotient	1.00	0.70	0.81
Energy equivalent per 1 liter of oxygen used	5.02	4.66	4.17

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Stages of energy exchange

Although the dissimilation and synthesis of protein, fat and carbohydrate structures have characteristic features and specific forms, there are a number of fundamentally common stages and patterns in the transformation of these different substances. In relation to the energy released during metabolism, energy metabolism should be divided into three main stages.

In phase I, large molecules of nutrients are broken down into smaller ones in the gastrointestinal tract. Carbohydrates form 3 hexoses (glucose, galactose, fructose), proteins - 20 amino acids, fat (triglycerides) - glycerol and fatty acids, as well as rarer sugars (for example, pentoses, etc.). It has been calculated that on average, 17.5 tons of carbohydrates, 2.5 tons of proteins, and 1.3 tons of fats pass through the human body during its lifetime. The amount of energy released in phase I is insignificant, and it is released as heat. Thus, about 0.6% of the total energy is released during the breakdown of polysaccharides and proteins, and 0.14% of fats, which is formed during their complete breakdown to the final metabolic products. Therefore, the significance of chemical reactions in phase I consists mainly in preparing nutrients for the actual release of energy.

In stage II, these substances undergo further breakdown by incomplete combustion. The result of these processes - incomplete combustion - seems unexpected. Of the 25-30 substances, in addition to CO2 and H2O, only three end products are formed: α-ketoglutaric acid, oxaloacetic acid and acetic acid in the form of acetyl coenzyme A. Quantitatively, acetyl coenzyme A predominates. In phase II, about 30% of the energy contained in the nutrients is released.

At stage III, the so-called Krebs tricarboxylic acid cycle, the three end products of phase II are burned to carbon dioxide and water. In this process, 60-70% of the energy of nutrients is released. The Krebs cycle is the general final path of the breakdown of carbohydrates, proteins and fats. It is a kind of nodal point in the exchange, where the transformations of various structures converge and the mutual transition of synthetic reactions is possible.

Unlike stage I - the stages of hydrolysis in the gastrointestinal tract - in phases II and III of the breakdown of substances, not only energy is released, but also a special type of its accumulation.

Energy exchange reactions

Energy conservation is achieved by converting the energy of food breakdown into a special form of chemical compounds called macroergic compounds. The carriers of this chemical energy in the body are various phosphorus compounds, in which the bond of the phosphoric acid residue is the macroergic bond.

The main place in energy metabolism belongs to the pyrophosphate bond with the structure of adenosine triphosphate acid. In the form of this compound, 60 to 70% of all energy released during the breakdown of proteins, fats, and carbohydrates is used in the body. The use of energy (oxidation in the form of ATP) is of great biological importance, since this mechanism makes it possible to separate the place and time of energy release and its actual consumption during the functioning of organs. It has been calculated that in 24 hours the amount of ATP formed and broken down in the body is approximately equal to body weight. The conversion of ATP to ADP releases 41.84-50.2 kJ, or 10-12 kcal.

The energy generated as a result of metabolism is spent on the basic metabolism, i.e. on maintaining life in a state of complete rest at an ambient temperature of 20° C, on growth (plastic metabolism), muscle work and on digestion and assimilation of food (specific dynamic action of food). There are differences in the expenditure of energy generated as a result of metabolism in adults and children.

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BX

In a child, as in all mammals born immature, there is an initial increase in basal metabolism by 1 1/2 years, which then steadily continues to increase in absolute terms and just as regularly decreases per unit of body mass.

Often, calculation methods are used to calculate the basal metabolic rate. Formulas are usually oriented towards indicators of either length or body weight.

Calculation of basal metabolic rate using body weight (kcal/day). FAO/WHO recommendations

Age	Boys	Girls
0-2 years	60.9 R-54	61 R - 51
3-9 years	22.7 R + 495	22.5 R + 499
10-17 »	17.5 R +651	12.2 R +746
17-30»	15.3 R +679	14.7 R + 496

The total energy received with food is distributed to ensure basic metabolism, the specific dynamic action of food, heat loss associated with excretion, physical (motor) activity and growth. In the structure of energy distribution, i.e. energy metabolism, a distinction is made between:

• Energy received (from food) = Energy deposited + Energy used.
• Energy absorbed = Energy received - Energy excreted with excrement.
• Metabolized energy = Energy received - Energy of maintenance (life) and activity, or "basic costs".
• The energy of the main costs is equal to the sum:
  • basal metabolic rate;
  • thermoregulation;
  • warming effect of food (WEF);
  • activity costs;
  • costs of synthesizing new tissues.
• Energy of deposition is the energy spent on deposition of protein and fat. Glycogen is not taken into account, since its deposition (1%) is insignificant.
• Energy deposited = Energy metabolized - Energy of basic expenditure.
• Energy cost of growth = Energy of synthesis of new tissues + Energy deposited in new tissue.

The main age differences lie in the relationship between the costs of growth and, to a lesser extent, activity.

Age-related features of distribution of daily energy expenditure (kcal/kg)

Age	BX	SDDP	Excretion losses	Activity	Height	Total
Premature	60	7	20	15	50	152
8 weeks	55	7	11	17	20	110
10 months	55	7	11	17	20	110
4 years	40	6	8	25	8-10	87-89
14 years old	35	6	6	20	14	81
Adult	25	6	6	10	0	47

As you can see, growth costs are very significant for a low-weight newborn and during the first year of life. Naturally, they are simply absent in an adult. Physical activity creates significant energy expenditure even in a newborn and infant, where its expression is sucking the breast, restlessness, crying and screaming.

When a child is restless, energy expenditure increases by 20-60%, and when screaming - by 2-3 times. Diseases make their own demands on energy expenditure. They especially increase with an increase in body temperature (for 1° C increase, the increase in metabolism is 10-16%).

Unlike adults, children spend a lot of energy on growth (plastic metabolism). It has now been established that to accumulate 1 g of body mass, i.e. new tissue, it is necessary to spend approximately 29.3 kJ, or 7 kcal. The following estimate is more accurate:

• Energy "cost" of growth = Energy of synthesis + Energy of deposition in new tissue.

In a premature, low-weight baby, the energy of synthesis is from 1.3 to 5 kJ (from 0.3 to 1.2 kcal) per 1 g added to body weight. In a full-term baby - 1.3 kJ (0.3 kcal) per 1 g of new body weight.

Total energy cost of growth:

• up to 1 year = 21 kJ (5 kcal) per 1 g of new tissue,
• after 1 year = 36.5-50.4 kJ (8.7-12 kcal) per 1 g of new tissue, or about 1% of the total energy of the nutrient content.

Since the intensity of growth in children varies in different periods, the share of plastic metabolism in the total energy expenditure is different. The most intensive growth is in the intrauterine period of development, when the mass of the human embryo increases 1 billion 20 million times (1.02 x 109). The growth rate continues to remain quite high in the first months of life. This is evidenced by a significant increase in body weight. Therefore, in children of the first 3 months, the share of "plastic" metabolism in energy expenditure is 46%, then in the first year it decreases, but from 4 years, and especially in the prepubertal period, an increase in the intensity of growth is observed, which is again reflected in the increase in plastic metabolism. On average, 12% of the energy requirement is spent on growth in children aged 6-12 years.

Energy costs for growth

Age	Body weight, kg	Weight gain, g/day	Energy value, kcal/day	Energy value, kcal/(kg-day)	As a percentage of the basal metabolic rate
1 month	3.9	30	146	37	71
3 »	5.8	28	136	23	41
6 »	8.0	20	126	16	28
1 year	10.4	10	63	6	11
5 years	17.6	5	32	2	4
14 years old, girls	47.5	18	113	2	8
16 years old, boys	54.0	18	113	2	7

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Energy consumption for hard-to-account losses

Losses that are difficult to account for include losses of fat, digestive juices and secretions produced in the wall of the digestive tract and glands with feces, with exfoliating epithelial cells, with falling off covering cells of the skin, hair, nails, with sweat, and upon reaching puberty in girls - with menstrual blood. Unfortunately, this issue in children has hardly been studied. It is believed that in children over one year old it is about 8% of energy expenditure.

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Energy expenditure on activity and maintaining body temperature

The share of energy expenditure on activity and maintaining body temperature changes with the child's age (after 5 years, this is included in the concept of muscular work). In the first 30 minutes after birth, the body temperature of a newborn decreases by almost 2° C, which causes significant energy expenditure. In young children, the child's body is forced to spend 200.8-418.4 kJ/(kg • day), or 48-100 kcal/(kg • day) to maintain a constant body temperature at an ambient temperature below the critical one (28...32° C) and activity. Therefore, with age, the absolute energy expenditure to maintain a constant body temperature and activity increases.

However, the share of energy expenditure on maintaining a constant body temperature in children of the first year of life is lower, the smaller the child. Then, energy expenditure decreases again, since the body surface per 1 kg of body weight decreases again. At the same time, energy expenditure on activity (muscle work) increases in children over one year old, when the child begins to walk, run, do physical education or sports independently.

Energy cost of physical activity

Type of movement	Cal/min
Cycling at low speed	4.5
Cycling at medium speed	7.0
Riding a bicycle at high speed	11.1
Dancing	3.3-7.7
Football	8.9
Gymnastic exercises on apparatus	3.5
Sprint running	13.3-16.8
Long distance running	10.6
Ice skating	11.5
Cross-country skiing at moderate speed	10.8-15.9
Cross-country skiing at maximum speed	18.6
Swimming	11.0-14.0

In children aged 6-12 years, the share of energy spent on physical activity is approximately 25% of the energy requirement, and in adults - 1/3.

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Specific dynamic action of food

The specific dynamic effect of food changes depending on the nature of the diet. It is more pronounced with protein-rich food, less so with fats and carbohydrates. In children of the second year of life, the specific dynamic effect of food is 7-8%, in older children - more than 5%.

Costs of implementation and overcoming stress

This is a natural direction of normal life activity and energy expenditure. The process of life and social adaptation, education and sports, the formation of interpersonal relationships - all this can be accompanied by stress and additional energy expenditure. On average, this is an additional 10% of the daily energy "ration". At the same time, in acute and severe diseases or injuries, the level of stress expenditure can increase quite significantly, and this requires consideration in the calculation of the food ration.

Data on the increase in energy requirements during stress are presented below.

States	Change in energy requirement
Burns depending on the percentage of body surface burned	+ 30...70%
Multiple injuries with mechanical ventilation	+ 20...30%
Severe infections and multiple trauma	+ 10...20%
Postoperative period, mild infections, bone fractures	0... + 10%

A persistent energy imbalance (excess or deficiency) causes changes in body weight and length at all developmental and biological age indices. Even moderate energy deficiency (4-5%) can cause a child's developmental delay. Therefore, food energy supply becomes one of the most important conditions for adequate growth and development. Calculation of this supply must be carried out regularly. For most children, recommendations for the total energy of the daily diet can serve as benchmarks for analysis; for some children with special health conditions or living conditions, an individual calculation is required based on the sum of all energy-consuming components. The following methods for calculating energy expenditure can serve as an example of using general age standards of supply and the possibility of some individual correction of these standards.

Calculation method for determining the basal metabolic rate

Up to 3 years	3-10 years	10-18 years
Boys
X = 0.249 kg - 0.127	X = 0.095 kg + 2.110	X = 0.074 kg + 2.754
Girls
X = 0.244 kg - 0.130	X = 0.085 kg + 2.033	X = 0.056 kg + 2.898

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Additional expenses

Damage compensation - the basal metabolic rate is multiplied: for minor surgery - by 1.2; for skeletal trauma - by 1.35; for sepsis - by 1.6; for burns - by 2.1.

Specific dynamic action of food: + 10% of the basal metabolic rate.

Physical activity: bedridden + 10% of basal metabolic rate; sitting in a chair + 20% of basal metabolic rate; patient confined to a hospital ward + 30% of basal metabolic rate.

Costs of fever: for every 1°C of average daily increase in body temperature +10-12% of the basal metabolic rate.

Weight gain: up to 1 kg/week + 1260 kJ (300 kcal) per day.

It is accepted to form some standards of age-related energy supply for the population. Many countries have such standards. All food rations of organized groups are developed on their basis. Individual food rations are also checked against them.

Recommendations for the energy value of nutrition for children of early age and up to 11 years

	0-2 months	3-5 months	6-11 months	1-3 years	3-7 years	7-10 years
Energy, total, kcal	-	-	-	1540	1970	2300
Energy, kcal/kg	115	115	110	-	-	-

Recommendations for energy standardization (kcal/(kg • day))

Age, months	FAO/WHO (1985)	UN (1996)
0-1	124	107
1-2	116	109
2-3	109	111
3^	103	101
4-10	95-99	100
10-12	100-104	109
12-24	105	90

Calculation and correction of energy metabolism are aimed at eliminating deficiencies of the main energy carriers, i.e. primarily carbohydrates and fats. At the same time, the use of these carriers for the specified purposes is possible only with consideration and correction of the provision of many fundamentally necessary accompanying micronutrients. Thus, it is especially important to prescribe potassium, phosphates, B vitamins, especially thiamine and riboflavin, sometimes carnitine, antioxidants, etc. Failure to comply with this condition can cause conditions incompatible with life, which arise precisely with intensive energy nutrition, especially parenteral.