54- Energy metabolism in the body. Energy values of nutrients. The energy equivalent of Oxygen. Measurement of metabolic rate- direct/ indirect caliometry. Basal metabolic rate and daily energy requirements und der different physiological conditions.

Energy metabolism in the body

• Our body needs energy for anabolic and catabolic pathways. • energy from the breakdown of nutrients- carb, fat and protein consumed • Their energy contents is measurable in that they are fully “oxidised” to CO2 and water under in presence of oxygen . • Energy = ability of a system to do work

Energy is storable in different forms such as kinetic energy, thermic energy, and electrical energy and so forth. Unit of energy is J (Joule) 1 kcal- 4.18Kj

Energy values of nutrients.

Substrate	Physiologic Calorific Value<br>(kJ/g)	Energy Equivalent of Oxygen<br>(kcal/L O₂)
Fats	39.0	4.69
Proteins	17.2	4.80
Carbohydrates	17.2	5.05

The energy equivalent of Oxygen.

• It refers to the amount of energy (in kcal) released during the oxidation of nutrients per litre of oxygen (O₂) consumed.

• This value depends on the type of substrate being oxidized (carbs, fats, or proteins).

• It is determined at a specific Respiratory Quotient (RQ):

  • RQ is the ratio of CO₂ produced to O₂ consumed during metabolism.

Measurement of metabolic rate- direct/ indirect caliometry

• Methods of measuring gas exchange are used to determine:

  • The amount of O₂ utilized

  • The amount of CO₂ released

  • Measured in litres per unit of time

• Metabolic Rate:

  • The rate at which metabolic reactions use energy

  • Energy is partly used to produce ATP

  • Some energy is released as heat

• Calorimetry:

  • Method of measuring changes in body state variables to determine heat transfer

  • Can result from chemical reactions, physical changes, or phase transitions

  • Performed using a calorimeter

• Direct Calorimetry:

  • Measures the actual amount of heat produced by a subject in a small enclosed chamber

• Indirect Calorimetry:

  • Calculates heat production by measuring:

    • O₂ consumption

    • CO₂ production

    • Sometimes nitrogen waste (for protein metabolism)

Energy Intake

Nutrient	Recommended Intake	Notes
Proteins	0.8–1.3 g/kg	May increase to 1.5–2.0 g/kg in athletes, illness, or pregnancy
Fats	0.8–1.0 g/kg	Should be ~20–35% of total energy intake
Carbohydrates	4–6 g/kg (can go up to 8–10 g/kg in endurance athletes)	Focus on complex carbs; limit added sugars

• Energy intake depends on:

  • Basal Metabolic Rate (BMR)

  • Physical activity level

  • Age, sex, body composition

  • Physiological states (growth, pregnancy, illness)

BMR

Basal Metabolic Rate (BMR)

• BMR: The minimum energy expenditure required to maintain vital body functions at rest (e.g., breathing, circulation, temperature regulation).

• Measured under standard conditions:

  • Subject supine (lying flat)

  • At least 12 hours post-meal (fasting)

  • Thermoneutral environment (20–22°C)

  • Subject at physical and mental rest (after lying down for ~30 mins)

• Lower BMR values occur only during sleep

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🧬 Functions Supported by BMR

• Circulation (blood flow)

• Ventilation (breathing)

• Excretion

• Neural activity

• Hormone secretion

• Thermoregulation

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📊 Factors Affecting BMR

• Sex: Men typically have higher BMR than women

• Age: BMR decreases with age

• Body mass & surface area: Larger bodies have higher BMR

• Height

• Physiological states:

  • ↑ BMR: Physical activity, pregnancy, lactation, fever

  • ↓ BMR: Sleep, fasting, hypothyroidism

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🧪 Hormonal Regulation

• ↑ BMR:

  • Thyroid hormones: T₃, T₄

  • Catecholamines: Epinephrine, norepinephrine

  • Growth hormone, glucagon (to a lesser extent)

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📈 Typical BMR Values

• Men: ~6600–7000 kJ/day (≈ 1600–1670 kcal/day)

• Women: ~6000–6400 kJ/day (≈ 1430–1530 kcal/day)