Introduction

To maintain physiologic functions, the human body continuously expends energy by oxidative metabolism. This energy is used to maintain chemical and electrochemical gradients across cellular membranes for the biosynthesis of macromolecules such as proteins, glycogen, and triglycerides, and for muscular contraction. Another part of the energy is lost as heat because of the inefficiency of metabolic transformations. Ultimately all the energy produced by the organism is dissipated as heat.

The energy expended by an individual can be assessed by two different techniques: indirect and direct calorimetry. The term indirect calorimetry stems from the fact that the heat released by chemical processes within the body can be indirectly calculated from the rate of oxygen consumption (VO2). The main reason for the close relation between energy metabolism and VO2 is that the oxidative phosphorylation at the respiratory chain level allows a continuous synthesis of adenosine triphosphate (ATP). The energy expended within the body to maintain electrochemical gradients, support biosyn-thetic processes, and generate muscular contraction

Bomb calorimetry (food, feces, urine)

Indirect calorimetry (VO2, VCO2)

Direct calorimetry (Total heat losses)

cannot be directly provided from nutrient oxidation. Almost all chemical processes requiring energy depend on ATP hydrolysis. It is the rate of ATP utilization that determines the overall rate of substrate oxidation and therefore VO2. With the exception of anaerobic glycolysis, ATP synthesis is coupled with substrate oxidation. The biochemical theory of oxidative phosphorylation considers that 3 mol of ATP are generated per gram-atom of oxygen consumed (i.e., a P:O ratio of 3:1). The energy expenditure per mole of ATP formed should be calculated from the heat of combustion of 1 mol of substrate, divided by the total number of moles of ATP generated in its oxidation. It is interesting to note that each mole of ATP generated is accompanied by the release of about the same amount of heat (~75kJ/mol ATP) during the oxidation of carbohydrates, fats, or proteins. Because there is a proportionality between VO2 and ATP synthesis, and because each mole of ATP synthetized is accompanied by the production of a given amount of heat, one understands the rationale of using VO2 measurement to calculate heat production within the body.

Since indirect calorimetry measures the heat released by the oxidative processes and direct calori-metry assesses the heat dissipated by the body, a relationship exists between the two: for a subject in resting conditions, the difference between metabolic heat production and heat dissipation represents the body heat balance (Figure 1). The heat production from oxidative processes is equal to the sum of

Energy balance

Heat balance

Substrate oxidation (CHO, fat, prot.)

Energy storage e ©

Energy mobilization

Heat gain (body temp.)

Heat losses (body temp.)

Indirect calorimetry (VO2, VCO2 , Nu)

Substrate oxidation (CHO, fat, prot.)

Figure 1 Heat balance, energy balance, and substrate balance: three different concepts.

Substrate mobilization (lean, fat tissue, glycogen)

Bomb calorimetry (food, feces, urine)

Indirect calorimetry (VO2, VCO2)

Direct calorimetry (Total heat losses)

Food analysis

Indirect calorimetry (VO2, VCO2 , Nu)

Figure 1 Heat balance, energy balance, and substrate balance: three different concepts.

the nonevaporative components (radiant heat exchange + convective + conductive heat transfer) plus the evaporative heat transfer. In order to assure the equality of the equation an additional term representing the rate of storage of body heat must be included:

Heat production = Heat losses ± Heat storage

Heat storage can be positive when excess heat is gained, resulting in a rise in internal body temperature. Heat storage can be negative when excess heat is lost, resulting in a cooling of the body. The rate of heat storage can be estimated from the body weight, the specific heat capacity of the body (which depends upon body composition), and the rate of change in internal body temperature. In practice, this calculation remains somewhat uncertain since the changes in temperature within the body are not uniformly distributed within each tissue.

Under most environmental conditions, heat is lost by all channels (i.e., radiative + convective + conductive + evaporative). However, except during immersion in water, the rate of heat gain or loss by conduction constitutes a small proportion of the total heat loss (typically 3%). Heat can be lost by convection (air currents) but it can also be gained in very hot conditions such as in a desert characterized by high movement of hot air.

Lower Your Cholesterol In Just 33 Days

Lower Your Cholesterol In Just 33 Days

Discover secrets, myths, truths, lies and strategies for dealing effectively with cholesterol, now and forever! Uncover techniques, remedies and alternative for lowering your cholesterol quickly and significantly in just ONE MONTH! Find insights into the screenings, meanings and numbers involved in lowering cholesterol and the implications, consideration it has for your lifestyle and future!

Get My Free Ebook


Post a comment