Physiological Application Two Examples Example

Expressing heat production relative to body mass is required when comparing energy expenditure rates between individuals that differ in size. Age and gender-specific resting energy expenditure (REE) norms based on body weight and stature-derived were developed in the early 1900s by Kleiber and showed that adult mammals differing widely in body size had similar metabolic rates relative to body weight raised to the 0.75 power. Two components are usually considered as representative of whole-body metabolically active tissue, body cell mass (BCM), and FFM. BCM is typically estimated as the exchangeable potassium space that can be measured by total body potassium. The FFM component can be measured using two-component body composition methods.

In studies assessing REE, FFM is considered the principal contributor to energy requirements, and is commonly used as a surrogate for metabolically active tissue. However, this practice is inherently flawed as it pools together numerous organs and tissues that differ significantly in metabolic rate. The brain, liver, heart, and kidneys alone account for approximately 60% of REE in adults while their combined weight is less than 6% of total body weight or 7% of FFM. The skeletal muscle component of FFM comprises 40-50% of total body weight (or 51% of FFM) and accounts for only 18-25% of REE. REE varies in relation to body size across mammalian species. Within humans, REE per kg of body weight or FFM is highest in newborns (^56 kcal kg-1 day-1), declines sharply until 4 years, and slowly thereafter reaching adult values (~25kcalkg-1day-1). Among adults,

REE is lower in the later adult years, to an extent beyond that explained by changes in body composition. That is, the loss of FFM cannot fully explain the decrease (5-25%) in REE in healthy elderly persons.

Recent attention has been given to modeling REE based on available information on organ- and tissue-specific metabolic rates combined (Table 3) with the mass of these tissues as determined by MRI. Whole-body REE can be calculated from organ- tissue mass (REEc) and then compared to REE measured using indirect calorimetry (REEm) for individuals or groups. REE (in kJday-1) of each organ- tissue component (subscript i) can be calculated using the following equation:

where OMR (organ metabolic rate) is the metabolic rate constant (in kJ per kg per day) for each organ-tissue component (Table 3) and M is the mass of the corresponding organ/tissue (in kg). Whole-body REE (in kJ per day) is calculated as the sum of the seven individual organ-tissue REE

The whole-body REE equation is:

REEC = 100S X Mbrain + S40 X Mliver X Mheart + 1848 X Mt├│dneys + x Msm + 19 X MAT + 50

Mresidual [3]

This approach has allowed for the hypothesis to be tested that the proportion of FFM as certain

Table 3 Organ and tissue coefficients used in developing models
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