Human Body Exergy Metabolism

Carlos Eduardo Keutenedjian Mady
3.681 1.032


The exergy analysis of the human body is a tool that can provide indicators of health and life quality. To perform the exergy balance it is necessary to calculate the metabolism on an exergy basis, or metabolic exergy, although there is not yet consensus in its calculation procedure. Hence, the aim of this work is to provide a general method to evaluate this physical quantity for human body based on indirect calorimetry data. To calculate the metabolism on an exergy basis it is necessary to define the reference reactions and obtain their exergy variation. The reference reactions of the energy substrates are represented by the oxidation of the glucose, palmitic acid and a representative amino acid. Hence, from the exergy variation of these reactions and the consumption rate of the substrates, the metabolic exergy is determined. Results, for basal conditions and during physical activities, indicate that the difference between exergy and energy metabolisms is lower than 5%. Moreover, the body converts approximately 60% of the exergy of nutrients into available exergy to perform work.


Exergy Analysis; Human Body; Metabolic Exergy

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Glucose Palmitic Ac. Amino Ac. Hayne (2008) ‒16506 ‒39141 ‒18964

Cortassa (2002) ‒16516 ‒39223 ‒17578

Finally, the metabolisms on energy and exergy basis using the data from Hayne (2008) and from Cortassa et al.

(2002) are 2 2 11371 2366 6891 Hayne O CO N

M m m m    (18) , 2 2 9501 3963 6979 M Hayne O CO N

B m m m    (19) 2 2 11179 2502 1580 Cortassa O CO N

M m m m    (20) , 2 2 9558 3928 1823 M Cortassa O CO N

B m m m    (21) 2 Energy and exergy metabolism 1 Basal conditions Results in Table 5 indicate the metabolism on energy and exergy basis, considering the oxidation of proteins (M and B M ) and disregarding the oxidation of proteins (M p and B Mp ) for the energy measurements obtained by Hardy & Du Bois (1938). For this condition, the authors obtained that the metabolism is 79.8W. In all cases the difference between this value and the ones calculated herein was not larger than 2%. Furthermore, the difference of the metabolism using the two different references of thermodynamic properties did not differ more than 2%. The ratio of metabolism on energy and exergy basis

(considering and disregarding the oxidation of proteins) did not exceed 1.02. Hence, as in Batato et al. (1990), the approximation B M ≈ M for basal conditions is valid.

Table 5. Metabolism in energy and exergy basis with the oxidation of amino acids (M and B M ) and without the oxidation of amino acids (M p and B Mp ). Hayne (2008) (W) Cortassa et al. (2002) (W) M 3 9 M p 3 1 B M 8 7 B Mp 9 0 2 Physical activities For the experimental results of Figure 2 the metabolism in energy and exergy basis was calculated, using the values of thermodynamic properties of Cortassa et al. (2002) indicated in Eqs. (20) and (21), due to the small difference between results in Table 5. Figure 3 indicates M, B M and B QM for the experimental data of Figure 2. The difference of B M and B QM is one order of magnitude, indicating that when the metabolism is calculated as B QM (taking into account only the thermal exergy), more than 95% of the exergy content of metabolism is disregarded. Therefore, most of the available exergy to perform work would be disregarded. In Figure 4 it is demonstrated the ratio of the metabolism on energy and exergy basis as a function of time. In the first five minutes, the ratio B M /M ranged from 04 to 1.01 (time when the subject controlled the Figure 5. Ratio of the metabolism considering and disregarding the oxidation of proteins during the treadmill test.

Figure 6 Result of B M , W MAX and B QM as a function of time during the treadmill test. The first two properties are indicated in the left axis, the last on in the right side axis. Table 6 indicates the metabolism on exergy basis, the maximum available work, the exergy rate associated with metabolism (B QM ) and the ratio B M /M. It is possible to note that the thermal portion of metabolism is approximately 5% of exergy metabolism and the maximum available work from ATP hydrolysis corresponds to 60% of exergy metabolism. Hence, for the 11 runners more than 95% of the exergy released from the oxidation of nutrients would be neglected if B QM be considered as the exergy source of the body, whereas in fact the body uses 60% of the exergy content of nutrients. The ratio of metabolism in energy and exergy basis ranged from 1.01 to 1.03, indicating that the approximation B M ≈M is valid for physical activities. Conclusions In this work analyses of the human metabolism on energy and exergy basis were performed and it was proposed a method and an equation to calculate the metabolic exergy. From the range of tests analyzed it was possible to conclude that: ▪ For basal conditions results the metabolism on energy and exergy basis did not differ more than 2%, for the different thermodynamic properties. For basal conditions, results obtained from Batato et al. (1990) were verified; ▪

For the treadmill running tests, the metabolism on energy and exergy basis are very close. The ratio is equal to 05 in only one point; in the rest of the test the ratio was lower. ▪ Finally, the contribution of proteins did not exceed 3% of the total metabolism (energy and exergy basis) during physical activities and in basal conditions. Hence, the oxidation of proteins may be disregarded in a healthy person in basal conditions and under physical activities. ▪ The exergy released in ATP hydrolysis is the maximum available work that the body can obtain from the oxidation of energy substrates. Hence, the body keeps approximately 60% of the exergy content of nutrients in the chemical bounds of ATP. Acknowledgments: The authors acknowledge FAPESP (São Paulo Research Foundation) for his PhD grant 09/17578‒0 and CNPq (National Research Council) for grant 306033/2017-7. The authors also acknowledge the Sports medicine group and FIFA Medical Center of Institute of Orthopedics and Traumatology of the University of São Paulo Medical School. Nomenclature: ADP adenosine diphosphate ATP adenosine triphosphate b specific exergy, J/kg B body exergy, J B exergy rate and flow rate, W g specific free energy, J/kg G

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