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Thermodynamic analyses are performed to quantify allocation of the nutritional energy and exergy to most of the life processes by pregnant mice. In these analyses, ‘internal work performance’ is calculated for the first time in the literature for metabolism during pregnancy and found substantially higher than the ‘external work performance’. Variation of the daily entropy generation rates and the daily internal work performance rates during the course of pregnancy showed a highly similar phasic behaviour. With the progression of the pregnancy, external work performance decreased and second law efficiency increased significantly. On the 13th day of pregnancy, net energy extracted from the food at the cellular energy metabolism subsystem was 15·0 kJ; approximately 3 kJ of it was employed for daily internal work performance, 0·8 kJ was allocated to daily external work performance and 0·8 kJ was stored in the adipose tissue without entering into the cellular energy metabolism subsystem. Heat generation in association with internal and external work performance was 9·1 and 2·2 kJ, respectively. Energy, pertinent to the first law, and exergy (useful energy), pertinent to the second law, balances are described graphically, and comparison of these plots showed that the total exergy of the nutrients allocated to internal and external work performance and heat generation is substantially smaller in magnitude when compared with those of energy balance.

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The consequences of energy storage in the body as fat and then reusing it in the metabolism are assessed for seven cases by referring to entropy generation as the criterion for assessment: Case 1: Glycogen and lipids are stored by a person by dieting and then reused. Case 2: Glucose is converted into fat and then reconverted it into glucose spontaneously. Case 3: 2400 kcal of high-fat diet induced fatty liver disease and then a low-fat diet is consumed to recover. Case 4: Intermittent fasting by skipping a meal up to 30 days/year. Case 5: Salmon store fat in its body and reuse it later during its journey back to its spawning place. Case 6: Entropic assessment of the female arctic squirrels in hibernation is done. Case 7: Mean exergy cost of active field metabolism of homeothermic/ endothermic birds and mammals is compared. Assessments show that energy storage and its reuse generated negligible aging entropy in humans, intermittent caloric restriction via skipping a meal for 30 days in a year may extend the life span approximately 3%. Migrant or hibernating animals store energy and then reuse it with absolutely different purposes than the humans and generate most of their life span entropy.

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