In the former, TEE is a linear function of physical activity, while in the latter, moderate increases in physical activity lead to increases in TEE, but after reaching higher levels of physical activity, the body compensates further increases in physical activity by reducing energy spent on other physiological activities 2. This debate is framed with two alternative models, additive and constrained 2. Another important debate focuses on the models explaining TEE in humans, activated by the recent findings that populations with different levels of physical activity presented similar TEE values 4, 5. Several hypotheses have been proposed to explain the origin of the extra energy needed to support these human traits (trade-off between organ’s size, energetic efficiency of locomotion, biocultural reproduction, and dietary changes), including the hypothesis of the evolution in humans of an accelerated metabolic rate and thus larger energy budget 1. The TEE is ultimately related to the blood available to the organism or cardiac output.įrom an evolutionary perspective, the study of TEE is linked to the human energetic paradox 1, namely, the unfolding during our lifespan of metabolically expensive traits like the growth and maintenance of a larger brain, higher rate of reproduction, high levels of physical activity, and longer lifespan than any other living hominid. In metabolic terms, this whole activity is measured by the total energy expenditure (TEE), and includes energy spent in basal metabolic processes for the correct functioning and maintenance of the body systems (basal energy expenditure), energy spent in physical activity, thermoregulation and digestion of food, and energy invested during growth and reproduction 1, 2, 3. It is a measure of the quantity of blood available for the whole organism physiological activity during its different life stages. The cardiac output is the product of the amount of blood pumped from a ventricle in a single heartbeat (stroke volume), and the number of heart beats per minute (heart rate). An increased adjusted cardiac output, underlying higher total energy expenditure, would have been a key process in human evolution. It is absent in great apes, and present in humans and Neanderthals, large-brained hominins with an extended life cycle. Finally, we present a first study of cardiac output in the skeleton through the study of the aortic impression in the vertebral bodies of the spine. The limited variation of adjusted cardiac output with sex, age and physical activity supports the compensation model of energy expenditure in humans. We also use data from the literature to show that over the human lifespan, cardiac output and total energy expenditure follow almost identical trajectories, with a marked increase during the period of brain growth, and a plateau during most of the adult life. When compared to gorillas and chimpanzees, humans present an increased body mass adjusted aortic root diameter. To show the relationship between cardiac output and energy expenditure in hominid evolution, we study a surrogate measure of cardiac output, the aortic root diameter, in humans and great apes. This budget is ultimately related to the cardiac output, the product of the blood pumped from the ventricle and the number of heart beats per minute, a measure of the blood available for the whole organism physiological activity. Humans have a larger energy budget than great apes, allowing the combination of the metabolically expensive traits that define our life history.
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