Abstract
Debate surrounds the issue of how the large, metabolically expensive brains of Homo sapiens can be energetically afforded. At the evolutionary level, decreased investment in muscularity, adiposity and the digestive tract allow for a larger brain. Developmentally, high neo-natal adiposity and preferential distribution of resources to the brain provide an energetic buffer during times of environmental stress. Through an experimental design, we investigated the hypothesis of a trade-off involving brain and muscle at the acute level in humans. Mental performance was measured by a free-recall test, and physical performance by power output on an indoor rowing ergometer. Sixty-two male student rowers performed the two tests in isolation, and then again simultaneously. Paired samples t-tests revealed that both power output and mental performance reduced when tested together compared to in isolation (t(61) = 9.699, p < 0.001 and t(61) = 8.975, p < 0.001). Furthermore, the decrease in physical performance was greater than the decrease in mental performance (t(61) = −2.069, p = 0.043). This is the first investigation to demonstrate an acute level trade-off between these two functions, and provides support for the selfish brain hypothesis due to the relative preservation of cognitive function over physical power output. The underlying mechanism is unclear, and requires further work.
Highlights
The development of an enlarged and elaborated brain is considered a defining characteristic of human evolution[1]
The adult brain almost exclusively derives its energy from the metabolism of glucose[28]
This, coupled with its high energetic demand, ensure that the brain metabolises the most glucose of any organ[29,30]
Summary
The glucose demands of skeletal muscle increase significantly with activation[36,37,38,39,40] In such circumstances, skeletal muscle thereby becomes a powerful competitor of the brain for glucose and oxygen[41]. At high levels of activation both are reliant upon glucose metabolism, and require a high rate of oxygen and glucose supply. Should both be challenged simultaneously, competition for these valuable yet limited resources may develop, with one or both organs receiving an insufficient supply for optimal performance. It is further hypothesised that the relative decrease in muscle power output will exceed the relative decrease in cognitive function
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