Figure 1. Human brain is the most metabolically demanding organ in the body.

The Selfish Brain: A Trade-off Between Cognitive And Physical Performance in Humans

By Maryna Mullerman ’20

Figure 1. Human brain is the most metabolically demanding organ in the body.
Figure 1. Human brain is the most metabolically demanding organ in the body.

The brain metabolizes more glucose a simple and widely accessible sugar than any other organ in the human body. Daniel Longman and researchers from the University of Cambridge in the United Kingdom explored the trade-off involving the brain and muscles, or physical performance, in people. The study investigated how mental and physical activities are affected when they are performed simultaneously and separately.

62 young male rowers were asked to participate in three protocols. Protocol A involved consistent, free-rate rowing for 3 minutes with an average power output. The next test included a mental task; for 3 minutes, the participants were shown a large screen with 75 words to remember. Each word appeared only once. In Protocol B, the rowers had to recall and write down as many words as possible in 5 minutes. In Protocol C, the words had to be recalled immediately after rowing, a physically-demanding activity. The power output for rowing was calculated for Protocols A and C.

The results suggested that the rowing pace and the number of words recalled were independent of testing conditions. The effectiveness of mental recall and physical activity declined when two tasks were performed at the same time. Also, the power output was found to be lower in Protocol C than in Protocol A. The number of correct words recalled displayed a decline from Protocol B to Protocol C.

The researchers supported their hypothesis by showing an impaired performance of cognitive and physical tasks when two activities were done simultaneously. Moreover, physical activity was found to be affected more than mental activity, showing that compared to power output the decrease in brain function was notably less. The researchers suggested that when brain and muscles compete for a finite supply of blood glucose and oxygen, the brain would receive more resources. Although our body accounts for the optimal distribution of resources, scientists think that the evolution of the human brain has a cost of muscle loss. This study’s results supported the selfish brain hypothesis, the mechanism of which can be the incentive for future research in this field.



  1. D. Longman, et. al., A trade-off between cognitive and physical performance, with relative preservation of brain function. Scientific Reports 7, (2017). doi: 10.1038/s41598-017-14186-2.
  2. Image retrieved from:

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