The Battle of Brains vs. Brawn

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Do brains trump brawn? A remarkable new study of how the human body prioritizes its inner workings found that if you intensely think at the same time as you intensely exercise, your performance in both thinking and moving can worsen. But your muscles’ performance will decline much more than your brain’s will, the study found.

The results raise interesting questions about the roles that our body’s wide-ranging abilities may have played in the evolution of humans and also whether a hard workout is the ideal time to be cogitating.

Compared to almost all other animals, we humans have disproportionately large brains for our size. Our supersized cranial contents probably provided an advantage during our evolution as a species. Smart creatures presumably could have outwitted predators and outmaneuvered prey, keeping themselves fed, uneaten and winners in the biological sweepstakes to pass on their genes.

But most other species eschewed developing similarly outsized brains during evolution, because large brains carry a hefty metabolic cost. Brains are extraordinarily hungry organs, requiring, ounce for ounce, more calories to sustain their operations than almost any other tissue, and these caloric demands rise when the brain is hard at work. Thinking demands considerable bodily fuel.

In order to feed and maintain these large brains, early humans’ bodies had to make certain trade-offs, most evolutionary biologists agree. Our digestive systems shrank during evolution, for one thing, since food processing is also metabolically ravenous.

But whether a similar trade-off occurred with our muscles has remained in doubt. Muscles potentially provided another route to survival during our species’ early days. With sufficient brawn, animals, including people, could physically overpower prey and sprint from danger.

But muscles are also very calorically needy and, like brain tissue, use blood sugar as their primary fuel. So scientists have wondered whether and how early humans’ bodies balanced the fueling needs of their brains and their brawn. Did one take precedence over the other? If so, that choice could tell us something about the underpinnings of human development and also how best, even now, to manage thinking and moving.

Since experiments on cavepeople are, however, not practicable, researchers at Cambridge University in England decided instead to focus on the bodily machinations of 62 elite, collegiate rowers for their new study, which was published this month in Scientific Reports.

The scientists hoped to suss out what happens when both muscles and minds are stressed, and if one of those operations gets preferential treatment from the body.

To find out, they asked the rowers, who were all young, male and fit, to visit a university lab on three separate occasions.

During one visit, the men sat quietly while dozens of words were displayed on a large screen in front of them. The men had three minutes to memorize the words and then, immediately afterward (when the screen went dark) write down as many as they could remember. This was their mental task.

On another day, they rowed on a rowing machine as intensely as they could for three minutes while the researchers tracked their power output, testing muscular prowess.

Finally, on the last visit, they rowed for three minutes while simultaneously viewing a list of new words and then, immediately afterward, writing down as many as they could recall.

Then the researchers simply compared their performance on each task. Almost uniformly, the men had been able to produce fewer watts and recall fewer words when they performed the muscular and mental tasks together.

But the falloff in physical functioning was much steeper than the mental slump. The rowers lost almost 13 percent of their power output, a decline that was about 30 percent greater than their loss in word recall after the combined session.

“Our proposed explanation for this finding is that they were both competing for the same resource,” which in this case was blood sugar for fuel, says Danny Longman, a postdoctoral research fellow at Cambridge who led the study.

And the brain won.

The implication of this victory is that thinking probably provided more advantage for us during evolution than brawniness, Dr. Longman says, and on those occasions that both systems needed to be fed, the brain got its portion first.

Of course, this study was very short-term and viewed the tug-of-war between brains and muscles only indirectly. The researchers did not track actual changes in blood sugar uptake by any tissues. They also looked only at quite-intense exercise and used memory recall as their sole marker for thinking.

But even with these limitations, the study to some extent advances our understanding of how we became the species that we are, Dr. Longman says.

“For me, the main message of this study is a bit philosophical,” he says. “An enlarged and highly functioning brain is one of the key factors that make us human. This study demonstrated, in a very simple way, this defining characteristic of our species.”

More humbly, the results also indicate that intense workouts may not be the optimal time to compose your next epic poem or calculate tax withholding.