What are the effects of exercise on the brain?

The theme of much of the research described in this chapter is how much influence your brain has on the way you exercise. But it works the other
way around too: the exercise you do has wide-ranging effects on your brain, with the power to alter mood, memory, and even the structure of the
brain itself. Over the long term, there’s not much doubt that exercise makes you smarter. Studies in rodents have shown that physical activity makes
brains develop denser and more complex connections between neurons and stimulates the growth of new brain cells. These effects are especially
important during adolescence and early adulthood, when your central nervous system is developing rapidly and taking the shape it will maintain,
more or less, for the rest of your life.
A massive Swedish study published combed through the records of 1.2 million 18-year-olds who had taken compulsory military
screening exams between 1950 and 1976. The first finding was that aerobic fitness, but not muscular strength, was associated with greater
intelligence. But it wasn’t just being fit that helped—getting fit also offered a major boost. Those who had gained the most aerobic fitness from 15
to 18, as assessed from their high school phys ed marks, scored far better on the cognitive tests than those who had lost fitness. Since 268,496 of
the subjects were brothers, the researchers were also able to determine that the links between aerobic fitness and intelligence were primarily due
to environmental factors like exercise, rather than genetic factors.
The fact that aerobic exercise improves intelligence but strength training doesn’t may come as a surprise. Researchers believe that many of
exercise’s neural benefits relate to whole-body effects such as increased blood flow: getting your heart pumping in a cardio workout carries more
blood, along with helpful growth factors, to your brain. A study by University of North Carolina researchers used magnetic resonance
angiograms to determine that elderly subjects who did regular aerobic exercise had more small blood vessels in their brains, and fewer twists and
turns in those vessels, compared with non-exercising controls. The benefits of strength training, in contrast, tend to be limited to the muscles you’re
using.Although it takes time to rewire your brain, you can tap into some of exercise’s brain-boosting benefits almost instantly. researchers at
the University of Illinois at Urbana-Champaign put 21 volunteers through a set of tests to assess working memory (the ability to remember
something and then retrieve it for use a short time later) immediately after a 30-minute session of either aerobic or resistance exercise, and then
repeated the tests half an hour later. The aerobic exercisers improved their reaction time on the post-exercise test and improved it even more on
the second test; the strength trainers, on the other hand, were no different from controls who hadn’t exercised at all. These findings apply only to the
specific working memory task that was tested, but they suggest that the mental benefits of aerobic exercise start right away.

There are also indications that more (or harder) exercise produces greater cognitive gains. Another study, from Taiwan’s National
Cheng Kung University, found that mice forced to run on a treadmill made greater cognitive gains than mice that ran at leisure on an exercise wheel
(though both groups did improve). But there are limits. Extreme exertion like running a marathon generates stress hormone levels comparable to
those seen in military interrogations and first-time parachute jumpers, which can interfere with some mental processes. Researchers tested 141
runners immediately after they completed the Boston or New York marathons and found that their “explicit memory,” which answers questions like
“What happened an hour ago?,” was impaired. On the other hand, their “implicit memory,” measured by the ability to complete partial words, was
enhanced.
New results in this area continue to be published on a regular basis, so it won’t be long before we’re able to say with certainty why the extreme
stress of a marathon helps some mental processes and hurts others, or which particular exercise-produced growth factors are key to generating
new brain cells. For now, the advice is simple: keep doing all the exercise that’s recommended for a healthy cardiovascular system, and you’ll get a
mental edge as a bonus.