This Is Your Brain on Exercise

Many of my students are obsessed with some form of exercise. They are driven to run 10 to 20 miles, or swim four hours, every day, no matter how inconvenient to their daily schedule or how much physical pain it produces. These are my obligatory athletes. Frequently, they come to class and promptly fall asleep from exhaustion. Needless to say, their classroom performance suffers due to their extreme exercise.

People exercise for many different reasons. Often, how you exercise is determined by who you are, young or old, male or female, or what you wish to achieve by exercising. Some people exercise to lose weight, to develop strength or endurance, to relieve boredom, to spend more time with friends or make new ones or, conversely, to get away from everyone and everything by taking a long walk alone. Except for losing weight, exercise easily achieves all of these goals. Exercise makes our bones denser and muscles stronger while it improves vascular flow and tissue oxygenation.

Your body clearly benefits from a daily moderate level of exercising. Does your brain care?

Yes, your brain does pay attention to whether you are contracting your muscles, just not always in the ways, or for the reasons, that you might think. In contrast to your body, your brain benefits most when you perform activities that it evolved to perform—to move around your environment with purpose; movement for diversion or sport is a waste of valuable energy. Your brain benefits when the movement addresses its unique evolutionary priorities related to survival and procreation. Your brain does reward you for moving to explore your environment as much as possible so that you can discover the two most important things out there: food and sex.

Personal survival and the continuation of the species are what brains evolved to achieve. In return for finding food and sex, your brain rewards you, very briefly, with euphoria. Exploration requires movement and movement requires food. The utilization of food for movement is why things get very complicated with regard to answering the question of whether your brain benefits from exercise. Obtaining energy from food requires the utilization of oxygen. Thus, determining whether exercise benefits your brain depends upon finding a balance between your body’s requirement for oxygen and your ability to defend yourself from it.

Moderate exercise adapts your tissues to the consequences of the increased oxidative metabolism required to support movement, while too much exercise, particularly over time, pushes metabolic demands to the point that your brain’s innate protective systems are overwhelmed; the increasing levels of toxic chemicals, called reactive oxygen species (ROS), that lead to cellular injury and accelerated aging. For the brain, the abrupt increase in ROS levels induced by extreme exercising is much more harmful due to its own very high metabolic rate. This is partially why excessive exercising is so harmful to the brain. Moderate levels of exercise are less harmful to the brain because it produces lower levels of harmful ROS molecules that the body and brain are capable of neutralizing.

What is moderate exercise? Brisk walking, a gentle swim, mowing the lawn, bicycling at a slow pace, tennis (doubles, not singles), ballroom dancing, holding most yoga poses, gardening, or almost any activity that you can maintain for one or two hours and that requires some effort but not so much that you cannot talk while doing them. Most of these exercises are shared experiences that are enjoyable and thus more likely to be included in your weekly schedule.

Moderate exercise also offers many benefits via the production and release of a complex blend of molecules from muscles that modestly improve a range of brain functions. Daily moderate exercise may also delay the functional decay associated with normal aging and many common neurodegenerative diseases that depend on the long-term presence of brain inflammation. The modest improvements in mood or learning and memory abilities in response to daily moderate levels of exercise are most likely related to the reduction in inflammation and augmentation of the blood flow following the influx of the protein VEGF into brain. However, many other molecules also play an important role in communicating between your muscles and brain.

One of the most studied, and certainly over-hyped, of these molecules is BDNF. BDNF influences bodily functions that are indirectly related to exercising, such as regulating energy homeostasis and by modulating glucose metabolism in peripheral tissues. Via these actions, BDNF mediates the beneficial effects of moderate exercise by stimulating glucose transport into muscle cells and their numerous mitochondria. This vital role might explain why exercising is associated with such high levels of BDNF in the blood and muscles. However, although muscles produce BDNF, they do not release it into the blood. The BDNF in the blood comes entirely from platelets. Science is still without a viable mechanism to explain why blood platelets release so much BDNF following exercise. Whatever the reason, it probably has nothing to do with the brain. Most recent studies have failed to find a robust relationship between the increase in blood levels of BDNF and learning and memory abilities. The main reason is because BDNF is not capable of passing through the blood-brain barrier.

The brain is informed about the activity of muscles by the entry of a diverse group of hormones, including beta-hydroxybutyrate, lactate, Cathepsin B, Lipocalin-2, VEGF, irisin, PGC-1alpha, and others during and after exercise that may significantly benefit brain health and cognitive function. One interesting benefit of exercise on the brain is an increase in neurogenesis in response to the entry of beta-hydroxybutyrate from the blood.

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BDNF is produced within the brain in response to exercise. The specific actions of BDNF in the brain remain a mystery. However, neuroscientists have discovered some potential mechanisms. One possible explanation is that BDNF, working together with VEGF, increases the volume of a brain region that is critical for learning and memory; this region is called the dentate gyrus. The increase in volume of the dentate gyrus is most likely due to the growth of new blood vessels and not the birth of new neurons—most of your newborn neurons probably do not survive very long.

The improvements in learning and memory abilities in response to regular moderate exercise are today believed to be due to the augmentation of the blood flow to regions of the brain that are responsible for these abilities. This makes intuitive sense given that regional changes in blood flow are directly related to which brain regions are being utilized at any point in time. The increased blood flow also reduces the level of inflammation due to ROS formation. Essentially, the benefits of exercise on the brain require a delicate dance between various molecules so that the benefits of exercise outweigh the risks.

Exercise is not a panacea. With regard to depression, exercise is not better than drugs; however, daily moderate exercise should be considered as an important adjunct therapy to any treatment plan for depressed or anxious adults. The best advice is to do both: get regular exercise and take your medications. With regard to the aged or injured brain, regular exercising, if the person is able, will offer modest, but usually reliable, cognitive benefits. The main caveat for most old adults is that the greater the degree of cognitive decline a person exhibits prior to beginning any exercise routine, the less likely is the person to benefit from exercise.

For most people, under most conditions of good general health, the best advice to achieve good brain health has not changed since it was first written down three millennia ago by Homer in the Odyssey: “In all things, moderation.”