How Exercise Changes Your Brain and Your Mood

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You exercise to keep your muscles strong and your stamina high, along with your buff ratings. Perhaps you also exercise for your cardiovascular fitness. Now here’s further motivation to move that body: exercise changes your brain chemistry to promote better mood and memory.

Exercise works, and not just because of the buzz you get from having gone out and done something. Yes, exercise triggers the release of endorphins, chemicals that create a sense of euphoria. But there is much more to it than that.

Beyond the runner’s high lies a biochemical process that is part of the brain’s growth and repair system. Central to that process is a protein called brain-derived neurotrophic factor (BDNF). BDNF protects existing brain cells from damage. It strengthens the synapses (a synapse is the structure between nerves, through which messages are transmitted), and helps form new synapses. BDNF is also involved in a process called neurogenesis, the creation of new brain cells.

Think of BDNF as a benevolent force that nurtures and cares for its charges, the neurons in your brain. It does so at every life stage, including old age.

In children, clinical studies have shown that exercise increases brain volume and improves cognition in those with reading difficulties. Children have highly plastic brains. Plastic in this sense means malleable and able to form new connections, especially in response to learning. This plasticity is BDNF-dependent.

Even in adulthood, the brain remains plastic. Exercise enhances plasticity, allowing the brain to change — and therefore heal, with the help of BDNF. However, low BDNF may be an indication of several psychiatric disorders. Those disorders where BDNF levels have been found to be reduced include major depressive disorder (MDD), bipolar disorder and schizophrenia. The lower the BDNF, the more severe the symptoms.

The low level of BDNF found in depression has been described as “remarkable”.

Major depressive disorder affects up to 20% of people in the United States, at some time in their lives. The World Health Organization predicts that by 2030, it may be the most common cause of disability and mortality worldwide.

More people than ever before are now on antidepressants. An estimated 10% of Americans and 17% of the adult population in England take presecribed medication for depression.

One of the ways that antidepressants work is by raising BDNF levels. When given to healthy rats, various classes of antidepressants, from SSRIs to monoamine oxidase inhibitors, increase BDNF activity in the brain.

The same happens to humans following treatment with antidepressants.

Depression is not the only outcome: low BDNF is associated with various neurological disorders, including Alzheimer’s disease, Parkinson’s disease, autism, and epilepsy.

BDNF is found abundantly in the hippocampus, the area of the brain associated primarily with memory. This seahorse-shaped structure has a left and right component and is part of the limbic system, located deep within the brain. It is involved in spatial memory and the transfer of short-term memories to long-term storage. In Alzheimer’s disease, the hippocampus is damaged and there is loss of short-term memory.

Exercise does two positive things for memory and cognitive function. First, it increases the size of the hippocampus, leading to improvements in memory and reversal of age-related loss in volume. Shrink that hippocampus and you risk impaired cognitive function, leading to dementia.

Second, exercise increases the amount of BDNF in the hippocampus and other regions. A sedentary lifestyle, on the other hand, results in reduced levels of BDNF.

Other areas of the brain that are important for cognitive function, including the prefrontal cortex, are also expanded by aerobic exercise. The prefrontal cortex is involved in decision-making, planning, problem-solving and behaviour.

That’s what we all want to know, though there is no definitive answer. When it comes to depression, a review of the literature on the subject, published in 2019, gives us a rough idea.

Trials where participants exercised for eight weeks or more, at least three times a week for an hour, or four times a week for 30–45 minutes, resulted in increased BDNF. Trials where the participants exercised for six weeks or less, three times a week, for 45 minutes, resulted, overall, in no significant change.

And what about cognitive faculties? One study, published in 2006, followed and assessed 1,740 individuals aged 65 or above and based in Washington. None had cognitive impairment, and all scored well in brain screening tests. Around six years later, it was found that those who exercised three or more times a week were significantly less likely to develop Alzheimer’s than people who exercised less than three times per week.

In another study, one group of 30 people jogged for at least 15 minutes a day, for 60 days. A second, control group of 30 did no exercise for 60 days. At the end of the test period, BDNF levels were measured, and cognitive function assessed. The exercise group scored significantly better on the Mini-Mental State Examination, a 30-point questionnaire widely used in clinical settings. The exercisers also had significantly higher levels of BDNF than the non-exercise group.

The type of exercise you do is important; it seems that it has to be aerobic. BDNF responds well to aerobic exercise but not to strength training, which appears to have no influence on BDNF. Even so, you don’t have to run yourself into the ground to raise those BDNF levels: high intensity activity is effective, but so too is moderate activity: both have been shown to have positive effects on cognitive functioning.

So, unfortunately it is not enough to sit around doing sudokus and crosswords; intellectual challengeswon’t mitigate your vulnerability to neurodegenerative disorders as you age.