It has long been recognized that exercise produces feelings of psychological and emotional well-being and that aerobic fitness helps prevent age-related physical decline. Only in recent years, however, have scientists begun to unravel the myriad of complex physiological relationships between exercise and brain health.
Exercise has profound effects on brain health in everyone from the youngest children to the elderly. Indeed, exercise may exert its strongest effects on the aging brain. Studies have consistently shown that exercise, especially aerobic exercise, improves cognitive function in children and adults, including memory, attention, processing speed, and the ability to form and act on plans. Exercise helps preserve memory and other cognitive functions that can deteriorate with age. It also helps prevent age-related neurodegenerative diseases and other disorders and can even improve their symptoms. Large studies have found that even modest activity and movement, such as cooking, cleaning, or gardening, can have remarkable effects on cognitive functioning and the maintenance of brain health. In addition to its psychological, emotional, and cognitive effects, exercise appears to contribute to mental fitness, resilience, and the ability to respond to mental challenges. Exercise can even alleviate some symptoms of mental illness. Because of the significant impact of exercise on physical and mental health, the Centers for Disease Control and Prevention (CDC) and the American College Sports Medicine (ACSM) recommend that all adults get at least 150 minutes per week of moderate-intensity aerobic exercise, along with strength training, and that children and adolescents get at least 60 minutes of daily physical activity.
Exercise affects brain health by both direct and indirect mechanisms that are only beginning to be understood. Exercise confers numerous psychological benefits, such as a sense of accomplishment and improved motivation and self-esteem. Anxiety and stress are known to contribute to cognitive impairment and neural degeneration, and exercise can reduce both, in part by improving both mood and sleep. Many types of exercise require focus, concentration, and other cognitive skills, and practicing and strengthening these skills can aid in other daily activities. For example, the focus required for strength training (resistance exercise) has been found to improve brain functioning. Thus, although most studies of the effects of exercise on brain health have focused on aerobic activities, skillbased activities may have even more impact on cognitive function than purely aerobic exercise.
Perhaps the most dramatic effect of exercise on brain health is increased brain volume. Numerous studies have shown that the prefrontal cortex and medial temporal cortex, areas of the brain that control thinking and memory, are larger in people who exercise. Furthermore, 6–12 months of regular, moderate-intensity exercise is associated with the growth of new blood vessels as well as increased volume of certain brain regions. One study of older adults reported that six months of aerobic exercise significantly increased both gray and white matter compared to six months of nonaerobic stretching and toning exercises. Gray matter is made up of nerve cell (neuron) bodies, dendrites, and axon terminals at synapses (the spaces between neurons), and more gray matter is generally associated with better brain health. A 2016 report analyzed data from the Cardiovascular Health Study of nearly 6,000 older men and women who had been followed since 1989. Brain scans of the top one-fourth most active individuals showed substantially more gray matter in areas associated with memory and higher-level thinking. Furthermore, individuals whose physical activity had increased over the previous five years showed significant increases in gray matter in those areas of the brain. Repetitive gross motor movements strengthen the branching of dendrites that conduct signals to nerve cell bodies, and the more branched dendrites, the more information that the brain is able to store and transmit. White matter consists of nerve fibers (axons) that connect neurons and is associated with mental processing speed. Exercise has been shown to increase the density as well as the volume of white matter.
Exercise stimulates blood flow throughout the body, including the brain, where it also stimulates the growth of small blood vessels. Increased blood flow in the brain improves transmission of nerve impulses across synapses. Improved blood flow to the brain promotes more and stronger connections between neurons and has been shown to improve memory, attention, and overall cognitive function. Strengthening the cardiovascular system with exercise also delivers more oxygen and glucose to the brain for fuel. Some researchers believe that increased blood flow to the brain may be the most important effect of exercise on brain health, in part because it improves blood flow throughout the day, even while at rest. Finally, exercise reduces inflammation in the brain and helps protect against the development of vascular (blood vessel) disease that can harm the brain.
Exercise stimulates the release of hormones called endorphins, which are natural opiates, and neurotransmitters such as dopamine and serotonin. These chemicals can relieve pain and promote motivation and feelings of pleasure and well-being. Serotonin influences mood. Dopamine is important for cognition, working memory, learning, and the ability to experience pleasure. Exercise also increases levels of the neurotransmitters glutamate, which stimulates nerve activity, and gamma-aminobutyric acid (GABA), which inhibits neuronal activity. Low levels of glutamate and GABA can lead to mood disorders including depression, and as little as 20 minutes of moderate-intensity exercise can raise their levels. A small study suggested that 12 weeks of yoga practice increased GABA levels, which may help account for yoga-induced feelings of serenity. Exercise also may stimulate firing of the body's endocannabinoid system (endogenous receptors for marijuana), which is involved in reducing pain and promoting feelings of pleasure.
Exercise-induced release of growth factors in the brain appears to be very important for preserving the health and survival of neurons. Exercise stimulates the production of brain-derived neurotrophic factor (BDNF), which increases and strengthens connections among neurons, speeds up nerve transmission, helps prevent neurons from degenerating, and promotes the growth of new neurons in the hippocampus. Higher levels of BDNF are associated with aerobic exercise–induced increases in hippocampal volume and changes in nerve connectivity in the brain. By improving the growth and functioning of neurons, BDNF enhances communication and learning. High BDNF in later life helps protect against AD. Exercise also appears to increase levels of glial cell–derived neurotrophic brain factor (GDNF), which promotes the development and survival of dopamine-producing neurons. Parkinson's disease (PD) is characterized by low levels of dopamine and GDNF, as well as low levels of BDNF in the midbrain.
Stress and anxiety cause the brain to release cortisol, a fight-or-flight stress hormone. Chronic high cortisol levels have been linked to memory loss as well as to a host of chronic illnesses. Exercise may reduce cortisol levels through exposure to controlled stress that tightens regulation of the brain's stress response so that it is less likely to launch inappropriately. Yoga is known to reduce stress, and a 2010 study reported that after eight weeks of daily yoga practice and meditation, brain scans revealed shrinkage of the amygdala that is involved in processing stress, anxiety, and fear.
A 2014 study reported that physical training causes the production of enzymes in skeletal muscle that can clear the blood of a harmful metabolite that accumulates during stress. This may be a biochemical mechanism by which exercise protects against and counteracts depression.
Research has demonstrated that 20–30 minutes of exercise before a mental task or exam can speed up reaction time and improve decision making in people of all ages. Studies have consistently shown that, compared to sedentary older adults, physically active seniors have better memory and higher mental processing speeds; are better at scheduling, planning, and multitasking; and are less likely to experience cognitive decline. They are also better at focusing attention, as when driving. Even previously inactive older adults have improved cognitive function after 6–12 months of regular physical activity.
Exercise can promote mental health and is well known to help protect against stress-induced depression and brain injury. One study demonstrated that exercise interventions improved attention and mood in children, and the largest improvements were in children who had been diagnosed with attention deficit hyperactivity disorder (ADHD). Another study reported that exercise can improve some symptoms of schizophrenia.
Anxiety disorders are the most prevalent psychiatric disorders in the United States. Regular aerobic exercise reduces tension, raises and stabilizes mood and self-esteem, and improves sleep, which reduces stress. Just five minutes of aerobic exercise can begin to lower anxiety, and some studies suggest that a 10-minute walk can be as effective as a 45-minute workout at temporarily relieving anxiety and depression. There is also evidence that physically active people are less likely to be affected by anxiety or depression. One study reported that people who regularly exercise vigorously have a 25% reduced risk of developing depression or an anxiety disorder over a five-year period. A 2013 meta-analysis of research studies indicated that both aerobic and resistance exercise appeared to be as effective as antidepressants and psychological treatments for relieving depressive symptoms.
A 2016 study reported that moderate or intense exercise may slow brain aging by 10 years, and exercise is known to reduce the risk of dementia. Physically active older adults are less likely to develop AD and other dementias even when they carry high-risk gene variants. One study reported that seniors with more gray matter and higher levels of physical activity had a 50% reduced risk of memory decline or AD over a five-year period, and simply walking has been shown to significantly reduce the risk of AD. In one study, vigorous exercise, such as walking two miles or biking 10 miles daily, was the most important contributor to decreasing the risk of both general cognitive decline and dementia; however, just 30–45 minutes of brisk walking three times per week can delay the onset of dementia, particularly in seniors who are physically active prior to experiencing the first signs of cognitive decline.
Although most studies have focused on the effects of aerobic exercise, even twice-weekly sessions of resistance training or one hour of dancing per week have been found to have positive impacts on cognitive functioning over time. Exercises for improving coordination, balance, and agility also appear to have a significant impact on brain structure and cognitive function in the elderly. A 2014 study reported that fluid intelligence, which is similar to IQ, as well as connectivity in certain areas of the brain declined more slowly with age in people who practiced yoga.
Exercise may even modestly improve symptoms of cognitive impairment and dementia. In fact, evidence suggests that aerobic exercise and associated increases in hippocampal volume may have the most benefit for people with mild cognitive impairment (MCI), which often precedes AD. One study found a 4% increase in total hippocampal volume over six months in 70–80-year-old women with MCI who followed a twiceweekly aerobic walking program. This increase was more than double that observed in a similar study of cognitively healthy older adults. Interventions, including yoga, tai chi, and dance movement therapy, can improve measures of cognitive and physical functioning in people with mild-to-moderate dementia. A 2016 study reported that aerobic exercise significantly improved mobility, cognitive functioning, and behavior in institutionalized patients with dementia.
Exercise is also beneficial for people with PD and multiple sclerosis (MS). Studies have shown that exercise can improve and sometimes even reverse symptoms or slow the progression of PD. MS patients who exercise perform better on cognitive function tests than those who are more sedentary.
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Margaret Alic, PhD