Physical activity, exercise and brain health!

As with all the organs of the human body, the functional capacities of the brain gradually decline during aging. This decline is manifested by a decrease in learning and memory abilities, decision-making speed, sensory perceptions (hearing, vision, smell, taste), motor coordination, and higher cognitive processes such as abstraction, judgment or problem solving.

The decline in brain performance during aging is correlated with changes in brain structure, including a reduction in brain volume and the hippocampus (the seat of memory), as well as the integrity of the white and gray matter of the frontal regions involved in higher cognitive processes. These phenomena seem to accelerate after the age of 50 and, as a result, considerably increase the risk of neurodegenerative diseases in people who reach an advanced age: from the age of 65, for example, the risk of developing Alzheimer’s disease doubles every 5 years, reaching almost 50% after age 85. This is a worrying situation, because with the aging of the world population, it is estimated that the number of patients affected by Alzheimer’s disease will triple over the next few years, from 50 million to around 130 million people in 2050.

However, this situation is not irreversible. Several studies conducted in recent years suggest that certain lifestyle habits could contribute to slowing down the functional structural changes in the brain that occur as we age. Currently available evidence suggests that several modifiable factors may help slow this age-related decline in cognitive function and thus prevent the onset of dementia, such as controlling chronic diseases, sleep quality, social interactions, hearing screening, cognitive stimulation, and regular physical activity.

Of all these factors, the protective effect of physical activity has probably been the most studied and best documented. Several studies carried out over the last 15 years have observed that people who are physically active are less affected by a deterioration in their cognitive functions as they age and are less at risk of being affected by different types of dementia. For example, in studies that compare the cognitive performance of older people to younger people, it is generally observed that the loss of performance associated with aging is less pronounced in older people who are physically fit than in those who are sedentary. A review of 16 prospective studies (163,797 participants in total) on this subject showed that high levels of physical activity were associated with a 28% reduction in dementia in general, a protection that even reached 45% for Alzheimer’s disease. Similar results were obtained in a meta-analysis of 26 studies on the subject, with the most active people showing a 35% reduction in the risk of cognitive decline and a 14% reduction in the risk of dementia compared to those who were sedentary.

The amount of exercise required to provide the maximum cognitive benefits remains uncertain, but a rigorous analysis of a hundred studies involving 11,061 participants (average age of 73 years) suggests that an hour of moderate physical activity (walking, cycling, dancing, etc.) three times a week is associated with a significant improvement in the speed of information processing and executive functions (planning, resistance to distractions, judgment). Although the positive effects are observed quickly, after 10-12 weeks of physical activity, the most pronounced and robust effects on cognitive performance are observed after 6 months of exercise three times a week and after one year on brain volume markers.

As for the type of exercise to recommend, studies suggest that cardiorespiratory physical activities, such as brisk walking or running, are essential. Indeed, a link between cognition and cardiovascular fitness has also been observed in many studies.

More recently, a study that followed a group of 1462 Swedish women aged 38 to 60 for 44 years (starting in 1968) reported that those with the best cardiovascular fitness had a 90% lower risk of developing dementia compared to those who were sedentary. Other studies also show that resistance training, aimed at increasing muscular strength, and mind-body exercises, such as yoga and tai chi, can also reduce the risk of cognitive decline and improve cognitive performance in older people.

At the molecular level, the many benefits of physical activity are increasingly attributed to “exerkines”, these signalling molecules that are released in response to exercise, which have the potential to improve cardiovascular, metabolic, immunological and neurological health (see our article on the subject).

Effects on neurons
The protective effect of exercise against cognitive decline associated with aging is the consequence of the multiple positive impacts of physical activity on the body. First, regular physical activity has a direct effect on the structure of the brain: for example, a randomized clinical study showed that people who were subjected to an aerobic exercise program for a year had an increase in approximately 2% of the volume of the hippocampus, while this volume had on the contrary decreased by approximately 1.4% in the control group, in which the participants had been assigned to do only simple stretching instead of being physically active (Figure 1). In other words, not only did physical activity manage to counter the normal decrease in the volume of the hippocampus that occurs with aging (1-2% per year), but it even managed to increase the volume of this region of the brain.

Figure 1. Change in the volume of the left hippocampus in people who had or had not undergone a one-year program of aerobic physical activity. Adapted from Erickson et al. (2011).

Studies also show that in older adults, the increase in cardiovascular fitness (maximal aerobic fitness) that results from regular exercise is associated with an increase in brain and hippocampal volume, as well as a less significant loss of white and gray matter in the frontal and prefrontal regions. Exercise also improves neuronal connectivity and increases the levels of certain growth factors such as BDNF (brain-derived neurotrophic factor), which promote brain plasticity (the brain’s ability to create or reorganize networks of neurons), particularly at the level of the hippocampus and the frontal cortex (for more details, see this review article). Moderate exercise is also associated with an improvement in the function and number of mitochondria, not only in muscle cells but also in the brain. Mitochondria are essential components of the cell whose main function is to provide cells with the energy necessary to ensure their survival and proper functioning. Exercise could therefore prevent the natural decline in mitochondrial function during aging and protect the brain against fatigue and certain dysfunctions associated with this decline. Overall, these findings suggest that regular exercise is associated with changes in the structure of the brain itself and that these changes translate into better cognitive function, including improved memory.

Physical activity can also improve cognitive function indirectly by promoting better overall health, for example by improving stress management, sleep quality and cardiovascular health. This connection between the heart and the brain is particularly important, because numerous studies (see here and here, for example) have shown that traditional risk factors for cardiovascular disease (smoking, high cholesterol, hypertension, hyperglycemia, obesity, etc.) are also associated with an increased risk of cognitive decline and dementia, including Alzheimer’s disease.

A healthy heart for a healthy mind
This close relationship between the heart and the brain is not surprising, as the brain is an extremely energy-intensive organ whose function depends heavily on an adequate supply of oxygen and nutrients carried by the bloodstream: even though it represents only 3% of total body weight, the brain alone receives 15% of cardiac output and uses approximately 20% of all oxygen consumed. Brain activities such as thinking, memory and reasoning are therefore very dependent on the health of the heart and vessels.

Several studies have also shown that the development of cardiovascular disease and the appearance of cognitive disorders can in both cases be linked to the presence of different cardiovascular risk factors (smoking, obesity, hypertension, sedentary lifestyle, etc.) in early or middle age (Figure 2 below, from this review article). Atherosclerosis, which results from chronic exposure to these factors, affects the function of all vessels, the first clinical manifestation of this damage most often being the appearance of coronary heart disease such as angina and myocardial infarction. However, the vessels of the brain are not spared, and brain lesions can appear as early as middle age and gradually accumulate to generate greater damage at later ages, which causes the onset of cognitive disorders and dementia. The earlier exposure to cardiovascular risk factors begins, the more quickly brain damage appears: for example, a study showed that the presence of numerous cardiovascular risk factors in young individuals (18-30 years old) is already associated with a decline of several cognitive functions in middle age (reduction of memory and executive functions). Conversely, the adoption of healthy lifestyle habits known to improve cardiovascular health, in particular the absence of smoking, a healthy diet and regular physical activity, is associated with an improvement in cognitive performance.

Figure 2. Parallel evolution of cardiovascular disease and dementia risks following exposure to cardiovascular risk factors over the life cycle. From Qiu and Fratiglioni (2015).

A link between vascular damage and brain health is also illustrated by the increased cognitive problems and risk of dementia observed in people who have had a myocardial infarction, particularly those who have suffered a stroke or require coronary artery bypass surgery after the infarction. It is possible that this increased risk reflects a reduction in blood supply to the brain (hypoperfusion) caused by a deterioration in cardiac function in these patients.

In short, whether because of its direct effects on the brain or its benefits on cardiovascular health, the potential for preventing cognitive decline and the onset of dementia through physical activity is quite phenomenal. There is no doubt that it is one of the best weapons at our disposal to reduce the incidence of these diseases and the heavy burden they impose on the people affected and those around them.