Exercise for better blood flow to the brain
The vascular system, composed of endothelial cells, extends all around the body’s periphery and into and around all structures of the brain. As we age, our arteries physically stiffen and the endothelial cells (the innermost cell layer of our blood vessels) become less responsive to molecular cues in the blood, leading to an overall decrease in blood flow and increase in cardiovascular dysfunction, which often manifests as cardiovascular disease (Bolduc et al., 2013). These changes not only happen peripherally but also centrally in our brains. That is as we age, a decrease in blood flow or hypoperfusion occurs in the brain. As neuronal activity is closely linked to cerebral blood flow, this leads to a decrease in functioning of the brain’s neurons and other cells, which contributes to age-related decline in cognitive function. In particular, vascular disease has been linked to an elevated risk for Alzheimer’s disease and other forms of age-related dementia (Tarumi & Zhang, 2014).
Fortunately, the risk of cardiovascular dysfunction can be modified through changes in lifestyle, such as diet and exercise. For example, physical activity decreases the risk for stroke and cardiovascular disease, and research is emerging which shows that this is due to changes at the cellular level. In fact, results from randomized controlled trials have shown that exercise is as effective as drug treatment in reducing the risk of mortality from diabetes, coronary artery disease, and stroke (Naci & Ioannidis, 2013). Gretchen Reynolds, exercise blogger for the New York Times, wrote a detailed piece on this meta-epidemiological study, which she says, “raises important questions about whether our health care system focuses too much on medications and too little on activity to combat physical ailments” (http://well.blogs.nytimes.com/2013/12/11/exercise-as-potent-medicine/).
At the cellular and molecular level, exercise has been shown to improve the cerebrovascular system. In rodents, wheel running promotes both vasculogenesis (formation of de novo endothelial cells) as well as angiogenesis (formation of blood vessels from already existing tissue) in a variety of different brain regions including the motor cortex, cerebellum and hippocampus (van Praag et al., 2005; Wahl et al., 2007). Additionally, exercise increases vascular endothelial growth factor (VEGF) (Fabel et al., 2003), which is a protein that promotes both vasculogenesis and angiogenesis.
In humans, using a technique known as positron emission tomography, scientists have been able to view changes in cerebral blood flow during exercise. They found that during the initiation of exercise, global cerebral blood volume increased on average 27.9%, with even greater increases (37.6-70.5%) seen in specific brain regions involved in motor control, such as the sensorimotor cortex and cerebellum (Hiura et al., 2014). As exercise progresses, this increase in blood flow returns to baseline levels (Hiura et al., 2014) and may even decrease with strenuous or high-intensity exercise (Ekkekakis, 2009). As cognition has been shown to improve after acute bouts of exercise, scientists have been curious about what happens in the brain after exercise cessation. One recent study examined the effect of an acute bout of moderate-intensity exercise on brain activation during an attention task (the Stroop task) using functional near-infrared spectroscopy (fNIRS) in 16 healthy older adults (ages 64-74) (Hyodo et al., 2012). fNIRS is a brain imaging technique that measures the amount of oxygenated blood in cortical areas of the brain. They found that acute exercise enhanced performance on the Stroop task, which was correlated with increased activation or the percentage of oxygenated blood in the prefrontal cortex, a region involved in higher-level cognitive processing.
High-intensity interval training (HIIT) provides perhaps superior benefits to the cardiovascular system than continuous aerobic exercise (Wisløff et al., 2009) and may be more beneficial than continuous training for patients with cardiovascular disease (Guiraud et al., 2012). In patients with a variety of cardiometabolic diseases, such as coronary artery disease, heart failure, hypertension, metabolic syndrome, and obesity, HIIT has been shown to be twice as effective as continuous training at increasing cardiopulmonary fitness (19.4% versus 10.3% increase in VO2 max) (Weston et al., 2014). As such, one could hypothesize that HIIT, as compared to other forms of exercise, can provide superior enhancements to the cerebrovasculature and subsequently to neuronal and cognitive functioning. However, no studies to date have examined the effects of HIIT on cerebrovasculature, even at the level of cerebral blood flow (Lucas et al., 2015). This is an obvious area of future research, especially in regards to how HIIT affects the compromised cerebrovasculature in elderly individuals.
In conclusion, the well-known benefits of exercise on the cardiovascular system are not only important for the body but for the brain. These exercise-induced improvements in the cerebrovasculature appear to contribute to the enhancements we see in cognitive functioning with exercise. Future research is needed to see if exercise regimens such as HIIT are more effective than continuous aerobic exercise at causing improvements in this important feature of the brain.
References:
Bolduc, V., Thorin-Trescases, N., & Thorin, E. (2013). Endothelium-dependent control of cerebrovascular functions through age: exercise for healthy cerebrovascular aging. American Journal of Physiology-Heart and Circulatory Physiology, 305(5), H620-H633.
Ekkekakis, P. (2009). Illuminating the black box: investigating prefrontal cortical hemodynamics during exercise with near-infrared spectroscopy. Journal of sport & exercise psychology, 31(4), 505.
Fabel, K., Fabel, K., Tam, B., Kaufer, D., Baiker, A., Simmons, N., ... & Palmer, T. D. (2003). VEGF is necessary for exercise‐induced adult hippocampal neurogenesis. European Journal of Neuroscience, 18(10), 2803-2812.
Guiraud, T., Nigam, A., Gremeaux, V., Meyer, P., Juneau, M., & Bosquet, L. (2012). High-intensity interval training in cardiac rehabilitation. Sports medicine,42(7), 587-605.
Hiura, M., Nariai, T., Ishii, K., Sakata, M., Oda, K., Toyohara, J., & Ishiwata, K. (2014). Changes in cerebral blood flow during steady-state cycling exercise: a study using oxygen-15-labeled water with PET. Journal of Cerebral Blood Flow & Metabolism, 34(3), 389-396.
Hyodo, K., Dan, I., Suwabe, K., Kyutoku, Y., Yamada, Y., Akahori, M., ... & Soya, H. (2012). Acute moderate exercise enhances compensatory brain activation in older adults. Neurobiology of aging, 33(11), 2621-2632.
Lucas, S. J., Cotter, J. D., Brassard, P., & Bailey, D. M. (2015). High-intensity interval exercise and cerebrovascular health: curiosity, cause, and consequence. Journal of Cerebral Blood Flow & Metabolism, 35(6), 902-911.
