As the carrier of life-sustaining oxygen and nutrients, blood is the fuel of the body. As the most energy intensive organ, the brain requires a lot of fuel. The brain needs around 20% of the body’s blood supply in order to sustain its activity. The brain and cardiovascular system are coupled such that the amount of blood sent to the brain adjusts to meet its energy demands [1]. Just as blood flow to the muscles increases during physical exercise, blood flow to the brain increases during mental exercise. As the engine that pumps this fuel throughout the body along a network of vessels, the heart plays a critical role in maintaining brain function. This heart-brain connection may underlie the overlap between risk factors for dementia and cardiovascular disease.
The normal range for resting heart rate is between 60 to 100 beats per minute (bpm). It was found that elderly individuals with a resting heart rate greater than 80 bpm had a 55% higher risk for dementia than those with a resting heart rate of 60 to 70 bpm [2]. Resting heart rate is a measure of how efficiently the heart pumps blood throughout the body, with lower rates representing better fitness. If the heart is strong and healthy, and there are no disruptions along the network of vessels, then each beat will reliably carry blood throughout the body. But if the heart is weak, each beat pumps an insufficient amount of blood, meaning the heart will have to beat several times to distribute the blood to the rest of the body, resulting in a higher heart rate.
If the blood vessels are clogged or constricted then the heart will have to work harder to overcome the resistance within the vessels. This can lead to a thickening of the wall of the left ventricle of the heart. This thickening makes the heart less elastic, so it becomes less efficient at pumping blood, leading to a higher heart rate [3]. High blood pressure is the most common cause of this heart wall thickening.
Signs of pathology are present in the brain many years before individuals show symptomatic signs of dementia. Recent findings indicate that some of this damage in the brain is preceded by changes to the function of the cardiovascular system which occur decades earlier, during young adulthood.
A study found that the presence of left ventricle heart thickening around age 30 was associated with worse global cognition 25 years later, during middle age [4]. A separate study found that chronically high blood pressure during young adulthood (ages 20 to 40) damaged the structure of the brain, leading to the loss of brain cells in some areas, during middle age [5]. Sustained high blood pressure may cause damage in the brain by inducing mechanical stress on the cells surrounding the blood vessels [6]. Over time this stress can lead to the degeneration of these cells, and may be the mechanism by which high blood pressure induces brain atrophy?.
Together these studies suggest that high blood pressure puts stress on the heart, thereby reducing its efficiency. This, in turn, impairs the ability of the heart to pump blood to the brain during times of high demand, and this reduction in blood flow negatively impacts cognitive performance. The cellular stress induced by the high blood pressure and reduced blood flow can damage the cells, resulting in brain cell loss. This accumulation of damage to the brain can eventually result in sustained cognitive impairment.
The heart-brain connection may contribute to other prominent dementia risk factors as well. For example, social isolation is associated with higher risk for cardiovascular disease, and reduced blood flow to the brain [7; 8].
These processes occur over the course of the lifetime, and highlight the importance of maintaining heart healthy habits throughout life. Adopting a lifestyle that promotes cardiovascular fitness by engaging in regular exercise and eating a diet rich in vegetables is a wise investment because it is beneficial for all of the organs of the body by ensuring they have an adequate source of fuel to perform at their highest capacity.
Betsy Mills, PhD, is a member of the ADDF's Aging and Alzheimer's Prevention program. She critically evaluates the scientific evidence regarding prospective therapies to promote brain health and/or prevent Alzheimer's disease, and contributes to CognitiveVitality.org. Dr. Mills came to the ADDF from the University of Michigan, where she served as the grant writing manager for a clinical laboratory specializing in neuroautoimmune diseases. She also completed a Postdoctoral fellowship at the University of Michigan, where she worked to uncover genes that could promote retina regeneration. She earned her doctorate in neuroscience at Johns Hopkins University School of Medicine, where she studied the role of glial cells in the optic nerve, and their contribution to neurodegeneration in glaucoma. She obtained her bachelor's degree in biology from the College of the Holy Cross. Dr. Mills has a strong passion for community outreach, and has served as program presenter with the Michigan Great Lakes Chapter of the Alzheimer's Association to promote dementia awareness.
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