We often hear the adage about the importance of drinking eight glasses of water a day to keep our bodies healthy, but how about our brains? The adult human body contains around 60% water. All the cells in the body, including our brain cells, depend on this water to carry out essential functions. Therefore, if water levels are too low, our brain cells cannot function properly, leading to cognitive problems.
The brains of dehydrated adults show signs of increased neuronal activation when performing cognitively engaging tasks, indicating that their brains are working harder than normal to complete the task [1]. In healthy young adults, this additional effort typically manifests as fatigue and changes in mood, but in populations with less cognitive reserve, such as the elderly, this can lead to a decline in cognitive performance [2]. Performance on complex cognitive tasks that require high levels of brain power is most likely to decline due to the strain of dehydration. A meta-analysis of 33 studies including a total of 413 participants found that dehydration corresponding to more than a 2% reduction in body mass (e.g. 3 lbs. of fluid loss in a 150 lb. person) was associated with significant impairments on attention, executive function, and motor coordination [3].
Women of all ages are more sensitive to the effects of dehydration, but elderly women are especially vulnerable. A study examining the hydration status of 2,506 adults over age 60 found that women with inadequate levels of hydration showed worse performance on cognitive tasks related to attention and processing speed [4]. The performance of dehydrated men also declined, but to a lesser degree.
In young women, cognitive deficits can be readily reversed by replenishing fluids [5], while in the elderly, the prolonged cellular stress of dehydration may promote brain pathology and continued cognitive decline. A study assessing the cognitive function and hydration status of 1,091 people over age 65 found that dehydrated individuals were at higher risk for dementia, while individuals with dementia were at higher risk for dehydration [6]. Additional studies indicate that dehydration can accelerate cognitive decline in people with dementia [7]. Decreased water levels in cells can cause proteins to misfold and prevent the clearance of these toxic proteins, causing them to build up in the brain. While it is clear that dehydrated cells are associated with brain dysfunction, it is not yet known whether dehydration is a cause or an effect of dementia.
In addition to being most vulnerable to dehydration related cognitive decline, the elderly are also at higher risk for dehydration. The levels of water stored in the body decline with age due to changes in body composition, namely the loss of muscle and gain of fat. Muscle tissue provides a large reservoir of water since it is made up of nearly 80% water, while fat tissue has a much lower water content around 10%. The lower percentage of muscle mass in women may contribute to their increased sensitivity to dehydration. The elderly are also less likely to notice they are dehydrated. The brain becomes less sensitive to the thirst sensor with age, so thirst is a less reliable indicator of hydration status in this population [7]. Due to changes in kidney function with age, the elderly are less able to concentrate urine to conserve water and regulate sodium levels, putting them at higher risk for complications related to dehydration or overhydration [8]. Furthermore, it is more difficult to accurately diagnose dehydration in older adults. Traditional physical signs of dehydration, saliva tests, and urine tests are often inaccurate or misleading due to the presence of other chronic conditions. Blood tests are the only reliable indicators of dehydration in the elderly.
WHAT YOU CAN DO
To keep your brain adequately hydrated, it is recommended that women consume 2 to 2.7 liters (8 to 11 cups) and men consume 2.5 to 3.7 liters (10 to 15 cups) of fluids per day, though individual needs may vary depending on activity level and medication use [9]. It can help to develop a schedule to keep track of daily fluid intake. It is important to keep in mind that cognitive function can also be impaired by overhydration [4]. Overhydration can lead to drop in sodium levels that can induce delirium and other neurological complications, so fluid consumption should not vastly exceed medically recommended guidelines.
Diet and exercise are also important components to remaining hydrated. The hydration guidelines refer to the consumption of all fluids, not simply how many glasses of plain water we drink per day. However, it is counterproductive to start drinking more beverages laden with sugar or artificial sweeteners, since they have their own health risks. Our bodies obtain water from multiple nutritional sources, including many healthy mineral rich foods, so it is possible to get adequate levels of hydration by incorporating more water rich foods into your diet. Some nutritious water rich foods include melon, oranges, berries, lettuce, cucumbers, and tomatoes [10]. You can also preserve your body’s stored water content through strength conditioning exercises which build muscle mass.
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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