Hydration may affect cognitive function in some older adults
December 12, 2019
Science Daily/Penn State
Among women, lower hydration levels were associated with lower scores on a task designed to measure motor speed, sustained attention, and working memory. They did not find the same result for men.
Not getting enough water is enough to make you feel sluggish and give you a headache, but a new Penn State study suggests it may also relate to cognitive performance.
The researchers investigated whether hydration levels and water intake among older adults was related with their scores on several tests designed to measure cognitive function. They found that among women, lower hydration levels were associated with lower scores on a task designed to measure motor speed, sustained attention, and working memory. They did not find the same result for men.
The findings were recently published in the European Journal of Nutrition.
"The study gives us clues about how hydration and related drinking habits relate to cognition in older adults," said Hilary Bethancourt, a postdoctoral scholar in biobehavioral health and first author on the study. "This is important because older adults already face increased risk of cognitive decline with advancing age and are often less likely than younger adults to meet daily recommendations on water intake."
Asher Rosinger, Ann Atherton Hertzler Early Career Professor in Global Health, said the researchers found similar results when the participants were overhydrated.
"We found a trend suggesting overhydration may be just as detrimental to cognitive performance as dehydration for older adults," said Rosinger, who also directs the Water, Health, and Nutrition Laboratory and was senior author on the study. "Because of this, being in the 'sweet spot' of hydration seems to be best for cognitive function, especially for tasks requiring sustained attention."
According to the researchers, scientists have long suspected that dehydration may have an effect on cognitive performance. However, previous studies have largely focused on young, healthy people who are dehydrated after exercise and/or being in the heat.
Bethancourt said that because exercise and elevated ambient and body temperatures can have their own, independent effects on cognition, she and the other researchers were interested in the effects of day-to-day hydration status in the absence of exercise or heat stress, especially among older adults.
"As we age, our water reserves decline due to reductions in muscle mass, our kidneys become less effective at retaining water, and hormonal signals that trigger thirst and motivate water intake become blunted," Bethancourt said. "Therefore, we felt like it was particularly important to look at cognitive performance in relation to hydration status and water intake among older adults, who may be underhydrated on a regular basis."
For the study, the researchers used data from a nationally representative sample of 1271 women and 1235 men who were 60 years of age or older. Data were collected by the Nutrition and Health Examination Survey. Participants gave blood samples and were asked about all foods and drinks consumed the previous day. The researchers calculated hydration status based on concentrations of sodium, potassium, glucose, and urea nitrogen in participants' blood. Total water intake was measured as the combined liquid and moisture from all beverages and foods.
Participants also completed three tasks designed to measure different aspects of cognition, with the first two measuring verbal recall and verbal fluency, respectively.
A final task measured processing speed, sustained attention, and working memory. Participants were given a list of symbols, each matched with a number between one and nine. They were then given a list of numbers one through nine in random order and asked to draw the corresponding symbol for as many numbers as possible within two minutes.
Bethancourt said that when they first plotted the average test scores across different levels of hydration status and water intake, there appeared to be a distinct trend toward higher test scores in relation to adequate hydration and/or meeting recommended water intake. However, much of that was explained by other factors.
"Once we accounted for age, education, hours of sleep, physical activity level, and diabetes status and analyzed the data separately for men and women, the associations with hydration status and water intake were diminished," Bethancourt said. "A trend toward lower scores on the number-symbol test among women who were categorized as either underhydrated or overhydrated was the most prominent finding that remained after we accounted for other influential factors."
Bethancourt said that because the data was cross-sectional, they can't be sure whether suboptimal hydration levels are causing cognitive impairment or if people with impaired cognition are just more likely to be under- or overhydrated. The researchers were also unsure why they failed to see the same associations among men. Still, she said the results raise interesting questions.
"It was interesting that even though the test of attention, processing speed, and working memory took only a few minutes, it was the one most strongly associated with lower hydration levels," Bethancourt said. "Other research has similarly suggested that attention may be one of the cognitive domains most affected by hydration status. This left us wondering what the effects of inadequate hydration might be on more difficult tasks requiring longer periods of concentration and focus."
Rosinger said the findings suggest older adults may want to pay close attention to their hydration status, by both consuming enough liquids to avoid dehydration as well as ensuring adequate electrolyte balance to avoid overhydration.
"Because older adults may not necessarily feel thirsty when their body is reaching a state of underhydration and may be taking diuretics that can increase salt excretion, it is important for older adults and their physicians to better understand the symptoms of being both under- and overhydrated," said Rosinger.
https://www.sciencedaily.com/releases/2019/12/191212142720.htm
Had enough water? Brain's thirst centers make a gut check
Mouse study suggests gut sensors monitor hydration potential of each drink you take
March 27, 2019
Science Daily/University of California - San Francisco
Water bottles are everywhere these days, along with all kinds of advice about exactly how much water you should be drinking. But how does your brain actually know when you've had enough and can stop feeling thirsty? A new UC San Francisco study -- published March 27, 2019 in Nature -- may have the answer.
Until recently, scientists believed that a brain region called the hypothalamus makes us thirsty when it detects a drop in the hydration of our blood. But UCSF neuroscientist Zachary Knight, PhD, an associate professor of physiology and Howard Hughes Medical Institute Investigator, realized that this couldn't be the whole story -- in particular because a refreshing beverage begins to quench our thirst almost as soon as it touches our lips, despite taking 10 minutes or more to actually change our overall hydration.
In a 2016 study, Knight lab graduate student Christopher Zimmerman helped to explain this phenomenon by showing that sensory signals from the mouth and throat make thirst neurons in the hypothalamus shut down immediately when mice take a drink. These sensors appear to predict how hydrating a drink will be, based on the volume of liquid an animal swallows, and they are particularly attuned to cold fluids, which may explain why an icy drink is so refreshing.
"This fast signal from the mouth and throat appears to track how much you drink and match that to what your body needs," Zimmerman said. "But we also knew that this fast signal couldn't explain everything."
In particular, the researchers wondered how the brain knows exactly how hydrating a drink will be. After all, sea water doesn't quench thirst, but it would activate many of the same receptors in the mouth and throat as ice water from the fridge.
In their new study, Zimmerman and colleagues used flexible optical fibers implanted near the hypothalamus to watch the activity of thirst neurons as mice drank salty water. In agreement with the team's earlier results, these neurons did go quiet as soon as the thirsty animals took a drink, but then quickly switched back on, as if some other sensor were testing the water the animal had just drunk and alerting the brain: "Too salty -- stay thirsty!"
To see whether these signals could be coming from the gut, the researchers infused liquid directly into the stomachs of thirsty mice while watching the activity of their thirst neurons. They found that infusing fresh water deactivated these cells just as well as taking a drink does, but after infusions of salt water thirst neurons remained active. When mice were given a salt infusion and then allowed to drink pure water, their thirst neurons initially went quiet as they drank, but soon switched back on, as if signaling the need to drink more to make up for the added salt in their stomachs.
These results suggest that the sensors in the mouth and throat that Zimmerman discovered in 2016 let the brain temporarily quench thirst to reward animals for taking a drink, but that the thirst neurons then review this decision based on a second level of sensors in the gut (probably at the beginning of the small intestine, the authors suggest) that predict how well the drink will rehydrate the animal and tell it whether it needs to keep drinking.
"Interestingly, salt water didn't drive drinking in well-hydrated mice, just in mice who were already thirsty," Zimmerman said. "This suggests that signals from the gut are needed to quench thirst, but that you actually need to become dehydrated to trigger thirst in the first place."
The researchers showed that the gut's hydration signals travel via the vagus nerve to activate thirst neurons. Using a technique called optogenetics, which lets scientists activate or shut down particular groups of neurons using beams of light, the researchers showed how these cells -- which are located in the hypothalamus's subfornical organ (SFO) -- pass messages to the nearby median preoptic nucleus (MnPO), which can respond by driving animals to drink and telling the kidney to conserve water in the bloodstream.
The researchers were intrigued to discover that a subset of individual neurons in the MnPO appeared to respond to and integrate drinking signals from the mouth and throat, satiation signals from the gut, and information about an animal's overall hydration level from the bloodstream. Other nearby cells also encoded other information like an animal's stress level or the availability of water sources.
"This is the first time we've been able to watch in real time as single neurons integrate signals from different parts of the body to control a behavior like drinking," Knight said. "This opens the door to studying how all these signals interact, such as how stress or body temperature influences thirst and appetite."
In addition to studying the normal function of the SFO and MnPO thirst neurons, the researchers hope to use these insights to understand whether defects in how these neurons regulate fluid balance within the body could explain the origins of diseases like high blood pressure.
"The hypothalamus is a critical center for keeping our physiology within a healthy range, whether it's hydration, appetite, making sure we're the right temperature, or controlling blood pressure -- and all of these needs compete with and modify one another," Knight said. "It has been difficult to study how all of these factors interact in the brain of a living animal, but studies like this are beginning to allow us to investigate this critical question."
https://www.sciencedaily.com/releases/2019/03/190327142026.htm