Everyone knows the feeling: you forget to drink water for hours on end and start to feel strange. Your throat might feel dry and scratchy; maybe your muscles start to feel tight muscles. But inside the body, even more is happening: cells start to shrink as water leaves them, and the kidneys start to conserve water, which makes urine become darker.
It’s an easy problem to fix by drinking more water. Yet so many of us don’t do this basic act of self-care; 24% of older adults in the U.S. are at least a little dehydrated, according to a recent meta-analysis. “Nothing happens in your body without water playing a role in it, and so to ignore it completely is just missing huge opportunities to improve health,” says Jodi Stookey, a nutrition epidemiologist at San Francisco Department of Public Health who studies hydration. Although official recommendations vary—the widely-cited eight glasses a day may not have strong scientific backing—Stookey recommends about a liter a day for most adults.
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Here, 5 strange signs that you need to drink more water.
Moodiness and brain fog
The brain is one of the first places affected when you don’t drink enough water. Even mild dehydration—meaning as little as losing 1 or 2% of body mass in water—can impair attention, working memory, and reaction time. People can still do tasks in this state, but they tend to feel more grouchy, irritable, and stressed while doing them—and this is particularly true for women, Stookey says. There is some evidence that people may feel more pain when they’re not properly hydrated. “The brain is highly sensitive to fluid balance, and reduced plasma volume can alter cerebral blood flow and neurotransmitter activity,” says Lawrence Judge, assistant dean at the Marieb College of Health & Human Services at Florida Gulf Coast University.
Headaches and dizziness
Inadequate hydration triggers headaches. That’s because reduced fluid intake can temporarily shrink brain tissue—particularly the parts of the brain that regulate pain—while also affecting electrolyte balance. Both of these effects may contribute to head pain and lightheadedness, Judge says.
Digestive slowdown and constipation
Water is essential for moving things through your gastrointestinal system. When intake is low, the colon absorbs more water from stool, making bowel movements harder and less frequent. Chronic low hydration is a common yet under-appreciated contributor to constipation, Judge says. One 2024 study also found that adequate water intake is critical for maintaining bacterial and immunological equilibrium in the gut, keeping up a solid defense against intestinal pathogens.
Elevated heart rate and less endurance
As hydration drops, blood volume decreases, forcing the heart to work harder to maintain cardiac output. This can raise resting heart rate, increase weakness and fatigue, and reduce endurance—even during everyday activities, not just exercise. Just walking the dog may bring a fluttering heart and a swimming head if you don’t have enough water in your system, and that gym session may feel twice as hard. “With more dehydration, muscles work less well,” says Stookey. “You tire faster when exercising.”
Skin changes that mimic aging
While drinking more water won’t erase wrinkles, dehydration can make skin appear dull, less elastic, and more prone to fine lines. In fact, squeezing skin and seeing how it bounces back to shape—testing its elasticity—is one way that medical professionals can tell if someone is dangerously dehydrated. Adequate hydration helps maintain skin’s elasticity and supports barrier function, which affects overall appearance.
Increased risk of getting sick
A lack of fluids hampers the body’s ability to fight pathogens and maintain immune cell function. Being persistently underhydrated is associated with an elevated risk for earlier death and chronic diseases.
A 2023 study looked at 30 years of data for nearly 12,000 adults and found a correlation between people’s sodium levels in their blood—which go up when fluid intake goes down—and chronic conditions. They found that adults with higher sodium levels were likely to die at a younger age or show signs of advanced biological aging than those with levels in the medium ranges. It all points to the importance of hydration over the long term—suggesting that hydration has a special role in simply keeping us healthy and alive in the long term.
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