Welcome to Dear Julia, a column where readers submit everyday health questions. Which over-the-counter painkillers work best? Is it better to run or walk for exercise? Why do the parents of young kids get sick more often? Julia Belluz sifts through the research and consults with experts in the field to figure out how science can help us live happier and healthier lives.
Dear Julia: Do I actually have to drink eight glasses of water a day?
Dehydration is in the news this week, after Hillary Clinton lost her balance at a 9/11 event — a bout of wobbliness her campaign said was caused, in part, by dehydration. In a forensic analysis of the Democratic presidential candidate’s sick day, Politico reported that Clinton doesn’t actually like to drink water, "a source of tension with her staff."
One reason it is so crucial to stay hydrated is that we are literally made of water. "Water comprises from 75% body weight in infants to 55% in the elderly and is essential for cellular homeostasis and life," this comprehensive 2010 review of the evidence on water, hydration, and health explains.
Dehydration happens when the body doesn’t have enough fluid to carry out its basic functions — and it does a pretty good job of signaling when you haven’t imbibed enough: You feel thirsty, your mouth and lips get sticky and dry, and you can even get dizzy and fatigued. After a few days without water, you face the risk of death.
So water is essential to life. The Institute of Medicine suggests women get about 2.7 liters (or about 11 cups) and men 3.7 liters (or 13 cups) of total water each day. (Those amounts will likely go up or down depending on how much you exercise and sweat each day, and what kind of climate you live in.)
What’s less clear, however, is the optimal amount of water people need to drink each day. But there’s good news for Clinton and others who don’t enjoy quaffing H2O: Science certainly hasn’t demonstrated that drinking eight or more daily glasses is necessary.
Many people believe that the source of this myth was a 1945 Food and Nutrition Board recommendation that said people need about 2.5 liters of water a day. But they ignored the sentence that followed closely behind. It read, "Most of this quantity is contained in prepared foods."
Indeed, you can get hydrated through a range of foods and drinks, not just water. "For example, broth soups and foods with high water content such as celery, tomatoes, or melons can contribute to fluid intake," the Centers for Disease Control and Prevention recommends.
Even coffee! Researchers have found that caffeine in moderate doses doesn’t cause dehydration — so those cups of java can count toward your day’s hydration effort. (Though you’ll want to be careful about replacing water with salty foods and alcohol, which can be dehydrating.)
Still, the myth persists. "If there is one health myth that will not die, it is this: You should drink eight glasses of water a day," Carroll concluded. "It’s just not true. There is no science behind it."
Drinking lots of water doesn’t reduce your chronic disease risk or make your skin look better — but it can help you lose weight
While you need to be hydrated in order to live, many of the magical claims people make about consuming additional glasses of water to boost health are unfounded.
This 2010 review goes through all the evidence for the effects of hydration status on certain health outcomes. The researchers found strong evidence that increased fluid intake reduced the risk of urinary tract stones and exercise-related asthma — but just about every other health claim they looked at was based on weak, inconclusive, or speculative evidence. So, for example, drinking a lot of water didn’t reduce the risk of heart failure, cancer, or UTIs, as some have claimed.
A final water shocker: Even though beauty magazines and celebrities may suggest overdosing on water to keep skin looking supple, researchers who looked into that claim found no scientific proof to back it up.
There is one place where drinking lots of water can be very helpful: weight management. Since water contains no calories, and many other beverages — like milk, juice, and soda — can be chock full of them, replacing these drinks with water can help you keep your calorie intake down. Staying hydrated and filling up on fruits and vegetables — rich in water, low in calories — can also help keep your hunger in check.