番茄社区

Drink eight 8-ounce glasses of water each day. Strive for very light yellow urine.

The messaging around proper hydration can be daunting -- but is it necessary?

Nephrologists and neuroscientists who study thirst and hydration don't think so. In general, healthy kids and adults can disregard this "advice," according to experts interviewed by 番茄社区.

"Your body has a highly tuned system to tell you how much water to drink -- it's called thirst," said Joel Topf, MD, medical director of St. Clair Nephrology Research in Michigan. "We don't tell people to hyperventilate, even though breathing is good for you."

Topf says people shouldn't pay too much attention to their urine color, unless it's very dark or has blood in it. "Trying to optimize for super pale urine is a fool's errand," he said.

"I'd be much more focused not on urine color, but on sense of thirst," he said. "The idea that we have recently gotten so much smarter than evolution and we know that drinking additional water is helpful; it's just not right. There's no evidence to point to that."

The body has a sophisticated system for staying hydrated, said Yuki Oka, PhD, a professor at Caltech in Pasadena, California, who studies the neural and molecular mechanisms of maintaining homeostasis. That includes a sensor region in the brain called the lamina terminalis that's constantly monitoring blood volume and salt and mineral content, as well as the kidneys, which are very effective at ensuring the body retains the proper amount of water.

Essentially, there are two ways the lamina terminalis senses dehydration, Oka said. Either the salt and mineral content of the blood goes up because water is lost, or blood volume decreases overall. Both of those situations "trigger the feeling of thirst," he said.

"Eating potato chips increases sodium levels in blood, but that won't change blood volume. In this case, the increase in osmolality will trigger thirst," Oka said. "The other case is after exercising, you lose water and sodium and minerals. The osmolality doesn't really change, but the volume is decreased. That's hypovolemic thirst as opposed to osmotic thirst."

Neurons in the laminate terminalis will begin to fire in both cases, ultimately resulting in a sense of thirst that triggers people to drink, he said.

Generally, that starts to happen when the body loses about 2% of its water, Oka said, adding, "That's very sensitive."

"For survival purposes," he said, "if you don't feel thirst, then the general rule of thumb is that you don't have to drink."

Neurons firing from the lamina terminalis also communicate with neurons that play a role in kidney function, telling the body how much urination is necessary, Oka said. The main mechanism for communication between those neurons and the kidney is the hormone vasopressin, which is produced in the hypothalamus, he said.

"Generally in younger people, if they don't drink water, the kidney is very effective in retaining water," Oka said.

Topf added that the kidney "is designed to concentrate and dilute urine to keep the water in your body regulated. Our kidneys are designed to compensate for wide ranges of water intake over periods of time, and I think we under-appreciate how good our kidneys are at that job."

Topf noted that there are certainly exceptions to the general rule to drink only when you're thirsty.

"There are some medical conditions where we think that you should drink excess water beyond your thirst," he said. "The long-standing one is kidney stones. The more urine that you make, the more dilute the chemicals in your urine that can form kidney stones. So if you have a history of kidney stones, you should be drinking a lot of water, in excess of two and a half liters per day."

The other group that should drink frequently is women with frequent urinary tract infections (UTIs), Topf said. He pointed to a high-quality randomized controlled trial that showed a significant reduction in urinary tract infections for women with recurrent UTIs who increased their water intake.

Older people, too, might benefit from paying attention to urine color, as sense of thirst can dull with age, Oka said.

"Older people are recommended to drink even if they don't feel thirsty," Oka said. "Their body osmolality is already higher, which stresses the heart and the kidney, and that can increase the chance of complications from any disease."

Topf said that if older people notice darker urine, drinking one extra glass of water should suffice.

Correction: An earlier version of this story incorrectly referred to hypervolemic thirst. It should be hypovolemic thirst.

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