It's a piece of doctorly advice you have probably heard dozens of times: "Drink plenty of fluids." But have you ever wondered what it actually means? How much is "plenty," what kinds of "fluids" are best and why do you need them?

"That's a great question," says Mitchell Rosner, a nephrologist who focuses on fluid and electrolyte disorders and is the chair of the University of Virginia Department of Medicine. "What is plenty of fluids? Is that six glasses of water, eight glasses of water?"

The truth is that "plenty" varies, depending on a person's size, their normal water intake, how much they sweat, what they eat and other factors. "I think the hard part for clinicians when they make that recommendation is we don't know how much is enough, and how much is too much, because there are so many variables," Rosner adds.

In addition, says Sterling Ransone, president of the American Academy of Family Physicians, "just saying you need to drink more fluids doesn't give people a lot of understanding into why they need to do that."

Here's a look at why doctors recommend fluids, as well as what kind of fluids you need and how to know when you're getting enough of them.

— Why do you need 'plenty of fluids' when you're sick?

Hydration is always important, but it's even more so when you have a cold, the flu, a stomach bug or a mild case of COVID-19. The main reason to pay attention to your fluid intake when you're sick is that you are probably neglecting your normal eating and drinking habits.

"When someone has a cold or feels crummy in general, they may not be eating or drinking very well," says Riley Lipschitz, assistant professor in the division of general internal medicine at the University of Arkansas for Medical Sciences. "You want to stay hydrated so you're continuing to give your body fluids and electrolytes that it needs to keep fighting off whatever illness you're fighting."

Being sick also can lead to insensible fluid loss, such as sweating from fever or blowing your nose, Rosner says, while vomiting or diarrhea can exacerbate fluid loss. So drinking more fluids helps replenish these losses.

Failing to keep up with fluid loss could lead to dehydration, which will make you sicker and could even require hospitalization. Not having enough fluids might also cause your blood pressure to drop, which, Ransone explains, will prevent blood from flowing (and delivering oxygen and nutrients) to your heart and other vital organs.

But there's another reason to consume fluids when you're ill. "When you're well-hydrated, my experience is that most patients feel better," Rosner says, adding that there's a "compelling indication" that good hydration could improve immunity, too, though not a lot of strong evidence.

It has been suggested that hydrating before getting a coronavirus vaccine will help minimize the likelihood of side effects, but there's no evidence to support that claim. However, experts say drinking plenty of fluids will likely help you feel better and manage side effects, such as fever and chills, after getting vaccinated.

— What kinds of fluids are best (and worst)?

Water is the best fluid to consume when you are ill, experts say. But if you don't like the taste of water or you're not eating much, drinks such as Pedialyte offer some flavor and electrolytes.

"Those products have a little bit of salt in them, and what the salt does is it keeps the fluid in your blood vessels and in turn, keeps your blood pressure up," Lipschitz says. "But water is never wrong."

Not everyone necessarily needs electrolyte replacement when they have a cold or flu, especially if they are generally healthy, Rosner says. But if drinks other than water are more palatable and make you feel better, there is nothing wrong with consuming them.

A study published in the American Journal of Clinical Nutrition measured how 13 different drinks affected hydration and found that cola, diet cola, hot or cold tea, coffee, orange juice, sparkling water, sports drinks, lager, and full-fat and skim milk were as effective as drinking water.

It is usually best to avoid too many sugary, caffeinated and alcoholic beverages when you're sick, though, experts say. They can make you urinate more, causing more fluid loss. But, in small quantities, these beverages are probably harmless. A good option for kids is apple juice diluted with water, Ransone says.

Opting for hot or cold fluids is a personal preference. Lipschitz says cold beverages may be best for staying hydrated since you can probably drink cold beverages more quickly than hot beverages.

What about the old wives' tales that say sipping hot drinks opens your airways and relieves sinus congestion? Research suggests there's some truth to this, Lipschitz says. A study published in the journal Rhinology found that hot drinks relieved runny nose, coughing, sneezing, sore throat, chills and tiredness, while room-temperature drinks just helped a runny nose, cough and sneezing.

Another option is chicken soup. Research has shown that chicken soup potentially has anti-inflammatory effects and could ease symptoms of an upper respiratory infection. "Chicken soup is warm; you're getting in extra fluids, you're getting some salts and electrolytes," says Ransone, who recommends soup to his patients.

— How do you know you're getting enough fluids?

You've probably heard that you should drink eight 8-ounce glasses of water a day to stay hydrated. But, Rosner says, that's just a guide. A person's size, activity level, metabolism, diet and location in a cold or warm climate can influence how much water they need. The Institute of Medicine of the National Academies found that women who were "adequately hydrated" consumed 91 ounces of water a day from food and beverages and men consumed 125 ounces.

Listening to your thirst signals is the best way to ensure you're getting enough fluids, Rosner says. "It sounds oversimplistic, but that's where your thirst comes in," he says. "It's individual, it regulates your own body, and it's going to be different for everybody."

Your urine is another tell. It should be mostly clear or slightly yellow, Lipschitz says, "If it's darker than that, I would say continue to drink more fluids until you get to that point."

If you're feeling lightheaded, dizzy or confused, or have a racing heart or dry mouth, you could be getting dehydrated. If you notice any of these signs, Lipschitz says, you should start consuming more fluids. These symptoms apply to kids, too, Ransone says. He urges parents to also pay attention if their child isn't urinating as often as usual, if their eyes appear sunken or if they don't have tears when they cry. If the symptoms for you or your child worsen, seek medical attention.

— Can you take in too many fluids?

It's rare, but it is possible to consume too many fluids in response to illness. In 2016, BMJ Case Reports published a case study of a 59-year-old woman who followed long-standing advice to drink fluids for a urinary tract infection and consumed several liters of water. As a result, she developed hyponatremia, a condition caused by water intoxication that occurs when the blood's sodium levels drop dramatically.

Frequent urination, muscle aches, nausea or vomiting, confusion, seizures and disorientation are signs that you've consumed too much liquid. Rosner says that this situation is more likely among marathon runners or in sports settings when athletes chug water, however.