Celebrities love their oversized water bottles, but how much water do we really need to drink? An expert weighs in.

The coolest accessory amongst celebrities might just be the oversized water bottle.

Olivia Wilde recently told Vogue that she makes sure she gets her H20 in by carrying around a water bottle everywhere, while former NFL quarterback Troy Aikman declared on Instagram that drinking one to two gallons of water a day was the “best” and “easiest” thing he’s done for his health. Stars like Kendall Jenner, Lucy Hale, and Dakota Johnson have all been spotted strolling with a huge water bottle in hand.

Celebrities celebrating hydration, however, can make one wonder exactly how much focus we should be putting on water in our own lives.

According to Dr. Heather Moday, a leading immunologist and functional medicine doctor who is the author of The Immunotype Breakthrough, “Hydration is absolutely crucial for optimal health because all of our cells require water to transport nutrients and to remove toxins and waste. However, just drinking water is not going to prevent health problems.”

One reason to carry around that water bottle, however, is to replace fluids that have been lost due to normal body processes, like sweating, urination and even breathing. That’s why, on hot days, it’s especially important to replenish water that’s lost — which might mean carrying around a water bottle on your errands.

Other benefits? Monday says that keeping an eye on water consumption can help keep skin healthy, and it may help you make better food choices, too.

“If you are dehydrated on a regular basis, your body will prioritize water for more vital functions in the body leaving skin looking dry,” she explains. “And although drinking water will not miraculously make you lose weight, it can help curb your appetite slightly when you are hungry by stimulating receptors in the stomach making you feel more full. This ultimately may help you cut back on calories, Often when you are feeling hungry you may just need a drink of water.”

So, how much water is enough? Or too much?

Yet exactly how much water one should consume depends. While many people have heard that one should drink eight glasses of water a day, Moday prefers an estimation based on the specific person and their lifestyle.

“Your activity level, body weight and weather can change how much water you need day to day,” Moday advises. “A good rule of thumb is half of your body weight in ounces, daily, or enough to urinate about seven to eight times daily.”

While you may naturally reach for water when you’re thirsty, there are certain signs that suggest you could be under hydrating.

“Headaches are a big sign, decreased urination, dark urine, dizziness, dry mouth and fatigue. Long term, kidney stones can be a sign,” says Moday.

Overall, listening to your body’s cues is the best way to know you’re staying hydrated.

“??Most of the time you can use thirst as a guide for drinking water,” she notes. “However, if you are in very hot and humid weather or exercising you may become dehydrated before your thirst kicks in, so in those cases I would drink before you are thirsty.”

Your fluids don’t have to come from plain water, either.

“A good rule of thumb is getting about 80 percent of your water needs through 100 percent water and the rest through other fluids like tea, juices, fruits etc.,” she explains. “Just be aware that coffee and other caffeinated beverages along with alcohol are diuretics, and may actually cause you to urinate more, which may dehydrate you.”

While it’s unlikely, Moday warns that it is possible for people to drink too much water, which can cause health issues.

“This sometimes occurs in athletes who are taking in too much water while not replacing electrolytes such as sodium and potassium, which are lost through sweating,” she explains. “This can cause hyponatremia (low sodium) which is a potentially life threatening state. Some signs include swelling of the extremities, headaches, dizziness and fatigue. A simple fix is to watch the color of your urine — it should be clear and straw colored, not colorless.”

The celebrity water bottle trend isn’t just for fashion — so if you find yourself thirsty while out and about, or if you’re heading out on a particularly hot day, it might be worth toting one yourself.