August 26, 2015 05:55 PM

Getty; Inset: Craig Sjodin/ABC

Harley Pasternak is a celebrity trainer and nutrition expert who has worked with stars from Halle Berry and Lady Gaga to Robert Pattinson and Robert Downey Jr. He’s also a New York Times best-selling author, with titles including The Body Reset Diet and The 5-Factor Diet. His new book 5 Pounds is out now. Tweet him @harleypasternak.

Labor Day is just around the corner but that doesn’t mean that we’ve seen the last of the dog days of summer. We’re on track to have the hottest summer in recorded history, and a heat wave only makes it more important that you stay hydrated. Moreover, any activity that makes you sweat and breathe heavily robs your body of fluids, so caution when exercising is critical.

I can’t say it often enough: Drink up!

Crossed Signals …
Amazing but true: depending upon your age and sex, somewhere between 55 and 65 percent of your body is comprised of water. Diminishing that amount can have serious consequences.

How do you know how much to drink and when to drink it? Most people are borderline dehydrated a good deal of the time — that means that waiting until you feel thirsty is an unreliable signal. However, a clear indication that you’re dehydrated is the color of your urine: It should be pale yellow, with the exception of your first morning pee. Additionally, it’s common to confuse hunger with thirst. If you feel like you’re starving, sip before you sup to see whether those supposed hunger pains disappear.

… and Clear Signs
Being dehydrated can also fatigue you, give you a killer headache or muscle cramps and/or make you feel lightheaded or confused. Extreme dehydration can lead to serious consequences, including fever, heart palpitations, low blood pressure, vomiting and more.

Most of these symptoms are caused by electrolyte imbalance. Your blood and other body fluids contain minerals, including sodium and potassium, known as electrolytes, which help ensure the body’s fluid balance. They also control proper muscle and nerve function. When you perspire or otherwise lose body fluids, you also lose electrolytes.

How Much (and When) to Drink
To avoid dehydration, I recommend that women drink about eight cups of water a day and men up to 10 cups as a matter of course. In hot weather and when you are exercising vigorously, you’ll need even more. If you spread your intake over the day, you’ll be less likely to overeat. Have your last glass of water a couple of hours before bedtime, and you’ll be less likely to interrupt your beauty sleep with middle-of-the-night trips to the bathroom. Getting a good night’s rest is one key to setting your internal time clock, which believe it our not, helps control your weight.

Don’t Drink Your Calories
One of my nutritional rules is that you should eat your calories, not drink them, which is a one reason I’m such a fan of water. That’s why OJ and other fruit drinks don’t make my list of recommended beverages unless it’s a treat — I love nothing more than a glass of fresh-squeezed OJ with my Sunday brunch.

If you find plain old H2O boring, add a spritz of lemon or lime juice, or opt for sparkling water or a sugar-free flavored beverage like vitaminwater zero, which uses a natural sweetener and comes in a bunch of flavors to keep your taste buds from getting bored.

Most of the time, sufficient and regular water (or one of the alternatives above) intake is all you need to stay hydrated. However, if you’re engaged in prolonged exhaustive cardio, such as training for a marathon, you’ll need an electrolyte beverage with a little sugar, such as Powerade or coconut water. At such times, I make an exception from my usual drink-no-calories stance.

5 Hot Weather Exercise Tips
If you’re careful, adhere to a few basic rules and make some modifications, there’s no reason why you can’t continue to exercise in hot weather. The trick is to prevent dehydration, rather than having to treat it.

-Check the weather forecast. Avoid being outdoors and/or exercising outdoors on high heat index days.

-Be an early bird. Take your walk, jog or run first thing in the morning before the sun is high in the sky. Or find an air-conditioned track where you can put your miles in. Or if you can do so safely, go out after the sun has set. Better yet, swap your jog for a swim.

-Fill your “tank.” Have a cup or two of water an hour or more before you go for a run or walk so you have some reserve on hand.

-Don’t leave home without it. The “it” is your water bottle, of course. Take a big swig every 20 minutes or so. If you’re sweating profusely, drink more each time or more frequently. During a 45-minute exercise class, expect to consume a whole bottle of water. I’m almost never without a smartwater in one hand and my phone in the other.

4 More Pieces of Hot Weather Advice
Even if you’ve decided to take a break from your usual fitness routine on sizzling days, simply being outdoors in such weather can stress your body. If you must venture out into the cold — correction: hot — cruel world:

-Go light and loose. Wear lightweight, light-colored and loose-fitting clothes that breathe. Knits and clothing that wick moisture away from your body are good options. This is not the weather in which to show off your new tight-fitting black spandex pants.

-Pace yourself. Cool off between exposures to heat throughout the day much as you would break up high intensity exercise with less intense moves. If you’re running errands on foot, take a break to enjoy an iced coffee — and the AC — before completing your errands.

-Avoid the hard stuff. Alcohol and extreme heat don’t mix well because wine and booze increase your water loss and make you less attuned to the first signs of dehydration. Drink 8 oz. of water in between alcoholic drinks to replace fluid.

-Don’t overdo the fluids. In your concern about dehydration, don’t swerve too far in the opposite direction. Flooding your body with fluids — consistently drinking more than 8 ounces in less than 20 minutes — can cause hyponatremia, a.k.a. “water intoxication.” The dangerous condition, which extreme athletes sometimes experience, results from severe dilution of sodium in the blood stream.

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