Experts Say This Simple Mistake Could Be Making Your Workout Feel Harder

Ever finish a workout feeling unusually sluggish, even though you brought your water bottle and drank throughout your session?

As a registered dietitian and personal trainer, I know hydration during exercise matters. But new findings suggest the real issue may be starting your workout dehydrated. In fact, research from the Gatorade Sports Science Institute recently found that nearly half (43%) of athletes arrived at workouts already dehydrated. That means they were starting exercise at a disadvantage before even warming up.

The study, which included more than 500 women across activities including basketball, soccer, and ice hockey, also found that many participants were not replacing enough sodium or consuming enough carbohydrates during exercise. And while elite athletes were the ones being studied, the lessons extend beyond competitive sports.

Whether you're training for a marathon, heading to a cycling class, or squeezing in a quick walk after work, staying hydrated all day long can make a noticeable difference in how you feel and perform.

Hydration isn't just about replacing the sweat you lose during exercise. It's about making sure your body has what it needs before your workout starts.

“When you show up already dehydrated, you're starting at a deficit,” says Kimberly Stein, Ph.D., senior principal scientist at the Gatorade Sports Science Institute (GSSI) and lead researcher of the study. “Having a hydration plan during a workout is important, but really hydrating throughout the day and showing up already hydrated is truly the best thing you can do for yourself.”

Even mild dehydration can make exercise feel harder than it should. You may fatigue more quickly, struggle to regulate your body temperature, notice your heart rate climbing faster, or just feel like you have less energy than usual. That’s because dehydration reduces blood volume, meaning your heart has to work harder to deliver oxygen throughout your body. But the good news is that this is one of the easiest workout mistakes to fix.

Thirst is an obvious clue, but it isn’t always the first one. Some signs you may be starting your workout dehydrated include:

• Dark yellow urine
• Dry mouth
• Headache
• Feeling sluggish or unusually tired
• Dizziness or lightheadedness
• Muscle cramps
• Feeling like your workout is harder than it should be

If you're regularly heading into workouts feeling drained before you've even broken a sweat, it may be worth paying closer attention to your hydration throughout the day.

One reason hydration advice can be confusing is that there isn't a one-size-fits-all recommendation. People sweat at different rates depending on factors like body size, fitness level, workout intensity, temperature, humidity, and even genetics. That's part of what inspired this recent study.

Stein says the researchers realized the existing sweat-rate classifications they were using were based largely on data from male athletes, particularly football and basketball players. As they tested more women, from collegiate athletes to professionals, they found those standards didn't accurately reflect women's physiology.

The team developed updated reference ranges so sweat-rate classifications better reflect differences in body size while still recognizing that women can lose significant amounts of fluid during exercise. While women have lower sweat rates than men on average, the research found significant variation among women themselves. That means hydration needs should be personalized based on your individual needs.

There's no magic number that works for everyone. Instead, your goal during exercise is to replace enough fluid to limit excessive dehydration without drinking so much that you overdo it.

According to Stein, losing a small amount of body weight during exercise is normal. But once fluid losses reach around 2% of your body weight, performance can begin to suffer. For a 160-pound person, a 2% loss of body weight is a little over 3 pounds. “You don't have to match every ounce you lose,” she explains. “A little dehydration is fine. We just want to avoid that threshold.”

For many people, aiming to stay within about a 1% body weight loss is a practical target. Stein adds that it’s surprisingly simple to calculate your sweat rate. Here’s how:

• Weigh yourself before your workout.
• Exercise for about an hour under typical conditions.
• Keep track of how much you drink.
• Weigh yourself again afterward in similar clothing.

As a general rule of thumb, every pound of body weight lost during exercise represents about 16 ounces of fluid. Knowing your sweat rate provides a personalized starting point for future workouts. “If your sweat rate is 1.5 liters per hour but you're only drinking one liter, we can see exactly how much body weight you lost and recommend increasing your intake next time,” Stein says.

Keep in mind that your sweat rate isn't fixed. It changes based on factors like the weather, humidity, workout intensity, and even the season. If you primarily exercise indoors in an air-conditioned gym, your sweat rate may stay relatively consistent. If you work out outdoors, though, it's worth reassessing it several times a year.

For most workouts lasting less than an hour, water is usually all you need to stay hydrated. But if you're exercising for longer than about 60 minutes, working out in hot or humid conditions, or you're a particularly heavy sweater, replacing electrolytes (especially sodium) becomes increasingly important.

Sports drinks, electrolyte beverages, powders, or tablets can all help replenish sodium and other electrolytes lost through sweat. These products can be especially helpful during endurance events, back-to-back training sessions, or long workouts in the heat. Some sports drinks also provide carbohydrates, which can help fuel muscles during prolonged exercise.

Keep in mind that electrolyte needs vary just as much as sweat rates do. Someone taking a leisurely walk may not need anything beyond water, while a marathon runner or someone exercising outdoors in high temperatures could benefit from a beverage that replaces both fluids and electrolytes. Knowing your sweat rate and paying attention to how you feel during and after exercise can help you determine what works best for you.

Hydration isn't just something to think about once you're halfway through a workout. One of the biggest mistakes may be showing up already dehydrated.

Research suggests that many athletes begin training without being fully hydrated, and the same challenge may extend beyond competitive sports. Fortunately, the fix is simple: drink consistently throughout the day, learn how your body responds to exercise, and don't rely on one-size-fits-all hydration advice.

Stefani Sassos, M.S., R.D.N., NASM-CPT is the director of the Good Housekeeping Institute Nutrition & Fitness Lab, where she oversees nutrition and fitness content, product testing and evaluation. A registered dietitian and personal trainer, she provides evidence-based guidance on nutrition, exercise and healthy living.