Training for a Spartan race can do a number on your body if you’re not smart with your nutrition and recovery. Most athletes take that to mean getting an adequate amount of protein and sleep, but what you may not be taking into account is whether you have low electrolyte levels. This can have a direct influence on your performance.

Related: This Is Why Hydration Is More Important Than You Think

In fact, research has found that losing as little as 1-2% of fluids and electrolytes through sweat can cause you to feel more fatigued. (And chances are that when you’re competing, you may be losing a lot more than that.) Here’s everything you should know about electrolytes, starting with what makes them so crucial in the first place.

How Low Electrolyte Levels Can Derail Even the Most Elite Athletes

“Electrolytes are certain minerals like sodium, potassium, chloride, and magnesium that humans need quite a precarious balance of in order to maintain adequate heart function, muscle function, pH balance, and hydration status,” Monica Auslander Moreno, MS, RD, LDN, a nutrition consultant for RSP Nutrition, says.

In the case of exercise and training, electrolytes help maintain proper fluid balance in your body in addition to contributing to muscle function (including your heart). When you sweat, that fluid loss includes water and electrolytes. And if you’re dehydrated, your muscles get fatigued faster on top of performing slower, and workouts feeler even tougher than usual.

Related: This Is the Key to Keeping Energy and Electrolytes High When Training

What’s the Right Amount of Electrolytes?

There’s no one-size-fits-all rule when it comes to proper electrolyte levels, but you can base much of it off of your body weight. To start, weigh yourself both before and after exercising. The American College of Sports Medicine (ACSM) recommends continually drinking during exercise to prevent excessive water weight loss (anything more than 2% of your body weight). If you find that you’re staying in this range, there’s a good chance your fluid and electrolyte levels are relatively stable.

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If you lose more than that, or feel dizzy, fatigued, shaky, have muscle cramping, or even vomit at any point before, during, or after training, these are warning signs that you may have low electrolytes, Moreno says.

Don’t forget to take into account the time and location of your training, as the climate, altitude (which may make you exert yourself more than usual), and duration can all increase the amount of electrolytes lost through sweat. The ACSM says that it’s possible to lose anywhere between four to 10 liters of water and 3,500 to 7,000 milligrams of sodium in these types of situations.

What to Do When You Have Low Electrolytes

While water may cut it for shorter workouts, Moreno says to consider sipping on an electrolyte-based sports drink for any workout session longer than 90 minutes.

Related: 4 Reasons Your Muscles Cramp Up (and What to Do About It)

Drinking electrolytes while exercising is important in maintaining nutrient and fluid levels, but in those instances where you’ve been sweating and losing more fluid than normal and are looking increase low electrolytes, you don’t have to resort to chugging sports drinks for the next 24 hours.

There are also plenty of foods that have high levels of electrolytes in them, including as bananas, celery, kale, beets, pumpkin seeds, potatoes, oranges, chocolate milk, and even pickles. Pack your post-workout meal with some of these foods to help refuel and rehydrate quickly.