Fluids and Electrolytes for Hydration in Sports Performance

Hydration is important for optimal sports performance. Fluid losses as little as 1% of body weight can affect the body’s ability to cope with exercise and heat stress. Further dehydration can also negatively impact performance. Hydration status is affected by environmental factors such as temperature and exercise and is also affected by fluid and electrolyte intake (1).

Electrolytes, such as sodium, potassium, calcium, chloride and magnesium play important roles in cells and their concentration within cells and body fluids are tightly regulated. When levels become too high or too low there can be serious consequences within the body. Sodium and chloride are the two electrolytes that are the most important to hydration as they are lost in sweat in high amounts. Sodium intake is especially important in hydration to prevent muscle cramping and avoid low sodium levels in the blood (1).

If you drink purely water without sodium or too little sodium during long periods of exercising and sweating this can lead to low sodium levels. Sodium and fluid are lost through sweat, and both need to be replaced. Low sodium level in the blood is called hyponatremia. Hyponatremia can be a medical emergency and usually presents with lightheadedness, dizziness, fainting, and can lead to seizures, coma and death (1,2). Many athletes that faint during a race are experiencing this. It is still considered a rare occurrence, but it does happen.  

On the other end of the spectrum, you can also intake too much sodium with not enough fluid. This can also lead to dehydration as water will be pulled out of cells. Think back to when you learned about osmosis in science class. Water will flow to the area of solutes with a higher concentration, in this case water is pulled from cells into your intestines to dilute the high sodium (1). This results in a loss of fluid within the body.

What you are probably wondering now is how much fluid and electrolytes should I have during exercise to prevent dehydration and hyponatremia? This is largely dependent on your sweat rate as well as the concentration of sodium and chloride in your sweat.  Sweat rates as well as concentrations of sodium vary from person to person. Athletes with higher sweats rates, high sodium concentrations in their sweat, exercise greater than 4 hours and small body size are all at a higher risk for developing exercise induced hyponatremia. Higher sweat rates and concentrations can be identified by white salt deposits on clothing. You can get an idea of your sweat rate by training in a similar environment to race conditions and weighing yourself before and after a workout. Accounting for fluids consumed and urine lost, the amount of sweat lost per hour can be calculated (2).

Sweat rate (L/h) = body weight difference (1 kg = 1 L) + water intake (L)—urine volume (L).

Fluid losses through sweat have been found to range anywhere from 300 mL/hour to 2400 mL/hr. To put this in perspective one 8 oz cup = 240 mL. Once you know your personal sweat rate for a set of given conditions you can determine how much fluid you need to replace per hour. Studies recommend limiting fluid intake to no more than 600-700 mL/hr2,3,4. This is because higher fluid intake may lead to gastrointestinal discomfort and hyponatremia (3).

In terms of sodium losses, the average sodium sweat content is 1 g/L but this will vary greatly based on the person4. There are ways you can measure your sweats' sodium concentration in a lab setting or with a patch. One source recommended replacing sodium at 300-600 mg/hr for those who are salty sweaters, have sweat rates over 1200 mL/hr or with prolonged exercise over 2 hours (4). The USA track and field advisory recommends 500-700 mg/L of sodium (1). Another source recommends endurance athletes take in 460 mg/L to 1150 mg/L in exercise over 2 hours (1). Most sports drinks and electrolyte drinks have between 230-690 mg/L of sodium when mixed correctly but you should check this on packaging (4).

To recap, you should ensure you are getting balanced fluid and sodium in your nutrition throughout your race for optimal hydration and to prevent hyponatremia. Fluid and sodium intake should be individualized as needs vary greatly depending on the person and conditions. You can work with a registered dietitian to optimize your race day hydration plan to keep you safe and well hydrated.  

References

1.      Karpinski C, Rosenbloom C. Chapter 6: Fluid, Electrolytes and Exercise. In: Sports Nutrition: A Handbook for Professionals. Academy of Nutrition and Dietetics; 2017:107-129.

2.      Armstrong LE. Rehydration during Endurance Exercise: Challenges, Research, Options, Methods. Nutrients. 2021;13(3):887. Published 2021 Mar 9. doi:10.3390/nu13030887

3.      Nikolaidis PT, Veniamakis E, Rosemann T, Knechtle B. Nutrition in Ultra-Endurance: State of the Art. Nutrients. 2018;10(12):1995. Published 2018 Dec 16. doi:10.3390/nu10121995

4.      Vitale K, Getzin A. Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations. Nutrients. 2019;11(6):1289. Published 2019 Jun 7. doi:10.3390/nu11061289