When your core temp starts to rise and inhibits performance, you need to take cooling measures. Maintaining adequate hydration is important for proper thermoregulation. If you are dehydrated, then you need to know how to hydrate fast.
Do you need to rehydrate?
Thermoregulation is compromised when total body water is decreased by 1%.1 When body water depletion is equivalent to 2-3% of body mass, endurance cycling performance is reduced in the heat, at least when little or no fluid is ingested. 2 Therefore, it is important to learn when you need to hydrate. Being aware of dehydration symptoms (thirst, fatigue, headache, core temperature rise, etc) is important. A more precise way is to determine your sweat rate under varying environmental conditions and exertion efforts.
Is water the best?
Many people assume that drinking water is the best way to hydrate fast. However, research has shown that a low-carbohydrate drink will be absorbed into the blood faster than water alone, faster than a high-carbohydrate drink, and faster than a medium-carbohydrate drink.3 Such a low-carb drink is considered hypotonic, meaning that it is of lower density or concentration than blood plasma. This low density allows the fluid to flow quickly through the gut walls into the blood stream.
The article Different types of sports drink and when to use them by Precision Fuel and Hydration describes the difference between these hypotonic, isotonic, and hypertonic drinks. The article Sports hydration: why water isn't enough by Skratch Labs digs a bit more into the physiology of hydration. Both explain why a hypotonic drink is your choice when you need to hydrate fast.
In short, a drink with less than 5% carbohydrate concentration is considered hypotonic, and is the best answer to, “How to hydrate fast?” A 5% carbohydrate concentration is 5 grams per 100 ml, or 20 calories per 100 ml. This means that a 500 ml (16.9 ounces) hypotonic drink bottle would have no more than 100 calories.
While a hypotonic drink (or water) will hydrate you fast, there is a risk of overhydration. Hyponatremia involves the reduced sodium concentration in the blood, and is caused by over-dilution by water. Hyponatremia can be a life-threatening situation. It is recommended that hyperhydration not exceed 1% of body mass.1
CORE and a hydration plan
The best way to maintain an optimal hydration level is to use CORE in conjunction with a hydration plan. The hydration plan will be based on your sweat rate at a given exertion level during the anticipated environmental conditions. It will consider the electrolytes and the calories you need. Weighing yourself before and after your training session will tell you how much your hydration balance has changed.
Monitoring your core temp while testing this hydration plan during training will help you know if it is providing adequate cooling. And if you find your core temp is still rising even while you’re adequately hydrated, you will need to adopt additional cooling strategies.
For further reading about how to hydrate optimally, please see the National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active. If you plan and execute your hydration, you will have no need to hydrate fast…because you will always be at an optimal hydration level for each phase of your workout.
- National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active
- Does Hypohydration Really Impair Endurance Performance? Methodological Considerations for Interpreting Hydration Research
- The Hydrating Effects of Hypertonic, Isotonic and Hypotonic Sports Drinks and Waters on Central Hydration During Continuous Exercise: A Systematic Meta-Analysis and Perspective