Unfortunately, there is no easy and universal answer to that question. According to Noakes, it is "the amount of sodium and potassium in the body that determines the water balance, not the other way around." Therefore, your electrolyte balance affects your hydration balance.
A good place to begin is to consume a sports drink containing electrolytes when training or racing for durations over two hours. If you believe, based on the salt mine on your clothing and helmet straps, that you lose a lot of electrolytes in your sweat consider supplementing with electrolyte tablets.
Begin with the low end of the manufacturer's recommended dosage AND the recommended fluid intake. Be very wary of water cooler or message board recommendations to pop a handful of electrolyte tablets because "top racer Ricky or Renee" does.
Remember: your body likes balance. Consuming too much water and no electrolytes during extended exercise is not good and can cause hyponatremia. Consuming too many electrolytes with too little fluid is not good and can cause your body to retain fluid rather than releasing it for cooling purposes.
Fluid Replacement
The average sweat rate is between 0.8 to 1.4 liters (roughly 27.4 to 47.3 ounces) per hour during exercise. The average fluid absorption rates range from 0.8 to 1.2 liters per hour (27.4 to 40.6 ounces). Unfortunately, while the sweat-rate range and the fluid absorption ranges are close, some athletes sweat at higher rates per hour than their fluid absorption rate.
Do you have to replace every drop of sweat you lose during exercise? The rule of thumb is that you should neither gain weight during exercise (consuming more fluid than you lose) nor lose excessive weight. Excessive weight loss is considered to be more than 2 percent of your body weight.
If you lose up to 2 percent of your body weight, performance is affected less if you are performing in a cool environment than if you are performing in a hot environment. Weight losses above 2 percent of your body weight should be avoided.
Gastric Emptying
Sloshing. It's a horrible feeling. Sloshing occurs when you are consuming fluids--and perhaps solids--but rather than moving to your intestines the mix just sits in your stomach. Sloshing is worse for runners and mountain bikers than road cyclists, but roadies get it too.
Eventually, sloshing is followed by slowed-to-a-snail's pace performance, barfing or both.
You can increase the rate of gastric emptying by:
- Keeping a small fluid volume in your stomach by consuming fluids every 15 to 20 minutes rather than attempting to consume larger quantities of fluid at 60-minute or more intervals.
- Keeping fat, protein and carbohydrate concentrations low. How much protein and fat you can tolerate depends on your exercise pace and individual tolerance level.
- Remaining hydrated. Dehydration decreases gastric emptying.
- Keeping exercise intensity low. Intensity above 75 percent of maximum decreases the emptying rate.
You can increase intestinal fluid absorption by:
- Using a low-to-moderate level of glucose plus sodium.
- Using a low-to-moderate level of sodium.
What Should You Do?
All of this information can seem overwhelming. To begin cracking your hydration code:
- Determine your sweat rate in various environmental and racing conditions.
- Once you know your sweat rates, hydrate at rates appropriate to each situation. Your sweat and hydration rates are not constant and will need conscious modification given your fitness, the environment and your pace.
- Hydrate so that you keep your body weight loss to less than 2 percent during training and racing sessions.
- Avoid over-hydrating and weight gain during exercise sessions.
Most importantly, know that you have a range of optimal fluid replacement and what is optimum for you may or may not work for the athlete standing next to you.