Predicting SWEAT RATE and electrolyte needs...

Other than lab testing there is no exact way in which you can measure sweat losses, but you can make an educated estimation. In hot weather the simplest calculation that you can make involves checking body weight changes before and after training, and correcting for fluid intake during training. Fluid loss from urine loss would also affect this equation, as would weight gain from any solid food consumed. For the example below, let's look at how you can calculate sweat losses during a bike ride when consuming a carbohydrate-electrolyte beverage.

1. Check your weight before and after training and calculate your weight loss.
160 lb. to 158 lb. (73 kg to 72 kg)
2 lb. weight loss during training (0.9 kg)
(To obtain weight in kilograms, divide weight in pounds by 2.2.)

2. Know the amount of fluid that you consumed during the training session. Fifteen ounces of fluid weighs about 1 pound (an easier estimate is 1000-ml fluid equals 1.0 kg body weight). You can also weigh the bottle before and after your training session to see how much the fluid you consumed weighs.

Let's say you consume 60 ounces (1800 ml) of fluid during a three-hour bike ride. This fluid weighs 4 lb. (1.8 kg)

. Add the answers in 1 and 2.
2-lb. weight loss during training plus you consumed 4 lb. of fluid during training
This equals 6 lb. of fluid lost.
(0.9 + 1.8 kg = 2.7 kg)

4. 6 lb. equals 90 ounces fluid divided by 3 hours = 30 ounces per hour for sweat losses.
(2.7 kg equals 2700 ml fluid divided by 3 hours = 900 ml/hr).

In this example the athlete sweats about 30 ounces or 3.75 cups of fluid per hour (900 ml). Sweat rates can vary from 24 ounces or 3 cups of fluid per hour (720 ml) to 80 ounces or 10 cups (2500 ml or more) per hour.

A two-pound fluid loss is fairly significant and indicates that you need to drink more when training. Athletes who gain weight during training, may actually be drinking too much. Overdrinking is a concern in the development of hyponatremia or low blood sodium. However, most athletes do not meet their fluid needs and chances are that you will not match 100 percent of your sweat losses. Check you weights before and after training, and determine if you need to step up your fluid intake efforts. Rehydration should also be a main priority after training. Aim for 20 ounces (600 ml) of fluid afterwards for every pound lost during training. You should also include some sodium in your rehydration fluids and food choices as part of your recovery meals and snacks to improve your fluid balance. This is especially important following training sessions where optimal fluid intake may be more challenging and your weight loss is greater.