Sweat Rate Testing – how much fluid do you need during a marathon?
Sweat Rate Testing
How much fluid do you need to consume each hour during your marathon? Remember aid stations with Gatorade and water are every 2 miles starting at Mile 2
Do you know how much you sweat and how much you need to drink, or are you just guessing and hoping it works out? Many runners’ under-hydrate and can lose significant amounts of fluid which is detrimental to performance. Sweat rate losses can range from .8-2 liters (27 – 67oz) per hour in the heat. Any loss greater than 2.5% body weight (3.75 lb for a 150 lb runner) can have a negative affect so it is recommended to drink enough to lose no more than this amount. Over-hydration also is problematic and can cause stomach upset or dangerous electrolyte imbalances such as hyponatremia (low blood sodium levels). Additionally, many factors will affect how much fluid you need like temperature, humidity, altitude, sun exposure, pace and fitness level. So how is a runner to know how much to drink? Answer: do a sweat rate test and try to mimic the same variables you will experience during your race like time of day, temperature, etc. This is not a precise test but it is a decent way to help you understand your own needs for a given set of conditions. In short you simply want to weigh yourself before and after a run (typically 1 hour) and see how your weight changed. Here is an example:
Step 1: weight nude before run 150 lb
Step 2: weight nude (towel off all sweat) after run 149 lb
Step 3: weight change during run lost 1 lb or 15.5 oz fluid
Step 4: account for fluid consumed during run drank 8 oz
Step 5: add step 3 plus step 4 15.5 + 8 = 23.5 oz
Fluid needs are then approximately 24 oz (or 6oz every 15 min if tolerated). Drinking less (18-20 oz) is fine in that it is not necessary replace 100% of losses. The goal is to drink enough to prevent a loss greater than 2.5% body weight over the length of your entire run.
This example may be typical for the first hour of a run that starts in the morning but your losses will be much greater as the temperature increases and your body has been pounding the pavement for 4 hours. You could do another sweat rate test on a long run and just test the final hour. Try to avoid urinating during these tests so perhaps run for 3 hours then go to the bathroom right before getting on the scale for your pre weight then run your final hour and weigh again. Or you could do a sweat rate test for 2 hours. After you have done several tests you should be able to get an idea of how much you need to drink in different circumstance. Then see if you are able to tolerate drinking this amount. Like sweat rates, the amount of fluid your gut can tolerate is highly individual. It may help to drink on a regular, scheduled basis like every 15 minutes. Pay attention to your thirst, as it is a good indicator of fluid needs. Also make sure you practice drinking while training and keep track of your intake so you can determine what works best for you.
Article by Jacque Maldonado, MS, RD, CSSD, CDE
Jacque has a B.S. and M.S. in Nutrition and is an avid endurance athlete having finished four full Ironman® Triathlons and over 25 marathons including qualifying for Boston. She is also a USA Triathlon coach Level 1 and a Certified Specialist in Sports Dietetics. Jacque currently works in Kaiser Permanente’s Population & Prevention Services department.