Nutrition Q &A: Fluid Needs During a Time Trial

Q: Do I need to drink fluids during a time trial that lasts longer than 20 minutes?

A: The answer to this question lies in the answer to this: Do you know your sweat rate? Generally speaking, you probably know if you sweat a lot while exercising, but it is the amount of sweat you lose that will determine if you need to drink or not during a TT.

A loss of body weight of just 1% (1.5 pounds in a 150-pound athlete) can adversely affect the body’s ability to cope with the stress of exercise (1). A loss of more than 1% of body weight can impair athletic performance. (2) If you can maintain proper hydration your heart rate, core temperature, and perceived exertion will be lower, while your stroke volume, cardiac output and skin blood flow will be higher, equating to better performance. (3)

The amount of fluids you need is based on environmental and physiological conditions. For example, if the weather is warm and humid your body will lose more body water and electrolytes via sweat. On warm days, strive for the upper limit of the fluid recommendations below. A time trial is a unique challenge for an athlete, so you must listen to your body and consider your own individuality and practice with different amounts of fluids during your training.

The following general recommendations for athletes are based on scientific research, but knowing your own needs is better than following these guidelines since sweat rate depends on numerous individual factors:

For every 15-20 minutes, aim for 5-10 fluid oz of a sports drink (14-19 grams of carbohydrate per 8 oz.(6-8% carbohydrate)). (2)

How do I determine my sweat rate?

During a practice TT, determine your sweat loss rate by weighing yourself naked before and after. Record these numbers, and the distance, time and weather conditions.

It is imperative you replace the fluids and electrolytes lost. For every pound of body weight lost, replace with 16-24 oz of a combination of a sports drink and plain water. Drink gradually over the course of an hour vs. all at once to aid in optimal absorption and to reduce bloating. You need to drink more than the weight you lost because some amount of whatever you drink will be converted to urine, even if you are dehydrated.

Once you know how much sweat you lost, add the weight of fluids you drank during your TT to find the total weight of water you sweated and breathed out. You must account for urine volume lost – so if you can wait to urinate until after you’ve weighed yourself, then do. Otherwise measure the amount.

Sweat Rate Example:

Pre TT Weight: 163 lbs Post TT Weight: 162 lbs

Fluids consumed during TT: 4 oz

163 lbs -162 lbs = 1 lb weight ∆ (16 oz) + 4 oz fluids ingested = 1.25 lbs (20 oz)

20 oz/40 minute time trial = 0.5 oz/minute = sweat rate

To maintain fluid balance and replace sweat loss, one would have to drink 20 oz during the 40 minute TT but a fluid loss of up to 1% of body weight is okay, and 1.25 lbs is less that 1.6 lbs (26 oz = 1% of 162 lbs), so this rider does not need to drink during a 40 minute TT in similar conditions to the test TT, but would need to drink during a longer TT of 52 minutes or more (26 oz lost at 0.5 oz per minute). In TTs over 52 minutes this rider needs about one oz of fluid for each extra two minutes.

Every mouthful is around 1 oz. Drinking a few ounces more than absolutely necessary won’t hurt you.

While drinking may be a task you haven’t mastered yet while on a TT bike, the benefits are far greater than the disadvantages of carrying a water bottle or hydration pack. Drinking fluids during a time trial can not only provide your body with water but with carbohydrates and electrolytes that are in high demand. Next time you prepare for a TT consider how fluids influence your body and how a lack of body water can be detrimental to performance. In one study, drinking one 16 oz bottle of water during a 1 hour TT saved the riders over one minute compared to the same riders doing the same TT without water.

Main points:

Determine your individual sweat rate and fluid needs during your TT’s. When weather conditions change be sure to modify amount accordingly.

Practice drinking the specified amount during your TT. Practice makes perfect!

Note changes in sweat rate, overall performance and monitor progress.

If you need assistance with determining how to calculate your fluid needs, contact a Wenzel Coach.

Heather D’Eliso Gordon, RD, CSSD


1. Bergeron MF. Sodium: the forgotten nutrient. Sports Sci Exch. 2000;13:1-8. Available at: Accessed February 9, 2005.

2. Rosenbloom CA. Sports Nutrition Client Education Handouts CD. American Dietetic Association 2006.

3. Murray, R. Fluids & Electrolytes. Rosenbloom CA,Editor. Sports Nutrition, A Guide for the Professional Working with Active People 3rd ed.1999; 100-103.