Sweat Much? Proper Hydration For Indoor Training

Bike Nashbar is pleased to introduce the first of a three-part series of articles, Get More Out of Your Indoor Trainer, written by well-known sports science researcher Dr. Robert Portman, the author of Nutrient Timing, The Performance Zone and Hardwired for Fitness.

As the weather turns colder, serious cyclists will be moving indoors to stay in shape. Indoor workouts require multiple decisions: which trainer to buy, since there are many options, and, what are the best workouts. All too often, in the selection of the best trainer and workout regimen, nutrition issues are often ignored. This article is the first in a three-part series designed to help you maximize the benefits of your indoor training. This article focuses on rehydration, while subsequent articles will show how to maximize fueling and recovery.


A cyclist riding hard on an indoor trainer.

Proper rehydration makes a world of difference for your indoor training sessions.

Hydration is the cornerstone of any athlete’s regimen. Although working muscles are extremely efficient in converting fuel to energy, they are not perfect. In general, about 60% of the energy released during exercise is wasted as heat rather than used to fuel muscle contractions. By comparison, a well-tuned engine is only about 30% efficient which means 70% of the energy is released as heat. In fact, without any ability to cool, 25 minutes of hard exercise could bring your body temperature to 106°, which is a life-threatening temperature. Generally, hydration is even more important during an indoor session. During outdoor cycling air flowing over the skin causes evaporation and cooling. Indoor cycling lacks air flow which is why you sweat much more on your trainer.

To prevent an unhealthy rise in temperature your body has developed a sophisticated system of cooling in which heat from muscles is transported to the surface of the skin as sweat. As sweat evaporates, it cools the blood and helps maintain body temperature. During this process significant amounts of body fluid is lost. The effects of even moderate dehydration are an increase in fatigue, an increase in heart rate, a decrease in endurance performance, and an increase in the stress hormone cortisol.

For each 1% loss in body weight due to sweating, heart rate increases 5-8 beats per minute. It is not unusual for a 150-lb. individual to drop three pounds during a hard indoor training session. That’s 2% of your body weight. The table below shows the consequences of fluid losses thru sweating on exercise performance.

Body Water Loss Effects
0.5% Increased strain on heart
1% Reduced aerobic endurance
3% Reduced muscular endurance
4% Reduced muscle strength; reduced fine motor skills; heat cramps
5% Heat exhaustion; cramping; fatigue; reduced mental capacity
6% Physical exhaustion; heatstroke; coma


Water is still the preferred rehydration drink for most athletes. However, water is not ideal in replacing fluids lost through sweating. Researchers showed over 4 decades ago rehydration could be significantly enhanced by adding sodium to water. The reason being that water absorption is dependent upon sodium transporters in the cell. As sodium is reabsorbed water follows. Additionally, the electrolytes in the drink maintained thirst to encourage an athlete to continue drinking.

A cyclist riding hard on an indoor trainer.

Do not overlook the unique hydration demands of working out on an indoor trainer.

Although the first sports drink only contained electrolytes, it was soon noted that the addition of carbohydrates could further improve rehydration.  Carbohydrates help absorb sodium using a different sodium transporter. A combination of carbohydrates and electrolytes produces a more complete rehydration than just electrolytes or water alone. We now know that there are a number of sodium transporters, a carbohydrate-dependent sodium transporter as well as a protein-dependent sodium transporter.

The benefits of activating multiple sodium transporters was clearly demonstrated in a study comparing rehydration properties of water, an electrolyte/carbohydrate beverage and electrolyte/carbohydrate/protein beverage (Accelerade®). The researchers found Accelerade was:

  • 15% more effective than the electrolyte/carbohydrate beverage
  • 40% more effective than water


  • Hydration needs during your indoor workout should be an integral part of your total regimen which includes selecting your workout and the TV program you will be watching.
  • An electrolyte/carbohydrate/protein sports drink, such as Accelerade, will optimize rehydration
  • It is better to consume your hydration drink in smaller but frequent sips, which is far easier to do on an indoor trainer. This puts less stress on your GI system and improves the efficiency of rehydration.

Stay tuned for Part 2 of Get More Out of Your Indoor Trainer, which will focus on how proper fueling can improve performance.

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