By Bob Seebohar, MS, RD, CSSD, CSCS, METS II
Just because it is summer doesn't mean cyclists can't enjoy a nice indoor trainer workout from time to time. In fact, VO2 and interval training workouts are quite popular to do inside under controlled conditions. If you are planning on spending some time in the saddle indoors, remember it should come with a bit of a paradigm shift in your nutrition approach.
When riding outside, we usually go through our normal checklist of items to bring before heading out: cycling kit, helmet, shoes, socks, computer, sunscreen, sunglasses, gloves, full water bottles, nutrition items, tire kit, CO2 cartridges or pump, check! It's quite a process that can take up to 10 minutes just to prep for an outdoor ride. You want to be as prepared as possible for any situation that arises.
However, put your bike on an indoor trainer and the elaborate preparation plan goes out the window. Grab some cycling shorts, your shoes, a top and turn on some music or television and hop on the bike. It's as easy as that. Unfortunately, this lack of preparation can affect performance.
While there is research that shows the perceived effort riding indoors is higher versus outdoors, it is common to have an increased core temperature while riding inside. This is due to having a lesser skin cooling effect from the wind. You may have a fan but if the temperature is still warm in the room, your body will not cool itself as efficiently. This can lead to a greater sweat rate and loss of fluid, which is often the case when riding a turbo trainer indoors.
Depending on your objective for the ride (aerobic, strength, drills, intervals) and the duration, you may or may not require a bit more fluid when on the indoor bike trainer. While there are no steadfast recommendations about how much more to drink while riding indoors versus outdoors, as it is so dependent upon indoor environmental conditions, acclimation to heat and sweat rate, it is apparent that more fluids should be consumed per hour under these conditions:
- Duration exceeding 60 minutes, especially threshold and sprint intervals
- Little or no air flow (fans, open windows)
- High sweat rate (average sweat rate among population data is roughly 1 liter per hour).
Now, we have identified the obvious fact that you may need to hydrate a bit more while riding inside and the fact that there are no exact recommendations for a hydration program. So, what is a cyclist to do?Measure and monitor.
The best way to understand your body's hydration needs is to do some measuring or testing. This type of testing that I will refer to is quite easy to do and only requires a scale. It is called sweat rate testing. Basically, you step on a scale nude before you hop on your bike and again after your ride. Subtract your pre-weight from your post-weight and add in how much fluid you drank for the duration of the ride and you can then determine how much fluid you lost and how much you need to drink per hour.
For example, you ride for 1.5 hours on the trainer and you weigh 160 pounds nude pre-ride. You weigh yourself nude right after the ride and you weigh 159. 5 pounds. You have lost 1/2 pound. One pound is equal to 16 ounces of fluid so you have lost 8 ounces. Perhaps you drank 22 ounces of water during the ride so you have to add that into the equation: 8 ounces + 22 ounces = 30 ounces of fluid loss. Divide 30 ounces by the duration of cycling (1. 5 hours) and you come up with a sweat rate of 20 ounces per hour. 
I have seen sweat rates much higher than this during indoor stationary trainer rides. In fact, I have seen values over 40 ounces per hour. This could be problematic because the body cannot absorb that much fluid per hour. Standard fluid recommendations per hour usually do not exceed 24-28 ounces. So, what do you do if you are a heavy sweater and lose more fluid than you can drink?Add electrolytes, specifically sodium.
Sodium acts to stabilize plasma volume which will help you maintain your performance indoors when the exertion could be higher, as I mentioned previously. Sodium supplementation is quite a tricky and controversial area in the field of sports nutrition. Depending on who you talk to, you may be recommended to use sodium supplementation or stay clear of it. Because sodium is needed to maintain plasma volume and because it is the main electrolyte lost in sweat, it makes sense to add this to your hydration regimen for indoor riding based on the conditions I mentioned earlier in this article (> 60 minutes in duration, especially for intervals). However, what is not known is exactly how much sodium to use.
Sodium concentration testing can be done but not many professionals offer this simple test due to not having the proper technology. If you do not have access to this testing, I would recommend experimenting with sodium supplementation at conservative doses to see how it affects your sweat rate. Start at 200 milligrams of sodium per hour and perform the sweat rate test pre- and post-ride. If you are losing less during that ride, then the sodium amount is probably pretty close to what you need. If you are still losing a copious amount of fluid, add 100 additional milligrams of sodium until you find the point where you don't lose more than 2-3% of your body weight. This will help maintain your performance while riding inside and will help you achieve your performance related goals.
Keep in mind that as your fitness level changes, the seasons turn and your perception of fatigue changes, so will your sweat rate. Be sure to measure this throughout the year so you get a better idea of your body's needs.
Bob Seebohar, MS, RD, CSSD, CSCS, METS II, is a Sports Dietitian and Endurance Coach. He traveled to the 2008 Olympics as the USA Team Sports Dietitian and the personal Sports Dietitian and Exercise Physiologist for the Olympic Triathlon Team. Currently, he owns eNRG performance, a performance center in Littleton, Colorado that provides sports nutrition services, physiological testing, a training and recovery center and endurance coaching to endurance athletes.
Bob's book, "Metabolic Efficiency Training: Teaching the Body to Burn More Fat" has helped thousands of endurance athletes improve health and performance and has redefined the sports nutrition field. Read more about Bob and eNRG performance at www.enrgperformance.com and contact Bob at bob@enrgperformance.com