Try starting with one large bottle of fluid (22 to 24 ounces) and somewhere between 250 to 350 calories per hour. These are general guidelines to give you a rough idea of where to start. If these numbers do not work for you, be willing to experiment and change.
You can also have metabolic testing done at a university laboratory to help you hone in on the numbers more quickly.
Recently, professional cyclist Rory Sutherland was tested at the Colorado University Sports Medicine Exercise Physiology and Human Performance Lab and he found out that his body burns 200 to 300 calories more per hour than other "typical" professional cyclists. He found out that he needs to eat significantly more in order to maintain his energy throughout the long races.
Consume What You Like
We all have slightly different palates. It is critical that you find fuel and drink combinations that taste good to you. If you don't like the taste of a fuel or hydration product, you're going to have trouble consuming enough nutrition on race day. This can certainly lead to GI distress.
If the race director offers products that you cannot tolerate or don't like, figure out a way to supply enough of your own products to get you through race day.
Even if you find something you like, find a back-up, too: It's common for athletes to tolerate a particular combination of fuel and hydration products for an extended time, only to have those products become unpalatable.
Pace Your Fueling and Hydration
There are conflicting opinions on whether or not to "pace" nutrition. Some experts believe that you should completely rely on your natural instincts—eat when you're hungry and drink when you're thirsty.
While many athletes do quite well at natural pacing, others do not. Unless you are extremely experienced at long-distance racing, I suggest you set an alarm on your watch to go off every 20 minutes. This alarm serves as a reminder that you should eat and drink. If you naturally consumed food and drink recently, ignore the alarm. If you got distracted and it's been an hour since you've consumed fuel or fluids, it's time to do something about it.
Your aim on race day is to consume fluid and fuel at the paces you used during training. You don't want to feel stuffed or restricted.
The use, or non-use, of electrolyte tablets during training and racing is as individual as fueling. Some experts advise that supplementing with electrolyte tablets during training and racing is completely unnecessary, while other experts disagree. Several athletes have reported that consuming these tablets along with adequate fluids has revived them from vicious cramping or halted vomiting.