Scientists aren't sure of all the causes, but they list the following as the main culprits:
Slowed gastric emptying. Both exercise and digestion require increased blood flow. When you ride with a lot of food or fluid in your belly, the stomach and working muscles battle for extra blood. The result is that food doesn't leave the stomach as quickly as it should, and nausea may occur.
We also know when your intensity approaches 75 percent to 80 percent of max, that blood is shunted away from the stomach. Scientists also suspect that gastric emptying may improve with training. In other words, the more you ride while eating and drinking, the more efficient your digestive system becomes.
Physical irritation of the stomach lining. Exercise-related nausea can be induced by liquid sloshing in the stomach for long periods, which irritates the mucous membrane. While road riders would be far less susceptible than runners (who do have a higher rate of nausea and vomiting), this could be an important factor for mountain bikers.
Lowered pH. Vigorous riding exhausts the energy substrates (fuel) in cells and produces acids. If the level of exercise is so high that the body is unable to buffer these acids, your pH level will fall, triggering nausea, headache, restlessness, and weakness.
Dehydration. This can compound the problem of lowered pH. If you are insufficiently hydrated to produce sweat for vital body cooling, the necessary fluid will be pulled from your blood. Thus, the pH effect worsens.
Anxiety. Those pre-race stomach butterflies can stress your system just as much as high-intensity exercise and heat. Whenever you stress the gastrointestinal system, it slows down. Because digestion isn't a priority in an emergency (such as an impending race), the brain slows stomach contractions, which impedes digestion. It isn't surprising, then, that a nervous cyclist with food and fluid in his stomach might get nauseous.
There are other more obvious causes of nausea, including excessive alcohol consumption the night before, too much caffeine (especially if you aren't accustomed to it), rich foods, food poisoning, and even flu. If nausea is a chronic problem when you ride, there may be something more serious at work and you should consult a physician.
If you're a hard-charger, occasional nausea may be inevitable. Often, however, it can be avoided by following a few simple precautions:
1. Eat sensibly the night before. Ingest bland, high-carbohydrate foods, and little or no alcohol and caffeine.
2. Eat lightly before the event. A small meal of 500 to 700 calories three hours before the start. This might consist of a bowl of oatmeal with nonfat milk, a banana, and a raisin English muffin. Or eat a 200-calorie snack, such as four graham crackers or a slice of bread and a glass of juice, one to two hours prior.
3. Before and during the ride, avoid foods high in fat and insoluble fiber. These are difficult to digest and stay in the stomach longer. Instead, eat easily digestible, complex carbohydrate such as breads, cereals, muffins, crackers, pretzels, and pasta.
4. Avoid highly acidic foods such as citrus fruits, which can exacerbate the problem of lowered pH. If you become nauseated after using citrus-flavored sports drinks, switch to other solutions.
5. If you're doing an especially long ride and you'll have to eat during it, practice doing so in training so your stomach gets used to absorbing things during exercise.
6. Use training sessions to test foods and fluids, and eliminate those that create problems. Don't experiment during an important event.
7. For efforts of two to four hours, sports drinks with a carbohydrate concentration of 5 percent to 8 percent are adequate. For ultra-events, higher concentrations may be necessary. Select glucose polymer or maltodextrin beverages, which may be more readily absorbed than simple sugar solutions.
8. Stay hydrated by drinking lots of water in the days preceding the event. If you're awake early enough, consume several glasses three hours beforehand and another 10 to 15 ounces about five minutes before the ride.
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