Drinking water without electrolytes during very long rides can lead to dilution of normal blood sodium concentration. Without the proper amount of sodium, as the blood system absorbs extra water, excess fluid can build up in the brain and lungs.

The condition is called hyponatremia and the consequences can be fatal.

"Water intoxication," as cyclists call it, can cause brain swelling and serious amounts of fluid accumulation in the lungs. When this happens, oxygen is not transported into the bloodstream efficiently, and you can become short of breath, nauseated and disoriented.

Researchers from Baylor College of Medicine in Houston studied the development and treatment of marathon and ultra-distance runners with pulmonary edema (essentially dry-land drowning as the lungs fill with fluid).

Seven fit and previously healthy marathoners, ranging in age from 29 to 46, became ill with respiratory distress, coughing pink frothy sputum, low oxygen levels, low serum sodium, and brain swelling. Five were female.

The runners received treatment with intravenous fluids containing large amounts of sodium. One woman died. Four additional female runners who competed in the Houston Marathon this year also had the syndrome.

Cycling shorter distances (two or three hours or less) does not pose the same risks, and water or a sports drink are fine for rehydration. But endurance efforts in which exertion exceeds four hours can set the stage for hyponatremia — low levels of sodium in your blood system.

Sweating causes a loss of water and sodium, which reduces the total blood volume. When you drink water, it further dilutes the blood. As you begin to feel sick, your natural response may be to drink more water, which can cause your sodium level to become extremely low.

As hyponatremia develops, the symptoms are easy to confuse with other conditions like heart attack or ironically, dehydration or heat stroke.

The cruelest irony of all is that the response to these conditions may be to give more water. Making this misdiagnosis, and giving low-sodium fluids, can be a fatal mistake. The lesson for each cyclist — if you are riding two or more hours, replace fluids with a sports drink that includes electrolytes.

If your exertion lasts longer than four hours, you need to increase your intake of sodium (salt) beyond that found in most sport drinks. Each some sports bars, gels or food that also contain sodium.

Although your body needs water in order to avoid dehydration, if your exertion is prolonged, water must be balanced with sodium and of course carbohydrate to help fuel your muscles for long efforts in the heat.

Good Luck!