Whether in training or racing, cyclists ride over distances that challenge their fluid and fuel levels. Sweat losses can be high in hot weather and when working hard, but you may not notice this if the sweat evaporates quickly in the wind. In a hot laboratory, a fluid deficit of as little as 2 per cent leads to measurable physiological impairments (heart function and temperature control) and reduced ability to cycle (perception of effort is increased and work output decreased).
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Some scientists argue that in real-life conditions, a fluid deficit has less impact. The cooling effects of wind and moving through the air are difficult to simulate with laboratory fans. In the field, they may provide extra assistance to keep body temperatures from overheating. Of course, if you are drafting or riding in the middle of a peloton, you are trying to avoid this movement of air over your body, so you may miss out on its cooling benefit.
Some cyclists feel that fluid lost through sweating is also a loss of weight to carry. There are reports that some top cyclists make a calculated decision to let themselves become dehydrated before hitting a mountain stage so the energy cost of riding uphill will be lower. A study organized by Tammie Ebert at the AIS compared full hydration with a 2 per cent fluid deficit, achieved by restricted drinking in the early part of a ride, when cyclists had to ride uphill in hot conditions.
The ‘uphill’ effect was simulated by riding on a treadmill with a steep 8 per cent incline. When they were lighter, cyclists were able to do the hill climb at the same speed as when fully hydrated but with a lower power output (308 W vs. 313 W), showing that they were more efficient. However, their ability to continue riding at this speed was reduced by ~29 per cent.
Therefore, the ill effects of dehydration appear to outweigh any physical advantages of weight loss, albeit under a laboratory simulation of race conditions. This finding needs to be tested in a race setting.