less muscle, less fluids?

RickF

That might be true if you were comparing a lean cyclist to a morbidly obese cyclist, but at 5'8.5", the difference between 175 pounds and 157 pounds will have a negligable effect on insulation. When you are down to that level, you are no longer talking about substantial differences in subcutaneous fat. The difference is in muscle mass, muscle glycogen, and abdominal fat.

If you were lifting weights before, my guess is that much of the weight you have lost is muscle and glycogen. Some loss might have been abdominal fat, but it is unlikely you have lost a significant amount of subcutaneous fat. The other thing you have lost with the weight loss, regardless of whether it was muscle, glycogen, or fat, is the ability to store water. Thus, while at 175 pounds, you could probably lose 5.25 pounds (2.4 liters) of water without having a major impact on your performance. At 157 pounds, an equivalent weight loss would be 4.7 pounds (2.1 liters). That means you have 300 mL (about 10 fl oz) less reserve water in your body at the start. Also, since you are losing what you consider to be "non-essential" muscle as far as cycling is concerned, where the body is going to start robbing muscle is the intercostal muscles (those responsible for breathing) and the legs.

The bottom line is that the role of subcutaneous fat in insullation, except in swimming and in the morbidly obese, is negligable. In losing weight, the power to weight ratio might have shifted more to your favor, but the weight loss will not mean you can drink less, and it could mean that you need to drink more.