For every gram of carbohydrate that is stored in your body, 3 to 4 grams of water are also stored. Therefore high-carb diets can lead to an increase in fluid storage. When starting a low-carb/high-protein diet like the Atkins diet for example, the increase of protein will stimulate fluid loss and the low amount of carbs reduce the water storage. You may lose some weight quickly from this diet at first but keep in mind that this is water weight and your fluid losses will stabilize after the first few days. As always, a more balanced diet combined with a solid exercise plan will lead to true fat loss.
Thirst is a poor indicator of hydration levels in your body. Thirst kicks in after a 1-2% loss in body weight and by that time your performance is already affected. Stay ahead of this by ensuring you are properly hydrated throughout the day and especially before, during, and after your workout. Most food we consume in a day will give us 1L of fluids so strive to consume at least another 2L of fluids and even more in hot environments.