Dehydration is more common in older adults than in younger adults because

As humans age their kidney function decreases and the feeling of thirst begins to diminish. Often instead of feeling thirsty an individual over 50 begins to feel tired and sluggish so instead of replenishing their body with water they opt for sleeping. As a result the aging adult is more inclined to suffer from dehydration due to misunderstanding their body's need for water. So the correct answer is THE THIRST MECHANISM DIMINISHES.

Fiber doesnt contain any calories i believe