Final answer:
Detectable dehydration is typically noticed at about 2% body water loss, with symptoms including thirst, dizziness, and low urine output.
Step-by-step explanation:
The lowest percentage of detectable dehydration in the human body is approximately 2%. Dehydration occurs when water loss exceeds the body's intake, leading to negative water balance. Symptoms of dehydration become noticeable after about 2% of total body water is lost, which can manifest in signs such as thirst, dizziness, headaches, and low urine output. It is critical to maintain proper hydration for bodily functions, and dehydration can be a medical emergency in severe cases.