Final answer:
Dehydration occurs when the body loses more water than it takes in. For a marathon runner, inadequate hydration during long training runs can lead to dehydration. If John's weight drops by more than 2% of his body weight, it could indicate a significant level of dehydration.
Step-by-step explanation:
Dehydration occurs when the body loses more water than it takes in. For a marathon runner like John, inadequate hydration during long training runs can lead to dehydration. A weight suggesting that John is very close to becoming dehydrated would depend on various factors such as the duration and intensity of his runs as well as external conditions like temperature and humidity. However, if John's weight drops by more than 2% of his body weight, it could indicate a significant level of dehydration.