In many cases rapid weight loss means dehydration. During some days the person can lose up to several kilograms. Rapid weight loss, more than 10% of the total weight is considered harmful.
Dehydration symptoms are difficult …

1% dehydration leads to thirst.
2% dehydration causes feeling of anxiety, reduced appetite and capacity for work by 20%.
4% dehydration brings feeling of nausea, dizziness, emotional instability, fatigue.
6% dehydration leads to loss of coordination and coherence …