How can you stop weight loss?

Reverse the cause. A person with normal body weight should not lose weight for no apparent reason. A doctor can help find the cause, and begin an corrective treatment. When the cause is stopped, the person's weight should become normal again. Causes include diseases such as thyroid disorders, cancers, digestion and absorption problems, eating disorders, infections, excessive exercise, medicine effects, etc...
Get tested. Know what is causing the weight loss. Balance your diet. Make sure you do a parasite cleanse. If you have any chronic illness need to manage them better.