My thoughts.
Is it unhealthy to loose 10 pounds in 5 days...well to me it depends. Some people might not agree, but when you are starting out on your weight loss journey, and I'm speaking in very general terms, you do lose weight faster than when you are smaller.
When I first started my weight loss journey in June of 2008, I did a juice feast where all I drank was juice that I made. I tried to get my daily calorie intake and then some. I did this for 40 days and lost 30 pounds. Now if you break it down I probably lost around 20 actual poundage and the rest of that is water weight and I did workout during that time, well as much as a 300+ woman can!
Now that is fast weight loss, but interestingly when I couldn't get used to the green juices (ew) I stopped drinking enough and had to stop the feast. My weight loss stalled completely right before I stopped. I decided I was done detoxing myself and moved into my solid food diet plans. Now did I gain back the weight? No...I gained back 8 pounds because I was eating solid food again and then I lost that and then some. The reason why? Because I didn't go back to my old habits. After my experience, which was a personal zen thing for me, I stopped eating cheese, most dairy, sugary things, and a lot of carbs. Meaning I wasn't eating pizza or Baja Fresh every other night lol.
I also should mention that other people that did this particular feast with me, or while I did mine, did not have as much success than I did. The reasons were simple. A lot of them were way smaller than me. One woman drank a lot more juice than me, her starting weight was around 250 pounds and she was loosing around 2-3 pounds per week after her water weight fell off.
I guess my point is that it does depend on the person. I've known many people who have lost weight slowly and have gained it back. The reason why? Because they didn't do a lifestyle change. They dieted and then went right back to where they started. Now I loose my weight in chunks...one month I'll loose 10 or 13 pounds. Then I'll loose 2 the next. It goes like that with me. That is how my body works. Right now I freak out if I gain like 3 pounds haha...i struggle with new issues than I did when I started.
I think sunflowergirl has a good point about just making changes like reducing calorie intake and talking to a doctor about your weight loss plans. I think that is a good step and personally while my doctor thought my weight loss was fast she thought, based on my health, that it wasn't unhealthy for me because of my starting point and where I am at now.