You sound pretty thin to begin with and you will probably lose some weight while fasting. I'm not sure about if or how much muscle mass you might lose. You might want to go on a cleansing diet instead that incorporates lots of organic vegetables and fruits, salads, and fresh vegetable juices. Although the cleansing process will be slower, you will continue to get vital nutrients. Since you're new to fasting you might want to try a 1-3 day fast and see how you do. There are many ways to detox including colon cleanse, liver cleanse, liver/gallbladder flush, parasite cleanse,saunas, dry skin brushing, in addition to fasting. I'm not trying to discourage you from fasting...just letting you know there are other ways to clean yourself up if detoxification is your primary goal.