Fasting... Well I think that if it is to satisfy a vain purpose such as focus on the cosmetics of the body it is not a good practice. I also think that if you think it will help you attain some higher purpose, enlightenment of mind, self control, or personal spiritual elevation, it seems a bit of an extreme way to achieve it, in fact it is very common in practices involving spiritualism. Jesus is our savior. Prayer and faith is very effective. I have seen many women really obsess with their bodies and it is not healthy- in this case it is an eating disorder. I have also seen men do the same. It seems that around this time of year people feel the pressure to drop weight and find a reason to do it. I would ask them...please do not use God in this type of endeavor while deceiving others about it. I think it can influence people or put pressure on others to do the same. Some people plan on doing it for "God" while internally it is for vanity. I do not think that the Lord intended us to harm our bodies or be overly consumed in the thought of them. I think it is really important to think of what it is that your really doing it for. Also...If you are under a lot of stress it may not be very good for the nervous system. If you are a nursing mother remember that you are not only depleting that nutrient supply, but affecting the acid/alkaline balance of the body, and releasing more toxins into the milk. I would not do this until your baby is at least a year old or you have a doctors approval.
For people who are healthy minded and physically able to fast...I would say to look into the Daniel fast. If you feel it is stressing you out to fast then maybe you need to just listen to that and pray that the Lord lead you to a more safe and appropriate approach to what you want to do.
If you want to lose weight...look to cutting out heavily processed foods, fast foods, carbonated beverages, sugars, artificial ingredients, dyes, and other junk foods. Consume more water, fresh fruits, and vegetables, other RAW foods, and look into interval training- appropriate physical fitness based on health and condition.
A person is even developing at 18 years old and really need to talk to their parents before doing a fast.