I know it's not a good idea, but are there any supplements that really will stop hair falling out if you do an extended fast? my hair always falls out about 3 months after it?

Depends on cause...: One's primary care doctor can check for medical causes. If none are found, then one can see a dermatologist, especially one specializing in hair restoration (some treatments: minoxidil, etc...). Medical causes may include: hormonal problems (such as low thyroid), seborrhea, alopecia areata, stressful illness. Other causes: genetic male-female-pattern baldness, telogen effluvium.