You can reduce or eliminate those first week detox crises by doing a pre-fast cleansing diet. For a week or two, eat ONLY fruits and vegetables, fruits in the am, veggies in the pm. I ate this way for 30 days prior to my 60 day fast and was glad I did!
If you have Sugar issues (candida), fruits are NOT a good thing. Sugar feeds the yeast. In this case you need to stick to green juices, which can include Granny Smith apples in moderation. Carrots are out if you have yeast issues.
If you are truly morbidly obese, you would do well to consult with a natural health professional to help you outline your goals and make a plan that will protect your health. Find one that has fasting experience-- they're out there but you may have to look around a bit. Having someone to support you will make all the difference in your results.
Remember you must do a colon cleansing routine EVERY fasting day to flush toxins and avoid detox crises. I have posted the variations many times on this forum.
Be sure to do all the research you can on juice fasting. READ UP and understand the process before you begin!
You can do it. Arm yourself with good professional support and lots of information and you will succeed.