That's tricky...everyone responds differently to fasting and any 'negative' effects depend on how toxic you are and how well-nourished you are to begin with. Some people have a pretty easy time with fasting and experience an abundance of energy...other people have some pretty dramatic cleansing reactions...fatigue, flu-like symptoms, mood changes, weakness, brain fog, etc. The general advice while fasting is to reduce your normal level of activity somewhat...you can still work, exercise, etc., but you don't want to overtax yourself. There are some additions you can make to a juice fast that will provide you with more nourishment while maintaining the benefits. Using spirulina or blue-green algae will provide you with some protein and help keep your energy up. Another option is to use a product called UltraClear Plus by Metagenics...it is classified as a medical food and supplies all the nutrients which are beneficial for detoxification...you mix it in water or juice. It also contains some protein and is formulated to help reduce cleansing reactions. Metagenics is only sold by healthcare providers, though...you need to find someone who carries it. If you run a search you can check out their website.