Is The Master Cleanse nutritious? Well...yes and no.
The Master Cleanse drink does contain the sugars and mineral salts that will help provide energy and keep the rate of detox down to a level that is manageable for most people. However, it does not contain the proper amount of carbohydrates, fats and protein to sustain a busy, vigorous or stressful lifestyle for many people. Any kind of fast is best done when you can get plenty of rest, especially in the early stages when you are detoxing heavily.

The weakness is probably from low blood sugar. You may have already depleted your glcogen reserves and are going into ketosis. If this is the case a little extra syrup may help you feel better. The headaches could be from caffeine or Sugar withdrawal, or from a heavy detox. It took many months of doing the Master-Cleanse for three to five days before I was able to complete ten days. Do the best you can and don't feel guilty if you can't finish the first time. The main thing is to keep eating healthy and don't give up. You might want to try using the drink as a meal replacement for one or two meals and then eating a salad and fruit for one meal. This is not the traditional MC, but it might help you continue to detox at a slower rate and not be nearly as hard on you. Some people require a lot of preparation before they can complete the Master Cleanse.