This Master Cleanse couldn't possibly be natural or healthy, could it? People, like other animals, are meant to eat. I can understand becoming a vegetarian for 2 weeks and eating nothing but salad or other raw veggies, but this seems a bit far and unnatural. It almost seems as natural as those weight lifters who seem to be able to bench press like 100 kg at the gym I go to. None of what we do seems natural.
I want to be healthy, but in the most natural way possible. I go to the gym and lift weights, but rarely. It's just to keep the muscle in all the right parts so that I can burn calories easily. I run every 2-3 days. I play sports. I eat some crap, but also eat a large salad with vinegar dressing every day. Surely that's better than just drinking a lemon and maple syrup.
Besides, the key to the "master cleanse" diet is the actual bowel movements, it seems. Taking in only things you need without preservatives, sweeteners, colourings, hormones, etc, and then going to the toilet 2-3 times a day. Of course you'll feel cleaner. You're starving, not taking anything else in, and just dumping out stuff that your body doesn't want. It doesn't sound like you really need this lemonade diet to do so. A raw diet may be just as good.