Hmmm...well, I don't want to step on anyone's toes, but I do think that juice fasts/fruit fasts are frontal assaults on the endocrine system. They put far too much stress on the adrenal glands. Plus the people doing them are usually not in the best of health, or else they wouldn't have turned to something so extreme (often because they feel that their doctors and Western medicine have failed them, which they may have). By definition their adrenal glands probably aren't up to snuff, since their adrenals have been fighting inflammation and marshalling the immune system without a break, often for years, and are likely to be in some degree of fatigue.
It's true that fasting has been a part of a lot of different cultures going back as far as we can trace, but those were different cultures. There was little or no noise, pollution, media barrages, traffic jams, working mothers, divorce, assembly lines, radiation, etc. It was a much slower, less stressful, more natural lifestyle.
I don't think that fasting is only a concern for reactive hypoglycemics. Fasting puts such an incredible strain on the endocrine system that I would recommend other avenues of pursuit first. For most people the smoking gun is the adrenals (and the underlying causes of adrenal fatigue), along with the liver/gallbladder, thyroid and mineral balancing. parasite and non-fasting colon and kidney cleanses and other healing modalities can be a benefit as well.
Some people will have cancer and other things that go bump in the night, but adequately addressing the above issues will provide a great deal of if not total improvement for most people, and much of it can help people with the bad stuff hold up while they fight the good fight.
Yes, if you're a reactive hypoglycemic one or more symptoms will probably have manifested itself. However, one of the symptoms might be fatigue, brain fog or headaches. People would have no reason to think that they are reactive hypoglycemics because fatigue, brain fog and headaches are often thought to be (and could be) caused by other conditions. Not to mention that most people haven't even heard of reactive hypoglycemia, much less its symptoms. It's also possible that someone whose adrenals are borderline could have no symptoms and could be pushed over the edge by the stress of a fast.
There are so many less painful and stressful ways for us to improve our health and well-being that I think that for the most part fasting should be considered only as a last resort.