Here are excerpts from someone's very detailed story on the negative effects fasting and fruitarianism had on her health, after many attempts. (This is not me) The site this is from is http://www.beyondveg.com/williamson-s/bio/sharrhan-bio-1a.shtml

"between about 1978 and 1983, I experimented with various approaches to vegetarianism with occasional lapses into protein-dieting when I became too frustrated with the difficulties I had--I realize now, in retrospect--in assimilating all those dense carbohydrates. Once [during 1978-1983]I fasted on carrot juice for 10 days and felt quite good--but upon returning to solid food I'd experience an enormous letdown as depression, lethargy, hunger, weight gain, bloating, etc., set in once again.
Finally, in 1983 or 1984 I somehow rallied and disciplined myself to accomplish a 28-day juice fast which left me feeling totally rejuvenated. And for about a year thereafter, I did quite well, though I remember eating quite large quantities of food at times...I didn't feel particularly energized anymore; nor did I at any time experience the thinness most people associate with raw-food eating. In fact, I never at any time reached my ideal weight, even on a juice fast, despite my consistent working out and walking everywhere. Neither did my cellulite magically disappear as raw-foodists are told it should. (Water-fasting, which I did later, dramatically reduced my weight temporarily, but with consequences that were detrimental to my health, such as muscle wasting, a slight increase in nearsightedness in one eye, and a "what's the use" kind of lethargic mental outlook.)
In 1997 I did a 40-day juice fast. (For health reasons mainly, not to lose weight--I only lost a few pounds, which by the end of the fast were piling back on again.) I DID feel absolutely ecstatic when I was on this fast, no doubt about it. (Though my husband reminded me later that I'd been very moody and spacy at many times, as I rode the Sugar highs and lows.) At times I had a lot of energy and felt great "joy."
However, I discovered shortly after it was over that my gums were very soft and were receding from some of my teeth, most alarmingly. I had gingivitis. Also, I could clearly no longer ignore my hair--which was getting finer and finer (it had originally been very coarse)--and my nails, which were almost as soft as paper. My complexion was once again incredibly dry and flaky. Worst of all, over the coming weeks I became increasingly alarmed about the state of my mental processes. My thinking was confused and muddled. And I was very depressed, feeling "flat" and joyless.
At present, I'm working out a suitable diet for myself that is low-carbohydrate but incorporates plenty of plant fiber. ... [now] my diet, which is based on the Protein Power plan of Drs. Michael and Mary Dan Eades, is now stabilized at a level of 40 to 60g of carbohydrate a day. Right now I'm eating fish, beef, and turkey (no hormones), soy burgers, tofu, raw or cooked organic eggs, raw organic cheese, nuts, salad, and green vegetables. ...I don't eat any fruit at this stage of the game. Later I hope to eat occasional berries or other low-glycemic-index fruits that don't wreak havoc with the blood sugar
I feel better than I've felt in years, like I've switched my life "ON" again after a period of utter stagnation. My thinking is clear, I don't get depressed, and my creative drive (I'm a professional musician and artist) has returned with great force after years of Iying almost dormant. I'm waking up happy every morning and am experiencing a clear sense of spiritual, mental, and physical well-being"