There were just a few stories in the news recently about how mainstream medicine has recognized the benefits of regular fasting. LA Times, AP.

There are a lot of different ways to cleanse (modified fasting, fasting plus enemas, laxatives, fiber supplements, blood detox supplements, liver supporting supplements, chelation therapy, any combination of the above).

I haven't fasted in ages, but 3-5 day juice fasts always made me feel really good (and I was able to do long rides at an easy pace during fasts).

There's another thing to consider, and that's food allergies. Something like 80% of Americans are allergic and/or sensitive to common foods. So some of the benefit you feel from fasting may really be from avoiding certain foods, not all foods. IMO everyone should try an allergy challenge diet at some point in their adult life (which is almost more of a hassle than fasting, but well worth it if you do identify food allergens).