Nature didn't intend for us to fast routinely and regularly such as we need to do in eating/breathing or activity and rest/sleep.
Fasting is only a last-resort measure provided by Nature in case of injury/disease and famine.
True health is built only by the body given the ideal circumstances of wholesome-food, fresh air, sunlight exposure, warmth, and other influences, where fasting is only secondary.
The limitations of fasting are those imposed by the body itself.
Fasting does nothing........only the body heals.
This has also been the philosophy of Natural Hygiene ever since its inception in the early to mid 1800's.