Isaac Jennings (November 7, 1788 – March 13, 1874) was an American physician and writer who pioneered orthopathy (natural hygiene).

In 1822, Jennings began to give his patients without knowledge, placebos of bread pills and coloured water tonics.

[10][11] He believed that nature does its best to keep the human system in health and to give the best opportunity for the "vital forces" to work, the patient must rest and not be disturbed by medicine or stimulants.

[9] In 1867, the Buffalo Medical and Surgical Journal described Jennings' methods of utilizing bread pills as "down right quackery and imposition" and a "disgrace to the regular profession".

[12] A review of his book The Tree of Life suggested it was "without a particle of merit" and the ideas would not be popular with people of the present day.

[1] Jennings influenced many natural hygienists including Felix L. Oswald, Russell T. Trall, Herbert M. Shelton, Hilton Hotema and many others.