“There is nothing so good for the inside of a man as the outside of a horse” (horse-riding adage)

“There is something about the outside of a horse that is good for the inside of a man” (that is, riding a horse improves a person’s mental or physical health) has often been credited to Winston Churchill (1874-1965), but the saying was popular before he was born. Lord Palmerston (1784-1865) has also been credited with the saying, but documentary evidence is lacking.
 
Theodore Parker (1810-1860), a minister of the Unitarian church, wrote from the West Indies in April 1859 that his health had improved. The Springfield (MA) Daily Republican added:
 
“He is able to bathe in the sea and ride horseback, and he says he finds the maxim true that the outside of a horse is good for the inside of a man.”
 
Theodore Parker’s letter was reprinted in many American newspapers and helped to popularize the saying, but Parker’s remarks indicated that the saying was already in circulation.
 
   
Wikiquote: Horses
Unsourced
There is something about the outside of a horse that is good for the inside of a man.
Winston Churchill
 
12 April 1859, Boston (MA) Daily Traveller, pg. 4, col. 3:
LETTERS FROM THEODORE PARKER.—In the Music Hall, Sunday, letters from Theodore Parker and Mrs. Parker were read. Mr. Parker writes that his health is materially improved. He bathes in the sea and rides on horseback. He remarks that he had heard that “the outside of a horse is good for the inside of a man,” and his experience had proved it true. He sleeps well and his strength has increased very encouragingly.
 
12 April 1859, Springfield (MA) Daily Republican, pg. 2, col. 2:
Rev. Theodore Parker writes home from the West Indies that his health is materially improved. He is able to bathe in the sea and ride horseback, and he says he finds the maxim true that the outside of a horse is good for the inside of a man.
 
Google Books
Medical Common Sense:
Applied to the Causes, Prevention and Cure of Chronic Diseases and Unhappiness in Marriage

By Edward Bliss Foote
New York, NY: Published by author
1863
Pg. 145:
Horseback-riding is good for gentlemen troubled with this disease. Theodore Parker once facetiously remarked that the “outside of a horse is good for the inside of a man.” This was said, of course, with more especial reference to dyspeptics and those who do not take much exercise, for the outside of a horse is equally good for the outside of a man.
 
Google Books
January 1881, The Popular Science Monthly, “Indigestion as a Cause of Nervous Depression” by T. Lauder Brunton, pg. 384:
I believe it is to the late Lord Palmerston that we owe the saying that ” the outside of a horse is the best thing for the inside of a man,” and it is very near the truth.  A brisk trot for fifteen minutes will cause more pressure upon, and stimulation of, the liver than a lazy lounge of an hour or more.
 
Google Books
An American Four-in-Hand in Britain
By Andrew Carnegie
New York, NY: Charles Scribner’s Sons
1885
Pg. 185:
As we bowled along the conversation turned upon horseback riding, and some one quoted the famous maxim, “the outside of a horse for the inside of a man.” “But what about a woman?” asked F. “Oh,” answered Puss, ” the outside of a horse for the inside of a woman and the outside as well, for in no other position can a woman possibly look so captivating as on a horse. Girls who ride in the park have double chances.”
 
Google Books
The Quote Verifier:
Who Said What, Where, and When

By Ralph Keyes
New York, NY: St. Martin’s Press
2006
Pg. 91:
There is nothing better for the inside of a man as the outside of a horse.”
(...)
Verdict: Longtime male equestrian wisdom.
   
Jane Bloomfield: truth is stranger than fiction…
FRIDAY, 1 MARCH 2013
I Could Eat a Horse
(...)
Winston Churchill loved horses; his passionate petition helped post World War I horses return to Britain. ‘There is something about the outside of a horse that is good for the inside of a man.’ He obviously wasn’t eyeing up a juicy flank steak to fill his belly when he said that.