First recorded belief that "Guinness gives you strength"?

Before long, Guinness Stout found its way to every part of Ireland. In 1782, Henry Grattan, one of Ireland's foremost parliamentarians, was writing to Arthur Guinness that he regarded his brewery as the "Nurse of the people and entitled to every encouragement". Quite early, too, Guinness Stout crossed the sea to England. It spread through the growing industrial towns of the North and at the same time appeared in London itself. In 1794, an issue of the London "Gentleman's Magazine" carried a picture of an artisan drinking Guinness which bore the caption "Health, Peace and Prosperity". In 1837 Guinness was well enough known to feature in one of "Phiz's" famous illustrations for the Pickwick Papers. Exactly when the first glass of Guinness was drunk on the Continent is not known. What I have just learned though however, is that by 1815, it had already reached Belgium because a British Cavalry Officer of Wellington's Army, who was severely wounded at Waterloo, recorded in his diary -

"When I was sufficiently recovered to be permitted to take nourishment, I felt the most extraordinary desire for a glass of Guinness, which I knew could be obtained without difficulty. Upon expressing my wish to the doctor, he told me I might take a small glass...It was not long before I sent for the Guinness and I shall never forget how I enjoyed it. I thought I had never tasted anything so delightful...I am confident that it contributed more than anything else to the renewal of my strength".