In later interviews and correspondence Green described himself as having "always been a moral sort of person" who had been shocked during the war by the bawdiness or "passion" of fellow sailors. By about 1958 he claimed, through personal experience, to have equated this passion, in both men and women, with an excessive consumption of protein-rich foods. The solution, which crystallised into a personal philosophy of "protein wisdom", was a regime of moderation enshrined in the doctrine of "less lust from less protein". In June 1968 Green began a public campaign of dietary reform among Saturday shoppers in Harrow. After six months he dedicated himself full-time to the cause, which he now took to central London. Between 1969 and 1993 he became an almost daily sight for the workers, shoppers, and tourists around Oxford Street. Saturdays were spent attempting to improve the habits of cinemagoers in Leicester Square.
Initially, Green's principal means of communication was a board, carried above the crowd, on which he warned against passion and its eight key sources: Less Passion; From Less Protein: Meat; Fish; Bird; Egg; Cheese; Peas incl. lentils, Beans; Nuts. To the original eight "passion proteins" Green later added "sitting" to warn of the link between sedentary living, enhanced protein build-up, and lust. But this was the only significant revision of the banner which he carried for 25 years.
From the early 1970s Green began producing literature to distribute to passers-by. Two editions of his "protein wisdom" pamphlet were printed commercially in 1973, after which he acquired a press and produced 50 revised editions from the front room of his council flat. On average he sold 20 pamphlets on weekdays and up to 50 on Saturdays; by February 1993 he claimed to have sold 87,000 copies, always at the lowest possible price (for example, 11 pence in 1985).
Green's campaign for social reform was as remarkable for its durability and privations as it was for the idiosyncrasy of its message. Until qualifying for a free travel pass, he cycled 12 miles daily from Northolt to central London with his board and publications, a journey that took up to two hours in a headwind. On weekdays and Saturdays he campaigned until 6:30pm and spent Saturday evenings in Leicester Square; Sundays were taken up with printing booklets for the following week. By 1985 he had reduced his visits to four days, and made the journey three times a week until the age of 78. In line with his own teachings, Green limited his daily protein consumption to one egg and a concoction of milk powder and barley water. Otherwise he survived on oatmeal porridge, infusions of root vegetables, and home-made bread.
For some of his correspondents, Green's "protein wisdom" offered a new direction; for others he was a charlatan or a potential menace, especially to young women. But he was tolerated by the majority and, over time, celebrated as one of London's true eccentrics. From the 1970s he gained wide publicity in the British and foreign press, acquired the nickname Protein Man, and appeared on TV and radio. His principal message - "less passion, less protein" - was also used in a design range by the fashion house Red or Dead, a move Green welcomed as a further opportunity to spread the word. Stanley Owen Green died, unmarried, at his home, in Northolt, Middlesex, on 4 December 1993.