John Harvey Kellogg developed a variety of vegetarian foods intended for an invalid diet, which were easy to chew and digest. Kellogg's foods were often bland and followed the teachings of Ellen G. White and Sylvester Graham, who recommended a bland diet to minimize excitement, sexual arousal, and other things. Kellogg also developed corn flakes by experimenting with a dough made from wheat, oats, and corn. The dough was baked at high temperatures and broken into crumbs, which led to the discovery of delicate flakes that could be baked. The process of tempering was patented by Kellogg, which was to be a fundamental technique of the flaked cereal industry. Will Kellogg continued to develop and market flaked cereal and started his own company, the Battle Creek Toasted Corn Flake Company. This marked the start of a decades-long feud between the brothers, with Will's company eventually becoming the Kellogg Company. Other competitors, including C.W. Post, also entered the market.
John Harvey Kellogg, the founder of Kellogg's, was not just a food innovator but also an inventor in the field of medicine. He designed medical devices that were widely used at the Battle Creek Sanitarium in surgical operations and physiotherapy. He specialized in developing instruments for light therapy, mechanical exercising, proper breathing, and hydrotherapy, among other things. Some of his notable inventions included an electrotherapy exercise bed, a vibrating chair, an oscillomanipulator, a window tent for fresh air, a loofah mitt, and an apparatus for home sterilization of milk. He also invented the radiant-heat bath, which was later manufactured and sold in Germany and spread to Vienna. Kellogg's work in phototherapy is detailed in his book Light Therapeutics, which describes the application of the arc light to various parts of the body. Kellogg believed that electric currents were an extremely valuable therapeutic agent, especially when used in connection with other physiologic methods. He utilized electrotherapy coils to treat cases of paresthesias of neurasthenia, insomnia, and certain forms of neuralgia. He also invented a vibrating chair used to stimulate vital organs in the lower abdomen, which is still available for public viewing in the Kellogg Discovery Center in Battle Creek, Michigan. Furthermore, Kellogg devised an electrotherapy exercise bed that produced muscular contraction without pain and achieved the stimulation of a brisk four-mile walk. Kellogg's massage devices included two- or four-person foot vibrators, a mechanical slapping massage device, and a kneading apparatus. Kellogg believed in mechanical massage for cases of anemia, general debility, and muscular or nervous weakness.
John Harvey Kellogg believed in the concept of "biologic living" which involved maintaining a healthy body, mind, and soul through appropriate diet, exercise, and recreation. He emphasized the value of fresh air, exercise, and sunshine, while warning against the dangers of alcohol, tobacco, and drugs. Kellogg was a prominent member of the anti-tobacco consumption campaign, serving as the president of the Michigan Anti-Cigarette Society, and believed that tobacco caused not only physiological but also pathological, nutritional, moral, and economic devastation onto society. He also opposed the usage of alcohol and caffeine, citing various physiological, developmental, and moral problems caused by these substances. Kellogg's views on biologic living were in line with his Adventist beliefs, and he presented his philosophy in seven textbooks for Adventist schools and colleges. He also served as the president of the American Health and Temperance Association, which aimed to expose the far-reaching dangers of tobacco, alcohol, tea, and coffee. Kellogg's work on several committees against smoking culminated in Utah Senator Reed Smoot introducing a bill to Congress in 1929 that aimed to put tobacco under the purview of the Pure Food and Drug Act.
In his later years, Kellogg continued to work on promoting healthy eating and ran several institutes, including one in Florida which was popular but not as successful as his Battle Creek institute. He also served as editor of the Good Health journal for many years. Kellogg was a supporter of the eugenics movement and helped found the Race Betterment Foundation in 1906, which advocated for racial segregation and believed that non-white immigrants would harm the gene pool of white Americans. Kellogg also had a long personal and business feud with his brother, which was not resolved before his death.