"I've been thinking about things and wondering if I've gone too far with this whole longevity thing," the tech billionaire writes.
Johnson has become known for a lifestyle of strict diet, daily exercise, hundreds of medical tests, dietary supplements and experimental treatments - for example, Johnson received blood plasma from his teenage son.
In July, Johnson revealed that he had autoimmune gastritis, a chronic inflammation in which the immune system attacks the cells of the stomach.
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