Bryan Johnson has revealed that his autoimmune gastritis—a condition in which the immune system attacks the stomach's acid-producing cells—was the hidden cause of his low iron levels for 11 years, despite eating meat, getting ample sunlight, and trying multiple iron supplements. He said his condition prevented proper iron absorption, making intravenous iron infusion the only effective treatment. Gastroenterologists explain that autoimmune gastritis is driven by immune dysfunction and genetics ra