The world has come to know Bryan Johnson for his unshakable dedication to unlocking immortality in this life. He is well known for his quest to slow aging, achieve longevity, and even try to outsmart death itself. He pours millions into his health, follows extreme routines, and tries out experimental treatments — all in the name of living longer.Him dealing with some kind of disease or illness? That’s not something we usually get to hear.That's why, when he came forward about his own health struggle, it caught a lot of people off guard.Johnson recently revealed in a post made on X (formerly Twitter) that he’s been diagnosed with autoimmune gastritis, or AIG. In short, his immune system is attacking the lining of his stomach. He put it this way on social media: “My stomach is eating itself.”It’s a shocking twist indeed, especially for someone so invested in health. According to Johnson, the disease was quietly building up for years, and his only clue was ongoing low iron levels. After a lot of searching, doctors finally connected the dots.Johnson’s journey with AIG is shining a light on a little-known condition. Autoimmune gastritis isn’t common, and because it sneaks up slowly and doesn't shout its presence with obvious symptoms, it often slips under the radar for years.So, what exactly is this disease? How do you get it? What are the early signs? And risk factors?Here’s all you should know about it and why catching it early makes a difference.What is autoimmune gastritis?Autoimmune gastritis (AIG) is a chronic disease. The immune system, which is meant to guard you from threats, misfires and targets parietal cells, which live in your stomach lining. These cells create stomach acid and intrinsic factor, which your body absolutely needs to absorb vitamin B12. Without enough intrinsic factor, you can’t make healthy red blood cells or keep your nervous system in good shape.As the immune attack continues, more of these stomach cells die off. That means dropping acid levels, dropping intrinsic factor, and a rising risk of vitamin and mineral shortages, especially B12 and iron. If things get bad enough, people run into anemia and sometimes complications with nerves or digestion.Johnson shared in his post, “As a kid, I ate sugar cereal, drank sugary soda, and gobbled down fast food. I had a few healthy years in my early 20s but then became a young father of three and began building a business.” He also revealed, “Juggling that stress and grind, I let my health slip and gained 40 lbs. Within a few years I’d fallen into a deep, chronic depression,” adding, “my body began developing an autoimmune process affecting my thyroid and then my stomach lining.”Now, research published in Nature Reviews Disease Primers details that in AIG, immune cells target the stomach's parietal cells and eventually destroy them. These cells produce both stomach acid and intrinsic factor, which is a protein required for vitamin B12 absorption. AIG is driven by an immune attack against the stomach's proton pump (H+/K+ ATPase). That makes it fundamentally different from common gastritis, which is largely caused by infections or dietary irritation.Early signs to spot AIGNow, this is where the trouble begins: early autoimmune gastritis usually keeps quiet. Most people don’t notice anything out of the ordinary in the beginning. If symptoms do show up, they’re often so subtle (think being tired, weak, dizzy, or not having much get-up-and-go) that people brush them off. These signs usually come from low iron.As time goes on, vitamin B12 drops, which can make your hands and feet tingle, mess with your balance, cloud your memory, or make concentration hard. Other people might deal with an achy stomach, bloating, nausea, indigestion, or feeling oddly full after eating just a bit. The symptoms are easy to miss and look like lots of common stomach troubles, so many cases go undiagnosed until doctors run blood tests or a stomach biopsy.Johnson’s experience is a textbook example.He wrote, “For 11 years, I’ve had low ferritin, without anemia. We continually tried to raise my iron levels with food and supplementation but nothing would work.”He went on, “On the surface, my low ferritin was easy to dismiss by most standards of care. My hemoglobin and hematocrit were normal. Ferritin measures stored iron, while hemoglobin measures circulating iron, and because the body drains its reserves first to keep hemoglobin normal, you can be fully iron deficient with a perfectly normal hemoglobin and hematocrit,” adding, “This is why my low ferritin kept getting dismissed: the numbers that define anemia looked fine, so no one asked why my iron reserves wouldn't refill.”What are the risk factors?Shockingly, doctors still don’t know what exactly tips the immune system off course, but they believe it’s a mix of genetics and immune misfires. If you already have one autoimmune disease, you’re more likely to pick up another. Autoimmune gastritis often runs with thyroid disease, type 1 diabetes, and vitiligo. Johnson, for example, also deals with hypothyroidism, a related immune issue, which likely made doctors look closer when things weren’t adding up.“My hypothyroidism got diagnosed when I was 21 years old with a routine blood draw. That enabled me to begin proactive management, supplementing levothyroxine and Armour Thyroid. They are the hormones my body should be producing on its own but wasn’t,” he wrote, “By taking these pills daily, my body was able to operate as though my thyroid was functioning properly. What I didn’t know was that something else was going on inside my body: my stomach had begun attacking itself. But there was no routine test to find out and I didn’t have any symptoms.”The condition hits middle-aged and older adults more, but it does show up in younger people too. Women get it more often than men, and a family history of autoimmune illness raises the odds.How rare is AIG?Nobody knows the exact numbers because so many people go undiagnosed. However, estimates say it affects about 2% to 5% of people, but the real figure could be higher since many cases stay hidden until there's significant stomach damage.Johnson, in fact, wrote, “I just discovered it in May,” adding, “I'm unsure how long I've had it. AIG causes irreversible damage: nutritional deficiency, anemia, and over a long horizon, elevated cancer risk. When AIG is discovered today, standard medical care concedes defeat, stating that nothing can be done except managing the condition, no matter how awful or lethal the effects.”Management and treatmentThere’s more to worry about than fatigue and an upset stomach. Ongoing stomach inflammation eventually thins out the stomach lining (atrophy), which raises the risk for some types of stomach tumors, including gastric adenocarcinoma and type I gastric neuroendocrine tumors. The chance of developing these tumors isn’t huge, but it’s enough that doctors usually want to keep an eye on the situation with regular blood tests and sometimes endoscopies.As for treatment, right now, there’s no cure for autoimmune gastritis. Treatment is really about dealing with the fallout and stopping shortages before they cause bigger problems. People who can’t absorb vitamin B12 need lifelong shots or strong supplements. If iron is low, doctors usually prescribe tablets. And if those don’t work, sometimes infusions instead. Patients need regular checkups to keep tabs on blood levels, anemia, and any changes in the stomach lining.Bryan Johnson has said he’s trying every avenue possible, including new diagnostics, artificial intelligence, and experimentation, to find better answers. But no one has come up with a treatment that fixes the root immune problem yet.Johnson’s journey is a good reminder that no matter how advanced the technology is or how much money one puts into it, nobody is bulletproof, not even extreme health enthusiasts. Living well, working out, eating right, and not smoking make a big difference, but it doesn’t give you total immunity to diseases that come from within.The real takeaway here? Don’t ignore mysterious fatigue, constantly low iron, recurring B12 problems, or digestive symptoms that don't get better. Sure, it’s often nothing serious. But sometimes these little signs are your body waving a red flag. Early testing and diagnosis matter a great deal.