What is autoimmune gastritis? Inside the disease that halted Bryan Johnson's $2 million-a-year longevity plan

For years, entrepreneur and longevity advocate Bryan Johnson believed one stubborn health mystery was simply an unexplained anomaly. Despite meticulous tracking of his body, strict nutrition and relentless testing, one marker refused to improve — his iron stores. Now, he says he finally knows why.

Johnson has revealed that he has been diagnosed with autoimmune gastritis (AIG), a condition in which the immune system attacks the stomach's acid-producing lining. The diagnosis came after a stomach tissue biopsy in May, ending what he described as years of unanswered questions.

But rather than accepting the diagnosis as a lifelong condition to manage, Johnson says he wants to attempt something far more ambitious: finding a cure.

What is an autoimmune disease? 

An autoimmune disease occurs when the body's immune system mistakenly attacks its own healthy tissues instead of defending against harmful bacteria, viruses or other pathogens. 

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Normally, the immune system can distinguish between the body's own cells and foreign invaders. In autoimmune disorders, that recognition breaks down, triggering chronic inflammation and tissue damage. 

There are more than 80 known autoimmune diseases, including rheumatoid arthritis, lupus, type 1 diabetes, multiple sclerosis, Hashimoto's thyroiditis and autoimmune gastritis. 

The exact cause is not fully understood, but experts believe a combination of genetics, environmental triggers, infections, hormones and lifestyle factors may contribute to their development. 

What is Autoimmune Gastritis? 

Autoimmune Gastritis (AIG) is a chronic autoimmune disorder in which the immune system attacks the parietal cells in the stomach lining. These specialised cells produce stomach acid and intrinsic factor, a protein essential for absorbing vitamin B12. 

As the disease progresses, the stomach gradually loses its ability to produce acid, making it harder to absorb nutrients such as iron and vitamin B12. 

The condition often develops slowly over many years and may remain undiagnosed because patients experience few or no symptoms in its early stages. 

According to Bryan Johnson, doctors discovered his condition only after investigating years of unexplained low ferritin (iron stores). Blood tests revealed elevated anti-parietal cell antibodies, while stomach biopsies confirmed early autoimmune damage. 

If left untreated, autoimmune gastritis can lead to: 

1. Iron deficiency  
2. Vitamin B12 deficiency  
3. Pernicious anaemia  
4. Fatigue and neurological problems  
5. Increased long-term risk of stomach cancer and gastric neuroendocrine tumours  

Autoimmune gastritis frequently occurs alongside other autoimmune disorders, particularly Hashimoto's thyroiditis, which Johnson has lived with since his early twenties. 

Is there a cure for Autoimmune Gastritis? 

No. There is currently no approved cure. Current treatment focuses on managing complications rather than stopping the immune attack itself. 

Doctors typically recommend: 

• Regular monitoring through blood tests and endoscopy  
• Iron supplementation or intravenous iron infusions  
• Vitamin B12 supplementation, often by injection  
• Monitoring for stomach cancer and neuroendocrine tumours  
• Treating nutritional deficiencies  

Bryan Johnson has said his team plans to explore experimental therapies, including immune-cell engineering, regulatory T-cell therapies and AI-designed biological treatments. However, these approaches remain experimental and are not approved treatments for autoimmune gastritis. 

What are the symptoms of Autoimmune Gastritis? 

Many people have no symptoms in the early stages. 

As the disease progresses, symptoms may include: Persistent fatigue, weakness, iron deficiency that does not improve with supplements, vitamin B12 deficiency, anaemia, pale skin, shortness of breath, dizziness, numbness or tingling in the hands and feet, difficulty walking or maintaining balance, memory problems or brain fog, sore tongue, digestive discomfort and loss of appetite.  

Because symptoms often develop gradually, many patients are diagnosed only after routine blood tests reveal unexplained iron or vitamin B12 deficiencies. 

Can aggressive biohacking worsen autoimmune gastritis? 

There is no scientific evidence that biohacking itself causes or worsens autoimmune gastritis. However, certain extreme health practices may complicate the management of the disease if they increase nutritional demands or interfere with nutrient absorption. 

For example: Intense endurance exercise can increase the body's iron requirements.  

Frequent sauna sessions may indirectly increase physiological stress and fluid loss, although they are not known to cause autoimmune gastritis.  

Hyperbaric oxygen therapy can raise metabolic demands but has not been shown to trigger autoimmune disease.  

Highly restrictive diets, if poorly planned, may worsen existing deficiencies in iron or vitamin B12.  

In Johnson's case, doctors initially considered whether his plant-based diet, heavy training schedule, sauna use and hyperbaric oxygen sessions explained his persistently low iron levels. Further testing eventually showed that the underlying cause was autoimmune gastritis, which prevented his body from absorbing iron effectively.