Controversial biohacker Bryan Johnson has made it his life's mission to 'defeat death' and live forever - even taking blood transfusions from his son.But the multi-millionaire has hit a roadblock after revealing that he has been diagnosed with an incurable condition, autoimmune gastritis. Announcing it on social media, the 48-year-old put simply: 'My stomach is eating itself.'He blamed the diagnosis on his childhood dietary habits - years of consuming sugary cereals, fizzy drinks and fast food - combined with stress that saw him develop chronic depression as an adult.Johnson added: 'Somewhere in that timeline, my body began developing an autoimmune process affecting my thyroid and then my stomach lining.'I'm unsure how long I've had it. AIG causes irreversible damage: nutritional deficiency, anemia, and over a long horizon, elevated cancer risk.' Despite it being incurable, Johnson has vowed to 'try and solve it' through a fresh series of daring experiments and expensive longevity science projects.But what is autoimmune gastritis? What are the symptoms to look out for, and who is most at risk of developing it? Read on to find out more. Bryan Johnson (right) once used blood transfusions from his teenage son (left) in his pursuit of everlasting lifeWhat is autoimmune gastritis?Also known as autoimmune atrophic gastritis, or AIG, autoimmune gastritis occurs when the immune system mistakenly attacks the cells lining the stomach - the process that Johnson described as the stomach 'eating itself'.In some cases, the immune system also targets intrinsic factor, a protein made by stomach cells that is needed to absorb vitamin B12.Do you want to lose a stone in six weeks?Hi, I'm Emma Bardwell, a nutritionist with more than 15 years' experience helping people improve their health and lose weight.I've been there. I was overweight, unhappy with my skin and stuck in a rut. Then I changed the way I eat. You can now do the same with my tried-and-tested healthy eating plan. And for DailyMail+ subscribers, it's completely free. Click here to sign up and start your journey Without enough B12, the body cannot make healthy red blood cells. This can lead to pernicious anaemia, a condition in which the body is unable to absorb vitamin B12 properly. If left untreated, B12 deficiency can also cause nerve damage.Over time, autoimmune gastritis can increase the risk of serious complications, including stomach cancer and neuroendocrine tumours in the stomach, although these tumours are usually non-cancerous.Experts say AIG remains under-recognised, with estimates suggesting it may affect between five and 20 in every 1,000 people.What causes autoimmune gastritis?Johnson blamed his diagnosis on diet, stress and depression, which he believes triggered the autoimmune response in his body.However, the exact cause of autoimmune gastritis remains unknown, and research into the condition is still relatively limited.Experts say it is more common in women around the age of 60, as well as in people of African or northern European descent.Those with a family history of autoimmune atrophic gastritis, or other autoimmune conditions such as autoimmune thyroid disease, type 1 diabetes, Addison's disease and vitiligo, may also be at greater risk.Some researchers have also suggested the gut microbiome – the community of bacteria living in the gut – may play a role.Dietitian Kellie Blake says an unhealthy balance of gut bacteria may increase inflammation and damage the gut lining, making it 'leaky'.'Leaky gut' is a controversial term used to describe the idea that the gut lining becomes too permeable, allowing substances to pass into the bloodstream that would normally be kept out. Johnson (pictured in his Netflix show) has made it his life's mission to defeat death and live foreverHowever, it is not a formally recognised medical diagnosis, and there remains a clear divide between conventional doctors and some functional medicine practitioners over how much it contributes to wider illness.Some practitioners believe increased gut permeability may help drive a range of inflammatory and autoimmune problems, from eczema, psoriasis and skin rashes to digestive symptoms, inflammatory bowel disease, joint pain, swelling, brain fog and fatigue.What are the symptoms to look out for?AIG often has few or no symptoms in its early stages, making it difficult to diagnose.If they do occur, these may only include pain in the upper abdomen, heartburn, bloating and feeling full after eating only a small amount of food - all of which are typically associated with other, less serious problems.As the disease progresses, however, damage to the stomach can make it harder for the body to absorb vitamin B12, leading to a deficiency. Johnson says it is unclear how long he has had AIG for before his formal diagnosis in May. Symptoms at a later stage can include fatigue, shortness of breath, weakness, problems with memory and concentration, tingling or numbness in the hands and feet, difficulty with balance, and mood changes such as depression or anxiety. How is it treated? With there being no cure for AIG, treatment focuses on managing symptoms, correcting vitamin deficiencies and reducing the risk of complications. Experts say bloating, pain and other digestive symptoms may be eased by eating a healthy diet rich in vegetables, whole grains and fruit. Patients are also sometimes advised to eat smaller, more frequent meals and avoid foods that trigger symptoms, such as spicy or fatty foods, very high-protein meals, as well as alcohol, caffeine and fizzy drinks. Some experts also suggest an elimination diet - which briefly excludes foods before gradually reintroducing them - may help identify individual food triggers that can worsen AIG.As the condition can make it difficult to absorb vitamin B12 and iron, many patients need supplements or vitamin injections to prevent anaemia and nerve damage. Keeping at a healthy weight, quitting smoking and managing stress may also help.Some people find natural remedies like acupuncture, yoga and relaxation techniques help ease their symptoms, but there is limited scientific evidence that these are effective.Anyone considering alternative therapies or treatments should speak with a medical professional first.In the meantime, researchers are continuing to study new treatments that could slow or prevent damage to the stomach lining and reduce the risk of stomach cancer.