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Stomach cancer is caused by changes in the cells of the stomach, although it's unclear exactly why these changes occur.
Cancer begins with a change (mutation) in the structure of the DNA in cells, which can affect how they grow. This means cells grow and reproduce uncontrollably, producing a lump of tissue called a tumour.
Left untreated, cancer can spread to other parts of the body, usually through the lymphatic system (a network of vessels and glands called lymph nodes located throughout the body).
Once the cancer reaches your lymphatic system, it's capable of spreading to other parts of your body, including your blood, bones and organs.
It's not known what triggers the changes in DNA that lead to stomach cancer and why only a small number of people develop the condition.
However, evidence suggests that a number of different factors can affect your chances of developing stomach cancer. These are discussed below.
Your risk of developing stomach cancer increases with age. Most cases occur in people aged 55 or over.
For reasons that are unclear, men are twice as likely as women to develop stomach cancer.
People who smoke are about twice as likely to develop stomach cancer compared with non-smokers. This is because you swallow some cigarette smoke when you inhale and it ends up in your stomach. Cigarettes contain harmful chemicals which can damage the cells in your stomach.
The more you smoke and the longer you've been smoking, the bigger the risk. In the UK, around 1 in every 5 cases of stomach cancer (20%) is thought to be caused by smoking.
Helicobacter pylori (H. pylori) is a common type of bacteria. In most people, these bacteria are harmless, but in some people an H. pylori infection can cause problems such as stomach ulcers, recurring bouts of indigestion or long-term inflammation of the stomach lining (chronic atrophic gastritis).
Research has found people with severe chronic atrophic gastritis have an increased risk of developing stomach cancer, although this risk is still small.
A diet rich in pickled vegetables, such as pickled onions or piccalilli, salted fish, salt in general and smoked meats, such as pastrami or smoked beef, increases your risk of stomach cancer.
Countries where this type of diet is popular, such as Japan, tend to have much higher rates of stomach cancer than the UK.
A high fibre diet with five portions of fruit and vegetables a day will help protect against stomach cancer, and a diet high in fats and processed food and red meat will increase your risk of getting stomach cancer.
You're more likely to develop stomach cancer if you have a close relative with the condition, such as one of your parents or a sibling (brother or sister). In such cases, it may be appropriate for your doctor to arrange genetic counselling.
It's not fully understood why stomach cancer seems to run in families. It may be because of shared risk factors, such as having similar diets or having an H. pylori infection, or because of certain genes you inherit from your parents.
In around one in 50 cases of stomach cancer, testing has found that people share a mutation in a gene known as E-cadherin.
Research into stomach cancer has also shown that you may be more at risk of getting the condition if you have the blood type A. Your blood type is passed on from your parents, so this could be another way in which family history may increase your risk of developing stomach cancer.
There's also a condition that runs in families called familial adenomatous polyposis (FAP), which may increase your risk of developing stomach cancer. FAP causes small growths, called polyps, to form in your digestive system, and is known to increase your risk of developing bowel cancer.
For men, the risk of getting stomach cancer is increased after having prostate cancer, bladder cancer, breast cancer or testicular cancer. For women, the risk of developing stomach cancer increases after having ovarian cancer, breast cancer or cervical cancer.
Having certain medical conditions can also increase your risk of developing stomach cancer, such as pernicious anaemia (a vitamin B12 deficiency, which occurs when your body can't absorb it properly), and peptic stomach ulcers (an ulcer in your stomach lining, often caused by H. pylori infection).
If you've had stomach surgery, or surgery to a part of your body that affects your stomach, you may be more likely to develop stomach cancer.
This can include surgery to remove part of your stomach (known as a partial gastrectomy), surgery to remove part of your vagus nerve (the nerve that carries information from your brain to organs such as your heart, lungs and digestive system), or surgery to repair a stomach ulcer.
There are three ways stomach cancer can spread:
Stomach cancer that spreads to another part of the body is known as metastatic stomach cancer.