Infections
Helicobacter pylori infection is an essential risk factor in 65–80% of gastric cancers, but only 2% of people with H. pylori infections develop stomach cancer. The mechanism by which H. pylori induces stomach cancer potentially involves chronic inflammation, or the action of H. pylori virulence factors such as CagA. It was estimated that Epstein–Barr virus is responsible for 84,000 cases per year. AIDS is also associated with elevated risk. Smoking
Smoking increases the risk of developing gastric cancer significantly, from 40% increased risk for current smokers to 82% increase for heavy smokers. Gastric cancers due to smoking mostly occur in the upper part of the stomach near the esophagus. Some studies show increased risk with alcohol consumption as well. Diet
Dietary factors are not proven causes, and the association between stomach cancer and various foods and beverages is weak. Some foods including smoked foods, salt and salt-rich foods, red meat, processed meat, pickled vegetables, and brackens are associated with a higher risk of stomach cancer. Nitrates and nitrites in cured meats can be converted by certain bacteria, including H. pylori, into compounds that have been found to cause stomach cancer in animals.Fresh fruit and vegetable intake, citrus fruit intake, and antioxidant intake are associated with a lower risk of stomach cancer. A Mediterranean diet is associated with lower rates of stomach cancer, as is regular aspirin use.Obesity is a physical risk factor that has been found to increase the risk of gastric adenocarcinoma by contributing to the development of gastroesophageal reflux disease (GERD).
Vitamin D deficiency is also more common after surgical procedures that reduce absorption from the intestine, including weight loss procedures. Critical illness
Vitamin D deficiency is associated with increased mortality in critical illness. People who take vitamin D supplements before being admitted for intensive care are less likely to die than those who do not take vitamin D supplements. Additionally, vitamin D levels decline during stays in intensive care. Vitamin D3 (cholecalciferol) or calcitriol given orally may reduce the mortality rate without significant adverse effects. Breastfeeding
Infants who exclusively breastfeed need a vitamin D supplement, especially if they have dark skin or have minimal sun exposure. The American Academy of Pediatrics recommends that all breastfed infants receive 400 international units (IU) per day of oral vitamin D.

Pathophysiology
Decreased exposure of the skin to sunlight is a common cause of vitamin D deficiency. People with a darker skin pigment with increased amounts of melanin may have decreased production of vitamin D. Melanin absorbs ultraviolet B radiation from the sun and reduces vitamin D production. Sunscreen can also reduce vitamin D production. Medications may speed up the metabolism of vitamin D, causing a deficiency.The liver is required to transform vitamin D into 25-hydroxyvitamin D. This is an inactive metabolite of vitamin D but is a necessary precursor (building block) to create the active form of vitamin D.The kidneys are responsible for converting 25-hydroxyvitamin D to 1,25-hydroxyvitamin D. This is the active form of vitamin D in the body.