What is gastrin?

Gastrin is a hormone that is produced by ‘G’ cells in the lining of the stomach and upper small intestine, and released into the blood circulation. During a meal, gastrin stimulates the stomach to release gastric acid (hydrochloric acid). Gastric acid is used to convert the inactive form of a protein digestive enzyme called pepsinogen into its active form pepsin, which allows the stomach to break down proteins swallowed as food and absorb certain vitamins such as vitamin B12. Gastric acid also acts as a disinfectant and kills most of the bacteria and microorganisms that enter the stomach with food, minimising the risk of infection within the gut.

Additionally, gastrin can stimulate the gallbladder to empty its store of bile and the pancreas to secrete enzymes. Bile and pancreatic enzymes help absorb food in the small intestine.

Gastrin also stimulates growth of the stomach lining and increases the muscle contractions of the gut to aid digestion.

How is gastrin controlled?

Before a meal, the anticipation of eating stimulates the vagus nerve within the brain, which sends a signal to the stomach and stimulate the release of gastrin. Gastrin release is also stimulated by the stretching of the stomach walls during a meal, the presence of certain foods (particularly proteins) within the stomach cavity and an increase in the pH levels of the stomach (i.e. the stomach becoming less acidic).

The production and release of gastrin is slowed by the hormone somatostatin, which is released by the pancreas when the stomach empties at the end of a meal, as well as when there is a drop in the pH levels of the stomach (when the stomach becomes more acidic).

What happens if I have too much gastrin?

An excess of gastrin can occur due to a gastrin-secreting tumour (gastrinoma, also known as Zollinger-Ellison syndrome) occurring within the small intestine (specifically within the upper part known as a duodenum) or in the pancreas. In gastrinomas, high levels of gastrin moving around the gut stimulate acid release, leading to stomach and small intestine ulcers (sores on the lining of the organs) that may burst. High levels of stomach acid can also cause diarrhoea because the lining of the small intestine becomes damaged. When a gastrinoma is diagnosed it is important to consider a genetic condition called multiple endocrine neoplasia type 1 (MEN1).

High levels of circulating gastrin can also occur when the pH of the stomach is high (i.e. not acidic enough), for example, in pernicious anaemia (a blood disorder caused by insufficiency of vitamin B12) or atrophic gastritis (when the stomach lining is chronically inflamed and unable to produce and release acid), and during treatment with antacid drugs.

As gastrin also stimulates growth of the stomach lining, it is thought that high gastrin levels may play a role in the development of certain cancers of the digestive tract such as gastric cancer. However, more work is required to suggest such a link.

What happens if I have too little gastrin?

It is rare to have too little gastrin. However, low levels of gastric acid may increase the risk of infection within the gut and may limit the ability of the stomach to absorb nutrients.