What are type 1, type 2 and gestational diabetes, and why do they differ?

Diabetes is not one disease but a group of them, all defined by sustained high blood sugar. The common thread is insulin: the body either makes too little of it, or its cells are unresponsive to it. Classic symptoms include excessive thirst, excessive urination and excessive hunger, together with weight loss and blurred vision.

In type 1 diabetes, the immune system destroys the beta cells in the pancreas that make insulin. In 70 to 90% of cases the cells are destroyed by the person's own immune system, and the cause is not completely understood: it is believed to involve both genetic and environmental factors. Treatment with insulin is required for survival, and there is currently no known way to prevent it. Type 1 makes up an estimated 5 to 10% of all diabetes cases.

Type 2 diabetes works differently. It is characterized by high blood sugar, insulin resistance, and a relative lack of insulin: the cells respond poorly to insulin, and the supply of it is relatively short. The source names obesity and lack of exercise as its main causes, and some people are genetically more at risk than others. It makes up about 90% of cases, and many people have no symptoms in the first few years and are diagnosed on routine testing.

Gestational diabetes is a third pattern: high blood sugar that develops in pregnancy in someone who did not have diabetes. It affects 3 to 9% of pregnancies depending on the population studied, and in 90% of cases it resolves after the baby is born, though the risk of later type 2 diabetes is higher.