Compare and contrast the causes of Type 1 (insulin-dependent) and Type 2 (noninsulin-dependent) diabetes mellitus

Type 1 diabetes mellitus occurs when the body's immune system attacks beta cells within the Islets of Langerhans, in the pancreas. The immune system recognises pancreatic self-antigens as foreign and begins to remove the associated cells, as if a pathogen. This is known as an autoimmune disease. Beta cells are usually responsible for producing insulin which subsequently controls blood glucose levels. Thus, the destruction of these cells leads to type 1 diabetes and the associated symptoms. It is unclear exactly what causes this autoimmune attack. It is most certainly a combination of genetic and environmental factors, with some scientists believing viral infections can trigger the disease. Studies show it is only around 30% heritable however, so more research is needed to identify environmental triggers. 

Type 2 diabetes is also caused by a combination of genetic and environmental factors. However, interestingly it has more genetic heritability than type 1 diabetes. The environmental factors include being obese and physically inactive. This, as well as genetics, leads to insulin resistance. This means liver cells, fat cells and muscle require increased levels of insulin to enable glucose to be absorbed. The beta cells cannot produce enough insulin and therefore, blood glucose levels begin to rise. This differs from type 1 diabetes as the cause is extra-pancreatic rather than due to pancreatic destruction. Despite this, beta cells do begin to reduce in number, even in type 2 diabetes.