Diabetes mellitus type 2 is a metabolic disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency.
This is in contrast to diabetes mellitus type 1, in which there is an absolute insulin deficiency due to destruction of islet cells in the pancreas. The classic symptoms are excessive thirst, frequent urination, and constant hunger. Type 2 diabetes makes up about 90% of cases of diabetes, with the other 10% due primarily to diabetes mellitus type 1 and gestational diabetes. Obesity is thought to be the primary cause of type 2 diabetes in people who are genetically predisposed to the disease.
Type 2 diabetes is initially managed by increasing exercise and dietary modification. If blood glucose levels are not adequately lowered by these measures, medications such as metformin or insulin may be needed. For those on insulin, there is typically the requirement to routinely check blood sugar levels.
The classic symptoms of diabetes are:
- Polyuria (frequent urination)
- Polydipsia (increased thirst)
- Polyphagia (increased hunger)
- Weight loss
- History of blurred vision
- Itchiness, peripheral neuropathy
- Recurrent vaginal infections
Other symptoms that are commonly present at diagnosis include:
Many people, however, have no symptoms during the first few years and are diagnosed on routine testing. People with type 2 diabetes mellitus may rarely present with non–ketotic hyperosmolar coma (a condition of very high blood sugar associated with a decreased level of consciousness and low blood pressure).
The development of type 2 diabetes is caused by a combination of lifestyle and genetic factors. Some factors are under personal control, such as diet and obesity. Other factors, such as increasing age, female gender, and genetics – we obviously have no control over. A lack of sleep has been linked to type 2 diabetes. This is believed to act through its effect on metabolism. The nutritional status of a mother during fetal development may also play a role, with one proposed mechanism being that of altered DNA methylation.
A number of lifestyle factors are known to be important to the development of type 2 diabetes, including: obesity (defined by a body mass index of greater than 30), lack of physical activity, poor diet, stress, and urbanization. Excess body fat is associated with 30% of cases in those of Chinese and Japanese descent; 60-80% of cases in those of European and African descent; and 100% of Pima Indians and Pacific Islanders. Those who are not obese often have a high waist–hip ratio.
Dietary factors also influence the risk of developing type 2 diabetes. Consumption of sugar–sweetened drinks in excess is associated with an increased risk. The type of fats in the diet are also important, with saturated fats and trans fatty acids increasing the risk and polyunsaturatedand monounsaturated fat decreasing the risk. Eating lots of white rice appears to also play a role in increasing risk. A lack of exercise is believed to cause 7% of cases.
Most cases of diabetes involve many different genes, with each being a small contributor to an increased probability of becoming a type 2 diabetic. If one identical twin has diabetes, the chance of the other developing diabetes within his lifetime is greater than 90%, while the rate for non–identical siblings is 25-50%. As of 2011, more than 36 genes have been found that contribute to the risk of type 2 diabetes. All of these genes together still only account for 10% of the total inheritable component of the disease. The TCF7L2 allele, for example, increases the risk of developing diabetes by 1.5 times and is the greatest risk of the common genetic variants. Most of the genes linked to diabetes are involved in beta cell functions.
There are a number of rare cases of diabetes that arise due to an abnormality in a single gene (known as monogenic forms of diabetes or “other specific types of diabetes”). These include maturity onset diabetes of the young (MODY), Donohue syndrome, and Rabson-Mendenhall syndrome, among others. Maturity onset diabetes of the young constitute 1–5% of all cases of diabetes in young people.
There are a number of medications and other health problems that can predispose to diabetes. Some of the medications include: glucocorticoids, thiazides, beta blockers, atypical antipsychotics, and statins. Those who have previously had gestational diabetes are at a higher risk of developing type 2 diabetes. Other health problems that are associated include: acromegaly, Cushing’s syndrome, hyperthyroidism, pheochromocytoma, and certain cancers such as glucagonomas. Testosterone deficiency is also associated with type 2 diabetes.