Home   >   Medicine   >   Obesity and Diabetes
Diabetes Overview

Diabetes is a disease in which the level of glucose (a form of sugar) in the blood is too high. It is a serious disease. If left untreated, it may cause serious complications in eyes, kidneys, nerves, and heart. About 6 percent of Americans have diabetes and two-thirds of people with diabetes die of heart disease.


Glucose is a main source of energy for the body. Most of the food we eat is broken down into glucose which then passes into the bloodstream and circulates throughout the body. Glucose can enter some cells directly, such as the cells of the retina, kidney and nervous tissues. However, for most body cells, such as muscle cells (myocytes) and fat cells (adipocytes), insulin is required to move glucose from the bloodstream into the cell.

Insulin is a hormone produced by the pancreas, a large gland behind the stomach. In people with diabetes, either the pancreas does not produce enough insulin or the cells do not use insulin properly, or both. Consequently, glucose builds up in the blood.

There are three main types of diabetes:

Type 1 diabetes
In type 1 diabetes, the immune system attacks the insulin-producing beta cells in the pancreas, which then produces little or no insulin. This type accounts for about 5 to 10 percent of diagnosed diabetes in the United States. It occurs mainly in children and young adults.

Type 2 diabetes
This is the most common form of diabetes. About 90 to 95 percent of people with diabetes have type 2. This form of diabetes is most often associated with older age, obesity, family history of diabetes and physical inactivity. About 80 percent of people with type 2 diabetes are overweight. It usually begins with insulin resistance, a condition in which fat, muscle, and liver cells do not use insulin properly. At first, the pancreas keeps up with the added demand by producing more insulin. In time, however, it loses the ability to secrete enough insulin in response to meals.

Gestational diabetes
Some women develop gestational diabetes late in pregnancy. Although this form of diabetes usually disappears after the birth of the baby, women who have had gestational diabetes have a 40 to 60 percent chance of developing type 2 diabetes within 5 to 10 years. Gestational diabetes is caused by the hormones of pregnancy or a shortage of insulin.

Symptoms and Diagnosis

The signs of diabetes are

  • increased thirst
  • increased urination, especially at night
  • increased hunger
  • weight loss
  • blurred vision
  • sores that do not heal

Doctors use the following tests to diagnose diabetes.

  • A fasting plasma glucose (FPG) test measures blood glucose in a person who has not eaten anything for at least 8 hours.
  • An oral glucose tolerance test (OGTT) measures blood glucose after a person fasts at least 8 hours and 2 hours after the person drinks a glucose-containing beverage.

Test results indicating that a person has diabetes should be confirmed with a second test on a different day.

FPG Test

The FPG test is the preferred test for diagnosing diabetes because of its convenience and low cost. However, it will miss some diabetes or pre-diabetes that can be found with the OGTT. The FPG test is most reliable when done in the morning. Results and their meaning are shown in Table 1. People with a fasting glucose level of 100 to 125 milligrams per deciliter (mg/dL) have a form of pre-diabetes called impaired fasting glucose (IFG). Having IFG means a person has an increased risk of developing type 2 diabetes but does not have it yet. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.

Table 1. Fasting Plasma Glucose Test

Plasma Glucose Result (mg/dL) Diagnosis
99 and below Normal
100 to 125 Pre-diabetes
126 and above Diabetes

Research has shown that the OGTT is more sensitive than the FPG test for diagnosing pre-diabetes, but it is less convenient to administer. The OGTT requires fasting for at least 8 hours before the test. The plasma glucose level is measured immediately before and 2 hours after a person drinks a liquid containing 75 grams of glucose dissolved in water. Results and their meaning are shown in Table 2. If the blood glucose level is between 140 and 199 mg/dL 2 hours after drinking the liquid, the person has a form of pre-diabetes called impaired glucose tolerance (IGT). Having IGT, like having IFG, means a person has an increased risk of developing type 2 diabetes but does not have it yet. A 2-hour glucose level of 200 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.

Table 2. Oral Glucose Tolerance Test

2-Hour Plasma Glucose Result (mg/dL) Diagnosis
139 and below Normal
140 to 199 Pre-diabetes
200 and above Diabetes



National Diabetes Information Clearinghouse, USA