Definitions of being overweight

What defines whether you are overweight or obese? This is commonly done using a measurement of body weight adjusted for height and expressed as body mass index (BMI). The problem with this calculation is that it does not directly measure the amount of body fat, which has been shown to be a more accurate indicator of obesity than weight alone. In situations where people have more muscle mass, such as athletes for example, being overweight on a BMI chart can be misleading. Another problem with using BMI is that some people such as the elderly may have lost muscle mass and may be misclassified as in the healthy weight group whereas they are actually nutritionally deficient. For this reason many clinical researchers use a measurement of waist-hip ratio as a more reliable marker of body fat and the health related problems associated with obesity.

BMI:

Calculating the BMI is simple and quick and for most people being overweight on a BMI chart means you have excess fat. According to the World Health Organization a

Healthy weight BMI is 18.5 to 24.9, Overweight is BMI 25 to 29.9 Obesity is a BMI over 30.

It is important to understand that these figures represent statistical guidelines about health risks and that if you fall into an undesirable group a further personalized assessment is necessary in order to calculate your specific health risk.
However, you can be fairly confident in saying that if the fat has accumulated more around the abdomen, so called central obesity, your risk of Type-2 diabetes is very significantly increased. If you already have diabetes and you remain overweight then the most significant issue is the increased risk of cardiac complications.

Insulin Action

Being overweight or obese interferes with how insulin controls blood glucose. Although there is a very complicated metabolic relationship between weight and diabetes also involving blood fats, the basic fact is that weight gain decreases the effectiveness of insulin to lower blood glucose. This is referred to as insulin resistance.
It does not mean an immediate rise in blood glucose because the pancreas can compensate by producing more insulin and overcome the resistance. Taking a blood sample in someone in this state will show a normal fasting glucose but a high insulin level. It is regarded as pre-diabetic because eventually the pancreas fails to keep up with the high level of insulin production. This in turn leads to a decompensation of the control of blood sugar with rising glucose levels causing symptoms of diabetes.

While some overweight people will not develop diabetes, those with a family history of Type 2 diabetes are at a greatly increased risk. Genetic studies have identified an inherited tendency to defects in pancreatic function and when this is combined with the insulin resistance caused by being overweight the risk of developing diabetes is extremely high. As such it's important to be particularly aware of weight gain if there is already a family history of Type 2 diabetes.