Whereas type 1 diabetes is characterized by the onset in young persons (average age at diagnosis = 14), type 2 diabetes usually develops in middle age or later. This tendency to develop later in life has given rise to the term "adult onset diabetes," although the prevalence of type 2 diabetes in younger people is rising, making this term somewhat inaccurate and outdated.
The typical type 2 diabetes patient is overweight ,although there are exceptions. In contrast to type 1 diabetes, symptoms often have a more gradual onset. Type 2 diabetes is associated with insulin resistance rather than the lack of insulin, as seen in type 1 diabetes. This often is obtained as a hereditary tendency from one's parents. Insulin levels in these patients are usually normal or higher than average but the body's cells are rather sluggish to respond to it. This lack of insulin activity results in higher than normal blood glucose levels.
Type 2 diabetes is the most common type of diabetes. This disease exists in all populations, but prevalence varies greatly, ie, 1% in Japan, and greater than 40% in the Pima Indians of Arizona. In Caucasians, the figure is somewhere between 1-2% of the entire population. The high incidence of type 2 diabetes in certain groups such as the Pima Indians appears to be a relatively recent development that followed a change in the type of food intake (from relatively little food to plenty of food). With this came the development of obesity within their culture which results in diabetes developing in those that are genetically predisposed. This "urbanization phenomenon" has been most carefully studied in non-white populations, but is probably ethnically and racially nonspecific. In other words, obesity tends to promote diabetes in those genetically predisposed regardless of where you live and what your racial background is.
Type 2 diabetes tends to be fairly hereditary in contrast to type 1 diabetes. Approximately 38% of siblings and one-third of children of people with type 2 diabetes will develop diabetes or abnormal glucose metabolism at some point. The degree of obesity also seems to be a factor, with a larger percentage of diabetes developing in those who are more obese. Studies with identical twins showed that 90-100% of the time when diabetes developed in one it would also develop in the other compared with 50% in type 1 diabetes.
Development of type 2 diabetes seems to be multi-factorial; that is, there are a number of issues to blame. Genetic predisposition seems to be the strongest factor. Obesity and high caloric intake seem to be another. Twenty percent of people with this type 2 diabetes have antibodies to their islet cells which are detectable in their blood resulting in the expected low levels of insulin, suggesting the possibility of incomplete islet cell destruction (see discussion about autoimmune diabetes in the type 1 diabetes section). These patients often tend to respond early to oral drugs to lower blood sugar but may need insulin at some point.