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The classification of diabetes proposed by the World Health Organization (WHO) in 1985 includes four categories: insulin-dependent diabetes mellitus (IDDM), noninsulin-dependent diabetes mellitus (NIDDM), malnutritionrelated diabetes mellitus (MRDM), and other types of diabetes associated with certain conditions and syndromes (1). The two major types of diabetes, IDDM and NIDDM, are considered to be etiologically different. In the classification by the National Diabetes Data Group (2) and WHO in 1980 (3), the terms type I and type II diabetes were also adopted as synonyms for IDDM and NIDDM. However, in the 1985 WHO classification, only the terms IDDM and NIDDM appeared because the terms type I and type II diabetes were suggested to represent the pathogenic mechanisms. Nevertheless, these terms are still being used synonymously.
For many years, two main types of diabetes have been recognized on the basis of clinical phenotypes. In the mid-1970s, the term type I diabetes was introduced to include typical insulin-dependent diabetes of juvenile and adult-onset and diabetes initially controlled by oral hypoglycemic agents but with positive islet cell antibody (4). The association of type I diabetes with specific HLA types was also noticed (5).
There are many features to distinguish IDDM (or type I) and NIDDM (or type II). These include age of onset; mode of onset (acute or slow); presence or absence of obesity; ketosis proneness; stability of blood glucose; family history; association with specific HLA types; presence of autoantibodies such as islet cell antibodies, insulin autoantibodies, and anti-GAD antibodies; association with other autoimmune diseases; blood C-peptide levels; need for insulin treatment; and so on. For each of these items, there are almost always exceptions. For example, IDDM can occur in adulthood and NIDDM can occur in young people of school age (maturity-onset diabetes of the young [MODY]). Anti-GAD antibodies are also occasionally observed in those who are diagnosed with NIDDM. The classification of diabetes is easy in typical cases, but there are a number of situations in which it is difficult to classify the disease into one or the other of these two main categories. In 1985, the WHO said, "In diabetic patients of European origin, the clinical distinction of IDDM from NIDDM...