Defining Impaired Glucose Tolerance Insulin Resistance And Diabetes Mellitus

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Impaired glucose tolerance (IGT), determined by the postload glucose value of the oral glucose tolerance test (OGTT), is found in individuals with moderately disturbed glucose metabolism (22). These individuals are generally hyperinsulinemic and have an increased risk of developing diabetes (22). The National Diabetes Data Group guidelines require at least one intermediate glucose level greater than 11.1 mmol/L for the diagnosis of IGT (22). According to current American Diabetes Association guidelines, a two-hour postload glucose > 140 (7.8 mmol/l) and < 200 mg/dl (11.1 mmol/l) is defined as impaired glucose tolerance (23).

Insulin resistance, manifested by a diminished ability to keep glucose levels low with insulin levels in the normal range, precedes the onset of noninsulin-dependent diabetes mellitus. The glucose-clamp technique is the gold standard for the assessment of insulin resistance (22, 24). In individuals with insulin resistance, the glucose level is slightly raised, but remains below the diabetes range at the expense of raised insulin levels (22). Insulin resistance is thought to be a key factor in a cluster of cardiovascular risk factors (syndrome X or insulin resistance syndrome) (22).

According to 1993 criteria, diabetes mellitus was diagnosed when plasma glucose exceeded 11.1 mmol/l (200 mg/dl) and classic symptoms of diabetes (polydip-sia, polyuria, polyphagia, and weight loss) were present, or when the fasting plasma glucose exceeded 7.8 mmol/l (140 mg/dl) (22). If diabetes was not confirmed by these, but still suspected, an OGTT was performed (22).

When a number of single glycemic measures are compared, the two-hour post-load glucose level seems to be the best indicator of the presence of diabetes (22, 25, 26). The OGTT is a nonphysiologic procedure with high interperson variability (27). In a population of elderly people studied over a five-year period of time, the reliability coefficient of the diagnosis was 0.62 (27). Researchers have found that only 50 percent of OGTTs are reproducible (28). In patients that are diagnosed with impaired glucose tolerance on the first OGTT, only 40 percent to 60 percent are diagnosed with impaired glucose tolerance or diabetes on a second test (29-31). Therefore, epidemiologic studies based on a single OGTT may overestimate the prevalence of diabetes (22).

More recent criteria regarding the diagnosis of diabetes include: classic symptoms of diabetes plus casual (casual defined as any time of day without regard to time since last meal) plasma glucose concentration > 200 mg/dl; fasting (no caloric intake for at least eight hours) plasma glucose > 126 mg/dl (7.0 mmol/l); and two-hour plasma glucose > 200 mg/dl during an OGTT using a glucose load containing the equivalent of 75 grams anhydrous glucose dissolved in water (23).

There are two major types of diabetes: type 1 diabetes (5 percent to 10 percent of all diagnosed cases in the U.S.), which classically involves children and is primarily characterized by an absolute deficiency of insulin secretion; and type 2 diabetes (90 percent to 95 percent of all diagnosed cases), which is due to a combination of resistance to insulin action and an inadequate compensatory insulin secretory response. There are several other, less common forms of diabetes (1 percent to 5 percent of all diagnosed cases), which result from genetic conditions, surgery, drugs, malnutrition, infections, and other illnesses (23, 32).

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