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NutrInsight • Slow-release carbohydrates: Growing evidence on metabolic responses and public health interest

3.3 Hyperglycemia during an OGTT as a risk factor
for metabolic diseases

Blood glucose tests for diagnosis of impaired glucose tolerance or type 2 diabetes

Fasting plasma glucose levels and 2-h oral glucose tolerance tests are regularly used to diagnose metabolic
dysfunction and even type 2 diabetes. The fasting plasma glucose (FPG) test, which is routinely performed
by physicians to measure the plasma glucose level after fasting for at least 8 h, reflects hepatic glucose
production. To conduct the 2-h oral glucose tolerance test (OGTT), the subject’s fasting glucose is tested; the
subject then drinks a 75 g glucose solution, and another blood sample is drawn 120 min later. For research
purposes, intermediate time points are added (at 30, 60, and 90 min after ingestion of the glucose solution).
The OGTT assesses the body’s ability to react appropriately to carbohydrate intake. Normally, the blood
glucose level rises after the glucose solution is ingested, which stimulates the pancreas to release insulin
into the bloodstream. Insulin allows the glucose to be taken up by cells. As time passes, the blood glucose
level is expected to decrease. When a person is unable to produce enough insulin, or if the body’s cells are
resistant to its effects (insulin resistance), then less glucose is transported from the blood into cells, and the
blood glucose level remains high.

OGTT in non-diabetics: a more predictive test than fasting glycemia
for detecting progression to type 2 diabetes

In the diabetes field, OGTT has been recognized as an important and relevant diagnostic test by the World
Health Organization (WHO) and the International Diabetes Federation (IDF), because 1) fasting plasma
glucose alone fails to diagnose around 30% of cases of previously undiagnosed diabetes; 2) an OGTT is the
only mean of identifying subjects with impaired glucose tolerance; and 3) an OGTT is frequently needed to
confirm or exclude an abnormality of glucose tolerance in asymptomatic subjects [WHO, 2006]. While the
OGTT has been used in population-based surveys, it is seldom used in daily practice.

Non-diabetic subjects can be classified into four categories (see Figure 14) based on their OGTT glycemic
response profiles:

• normal glucose tolerance (NGT) subjects whose blood glucose concentration at fasting level is
<6.1 mmol/l (110 mg/dl), reaches a peak 30–60 min following glucose ingestion, and then declines
toward the fasting glucose level. The 2-h plasma glucose level is below 7.8 mmol/l (140 mg/dl);

• Subjects with impaired glucose tolerance (IGT) have a normal fasting plasma glucose (<6.1 mmol/l
(110 mg/dl)), but their glycemic curve manifests two abnormalities during OGTT: a rapid and continuous
rise in plasma glucose concentration and a lack of decline in plasma glucose at 2 h (2-h plasma glucose
≥7.8 mmol/l and <11.1 mmol/l or ≥140 mg/dl and <200 mg/dl);

• Subjects with impaired fasting glucose (IFG) have an elevated FPG (from 6.1 mmol/l to 6.9 mmol/l or
110 mg/dl to 125 mg/dl), an exaggerated early rise in plasma glucose concentration following glucose
ingestion, and a plasma glucose concentration similar to NGT at 120 min (<7.8 mmol/l or 140 mg/dl);

• Subjects with combined glucose intolerance (CGI) have an elevated fasting plasma glucose level
(from 6.1 mmol/l to 6.9 mmol/l or 110 mg/dl to 125 mg/dl) and elevated 2-h plasma glucose levels
(≥7.8 mmol/l and <11.1 mmol/l or ≥140 mg/dl and <200 mg/dl) [Abdul-Ghani et al., 2006a].

To be diagnosed as type 2 diabetic, the 2-h plasma glucose level must be above or equal to 11.1 mmol/l
(200 mg/dl) during an OGTT [WHO, 2006].

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