Normal fasting glucose levels
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Normal Fasting Glucose Levels: Definitions and Ranges
Normal fasting plasma glucose (FPG) levels are generally defined as less than 100 mg/dL (5.55 mmol/L) for adults. Average arterial plasma glucose values throughout a 24-hour period are about 90 mg/dL, rarely exceeding 165 mg/dL after meals and typically staying above 55 mg/dL even after extended fasting or exercise . During pregnancy, fasting glucose levels tend to decrease slightly, with median values dropping from 81 mg/dL preconception to 76–78 mg/dL in the third trimester, and returning to about 81–84 mg/dL postpartum. The same fasting glucose cut-off is recommended throughout pregnancy for diagnosing gestational diabetes .
Risk of Diabetes Within the Normal Fasting Glucose Range
Multiple studies show that even within the normal range, higher fasting glucose levels are linked to a greater risk of developing type 2 diabetes. For example, men with fasting glucose levels of 87 mg/dL or higher had a progressively increased risk of diabetes compared to those with levels below 81 mg/dL . Similarly, individuals with fasting glucose between 91–99 mg/dL had about twice the risk of developing diabetes compared to those with levels below 85 mg/dL, even after adjusting for other risk factors 35. This graded relationship holds true across different populations and age groups 25.
Cardiovascular Risk and High-Normal Fasting Glucose
Higher fasting glucose levels within the normal range are also associated with increased risk of cardiovascular disease (CVD). People with fasting glucose in the high-normal range (95–99 mg/dL) have a 53% higher risk of developing CVD compared to those with levels below 80 mg/dL 29. This association remains significant even after accounting for other risk factors such as age, blood pressure, cholesterol, and body mass index 910.
Fasting Glucose in Obese Youth and Insulin Resistance
In obese children and adolescents, higher normal fasting glucose levels are linked to lower insulin sensitivity and reduced beta cell function, which are early signs of diabetes risk. Each small increase in fasting glucose within the normal range raises the odds of impaired glucose tolerance, suggesting that even in youth, high-normal fasting glucose can be a warning sign .
Variability in Fasting Glucose Measurements
Fasting glucose levels can vary significantly from day to day within the same person. Continuous glucose monitoring in adults showed that, based on repeated measurements, a substantial number of people classified as having normal fasting glucose on one test could be reclassified as prediabetic or diabetic on subsequent tests. This highlights the importance of considering variability and trends over time, rather than relying on a single measurement .
Limitations of Fasting Glucose for Diabetes Screening
Relying solely on fasting glucose for diabetes screening can miss a significant number of cases, as many people with normal fasting glucose may still develop diabetes or have abnormal glucose tolerance. Monitoring both the absolute value and the rate of increase in fasting glucose over time can help identify those at higher risk, even if their levels remain within the normal range .
Conclusion
Normal fasting glucose is typically defined as less than 100 mg/dL, but higher values within this range are associated with increased risks of type 2 diabetes and cardiovascular disease in both adults and youth. Fasting glucose levels can fluctuate, and relying on a single measurement may not capture true risk. Regular monitoring and attention to trends, especially in those with high-normal fasting glucose, are important for early identification and prevention of diabetes and related complications 123568910.
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