Glycaemia: Regulation and Diagnosis (OGTT, Glycated Haemoglobin)

Glycaemia: Regulation and Diagnosis (OGTT, Glycated Haemoglobin)

Introduction

Glycaemia refers to the concentration of glucose in the blood. Its regulation is critical for energy homeostasis and cellular function. Dysregulation of glycaemia is a hallmark of metabolic disorders such as diabetes mellitus. Several diagnostic tools exist to evaluate glycaemic control, including the oral glucose tolerance test (OGTT) and glycated haemoglobin (HbA1c).

Regulation of Blood Glucose

Blood glucose levels are tightly regulated by a balance between glucose input (from diet and hepatic production) and glucose utilization (by tissues):

Key Hormones:

- insulin (from β-cells of pancreas): Lowers blood glucose by stimulating glucose uptake, glycogenesis, glycolysis, and lipogenesis.

- Glucagon (from α-cells): Increases blood glucose by promoting glycogenolysis and gluconeogenesis.

- Epinephrine, cortisol, growth hormone**: Act as counter-regulatory hormones that increase glycaemia during stress or fasting.

Target Tissues:

- Liver: Central organ for glucose storage and production.

- Muscle: Utilizes glucose for energy but lacks glucose-6-phosphatase.

- Adipose tissue: Responds to insulin by increasing glucose uptake and triglyceride synthesis.

Diagnostic Methods for Glycaemia

1. Oral Glucose Tolerance Test (OGTT)

- Used to assess glucose handling by the body.

- After overnight fasting, the patient consumes 75g of glucose orally.

- Blood glucose is measured at baseline and 2 hours post-glucose ingestion.

Interpretation (2-hour value):

- Normal: < 7.8 mmol/L

- Impaired glucose tolerance: 7.8–11.0 mmol/L

- Diabetes mellitus: ≥ 11.1 mmol/L

2. Glycated Haemoglobin (HbA1c)

- Reflects average blood glucose over the past 2–3 months.

- Formed by non-enzymatic glycation of haemoglobin.

- Not affected by acute changes in glucose levels.

Interpretation:

- Normal: < 5.7%

- Prediabetes: 5.7–6.4%

- Diabetes mellitus: ≥ 6.5%

HbA1c is especially useful for long-term monitoring and diagnosis of diabetes.

Clinical Relevance

Understanding glycaemic regulation and diagnostic markers is essential for early detection and management of diabetes mellitus. Combining OGTT and HbA1c provides comprehensive information on both acute and chronic glucose handling. Lifestyle, medication, and insulin therapy aim to restore and maintain normoglycaemia.

Conclusion

Blood glucose regulation involves a sophisticated hormonal network ensuring a balance between supply and demand. Diagnostic tools like OGTT and HbA1c are invaluable in identifying disturbances in glucose metabolism and guiding therapy.

References

1. Guyton and Hall Textbook of Medical Physiology, 14th Edition

2. Lehninger Principles of Biochemistry, 7th Edition

3. Lippincott Illustrated Reviews: Biochemistry, 7th Edition