Notes on Carbohydrates in Clinical Chemistry

Clinical Chemistry: Carbohydrates

I. Introduction

A. Definition
  • Carbohydrates are hydrates of aldehyde or ketone derivatives, defined by the position of the CO functional group.
  • Types of Carbohydrates:
    • Monosaccharides
    • Disaccharides
    • Oligosaccharides
    • Polysaccharides
  • Glycol aldehyde is the simplest carbohydrate (CHO).
  • Glucose is the only carbohydrate used directly for energy or stored as glycogen.
  • Glucose does not accumulate in muscle; it requires insulin to enter muscle cells and is quickly metabolized.
  • The brain relies entirely on blood glucose for energy; about 2/3 of glucose utilization in resting adults occurs in the central nervous system (CNS).
  • Glucose metabolism produces intermediates like pyruvic acid, lactic acid, and acetylcoenzyme A.
  • Complete oxidation of glucose results in carbon dioxide, water, and ATP.
B. Reducing and Nonreducing Sugars
  • Reducing Sugars: Glucose, maltose, fructose, lactose, and galactose, which have a double bond and a negative charge in their enol anion that makes glucose an active reducing substance.
  • Nonreducing Sugar: Sucrose is the most common, lacking an active ketone or aldehyde group.

II. Pancreas

A. Definition
  • The pancreas serves as both an endocrine (hormone secretion) and exocrine (digestive enzyme secretion) organ involved in carbohydrate metabolism.
  • Hormones secreted from the islets of Langerhans:
    • Insulin: Lowers blood glucose levels, promotes glucose uptake.
    • Glucagon: Raises blood glucose levels, promotes glycogenolysis.
    • Somatostatin: Inhibits insulin and glucagon secretion.
B. Hormones
  • Insulin
    • Synthesized by B-cells; released when blood glucose is high; enhances cell membrane permeability.
    • Promotes glycogenesis, lipogenesis, and glycolysis while inhibiting glycogenolysis.
  • Glucagon
    • Synthesized by a-cells; released during fasting; promotes glycogenolysis.
  • Other hormones influencing glucose levels:
    • Cortisol: Promotes gluconeogenesis and lipolysis.
    • Catecholamines: Inhibit insulin secretion, promote glycogenolysis.
    • Growth Hormone: Decreases glucose entry into cells.
    • Thyroid Hormones: Promote gluconeogenesis and intestinal glucose absorption.
    • Adrenocorticotropic Hormone (ACTH): Stimulates cortisol release and promotes glycogenolysis.

III. Clinical Conditions of Carbohydrate Metabolism

A. Hyperglycemia
  • Defined as increased blood glucose levels; toxic to beta cells, impairing insulin secretion.
  • Causes include: stress, infection, dehydration, insulin deficiency, etc.
  • Fasting Blood Sugar (FBS) Level: ≥ 126 mg/dL.
  • Laboratory Findings:
    1. Increased plasma and urinary glucose
    2. Increased urine specific gravity
    3. Presence of ketones in serum and urine
    4. Decrease in blood/urine pH (acidosis)
    5. Electrolyte imbalance (↓ Na+, ↑ K+, ↓ HCO3)
B. Hypoglycemia
  • Results from an imbalance in glucose utilization vs. production; low blood glucose levels.
  • Diagnostic Tests: Fulfil Whipple's triad
    • Low blood glucose
    • Symptoms of hypoglycemia
    • Symptoms alleviated by glucose administration.
  • Diagnostic Values:
    • 65 to 70 mg/dL: Glucagon & glycemic hormones released.
    • ≤ 60 mg/dL: Strongly suggests hypoglycemia.
    • <50 mg/dL: Severe hypoglycemia, especially requires evaluation in non-diabetic individuals.
C. Diabetes Mellitus (DM)
  • DM is a metabolic disorder characterized by hyperglycemia due to insulin secretion/receptor defects.
  • Diagnostic Criteria:
    • Fasting plasma glucose ≥ 126 mg/dL on multiple tests.
    • Glucosuria occurs when plasma glucose > 180 mg/dL.
    • Ketosis in severe DM from excessive fat metabolism due to inadequate carbohydrates.
  • Classification:
    1. Type 1 DM: Autoimmune destruction of pancreatic B-cells leading to insulin dependence.
    • Symptoms: polyuria, polydipsia, weight loss.
    • Common with autoantibodies (GAD65, IAA).
    1. Type 2 DM: Insulin resistance; the more common form, associated with obesity and lifestyle factors.
    2. Gestational DM: Impaired glucose metabolism diagnosed during pregnancy, resolved post-delivery.
    3. Specific Types of DM: Related to pancreatic or endocrine disorders.

IV. Glucose Methodologies

A. Introduction
  • Standard specimen for measurement is venous plasma glucose.
  • Methods include enzymatic, oxidation-reduction, and condensation methods:
    • Enzymatic Methods: Glucose oxidase method (colorimetric), Hexokinase method (most specific), etc.
B. Samples for Glucose Measurement
  • Include Random Blood Sugar (RBS), Fasting Blood Sugar (FBS), and Glucose Tolerance Tests (GTT).
  • Specific criteria for diagnosis, e.g., Fasting glucose <100 mg/dL (non-diabetic), 100-125 mg/dL (impaired).
  • Normal ranges for HbA1c and fructosamine measurements indicative of long and short-term glucose control, respectively.

V. Inborn Errors of Carbohydrate Metabolism

A. Galactosemia
  • Congenital deficiency of enzymes in galactose metabolism; includes jaundice, hepatomegaly, etc.
B. Essential Fructosuria
  • Caused by fructokinase deficiency; presence of fructose in urine.
C. Glycogen Storage Diseases (GSD)
  • Inherited disorders leading to abnormal glycogen metabolism; divided into types based on affected enzymes and clinical manifestations.

VI. Other Information

A. C-Peptide Test
  • Indicates insulin production; also used to monitor responses to pancreatic surgery.
B. Ketone Test
  • Monitors ketoacidosis and carbohydrate metabolism defects; abnormal levels noted during diabetes crises.