Carbohydrates Overview and Classification

Carbohydrates

• Carbohydrates are compounds that provide chemical energy to cells
• They are primarily sugars and starches
• Carbohydrates are a primary energy source for the brain, erythrocytes, and retinal cells
• Carbohydrates contain carbon (C), hydrogen (H), and oxygen (O)
• "-ose" suffix signifies a sugar
• Classification is based on:
  • Number of sugar units (monosaccharides, disaccharides, oligosaccharides, polysaccharides)
  • Size of the base carbon chain (trioses, tetroses, pentoses, hexoses)
  • Location of the carbonyl group (aldehyde or ketone)
  • Stereochemistry of the compound

Classification of Carbohydrates

• Trioses - 3 carbons
• Tetroses - 4 carbons
• Pentoses - 5 carbons (e.g., DNA)
• Hexoses - 6 carbons (e.g., glucose)
• Aldose: carbonyl group at the end (aldehyde group)
• Ketose: carbonyl group in the middle (ketone group)

Number of Sugar Units

• Monosaccharides: one sugar unit (e.g., glucose, fructose, galactose)
• Disaccharides: two sugar units (e.g., maltose, lactose, sucrose)
• Oligosaccharides: 3-10 sugar units
• Polysaccharides: >10 sugar units (e.g., starch, glycogen, cellulose)

Chemical Properties of Carbohydrates

• Reducing carbohydrates: contain ketone or aldehyde groups (e.g., glucose, maltose)
• Non-reducing carbohydrates: do not contain ketone or aldehyde groups (e.g., sucrose)

Carbohydrate Metabolism

• Begins in the mouth with salivary amylase breaking down polysaccharides
• Monosaccharides are absorbed by the gut and transported to the liver
• Glucose is the only carbohydrate directly used for energy or stored as glycogen
• Pathways of glucose metabolism:
  • Liver and muscle glycogen storage
  • Conversion to amino acids and proteins
  • Conversion to fats
• Ultimate goal is metabolism to CO2 and H2O

Glucose Breakdown

• Glycolysis: Conversion of glucose to lactate or pyruvate, providing energy
• Glycogenesis: Conversion of glucose to glycogen for storage (primarily in liver and muscle)
• Glycogenolysis: Breakdown of glycogen to glucose when blood glucose levels are low

Glucose Formation

• Gluconeogenesis: Formation of new glucose from non-carbohydrate sources (e.g., amino acids, glycerol, fatty acids)
• Occurs primarily in the liver
• Important in starvation and weight loss
• Protects the body, especially the brain

Carbohydrate Metabolism (Lipolysis, Lipogenesis)

• Lipogenesis: Conversion of carbohydrates to fatty acids, Excess glucose is stored as fat
• Lipolysis: Decomposition of fat

Regulation of Plasma Glucose

• Glycogenolysis: Releases glucose into the blood (quick response)
• Gluconeogenesis, and lipolysis: Longer response, Release glucose into the blood
• Glycogenesis: Liver stores glucose
• Insulin decreases glucose levels
• Glucagon increases glucose levels
• Epinephrine and Cortisol increase glucose levels
• ACTH increases glucose levels

Hormones Affecting Glucose Levels

• Insulin: Decreases blood glucose by facilitating glucose entry into cells and promoting glycolysis
• Glucagon: Increases blood glucose by stimulating glycogenolysis and gluconeogenesis
• Epinephrine and Cortisol: Increase blood glucose by stimulating glycogenolysis, gluconeogenesis, and lipolysis
• Adrenocorticotropic Hormone (ACTH): Increases blood glucose by stimulating the adrenal cortex to release cortisol

Other Hormones Affecting Glucose Levels

• TSH (thyroid-stimulating hormone): Increases glucose absorption from intestines
• Somatostatin: Inhibits insulin, thus increasing glucose

Types of Diabetes

• Type 1 Diabetes: Insulin-dependent diabetes mellitus (IDDM), autoimmune destruction of islet beta cells, usually in children and adolescents
• Type 2 Diabetes: Non-insulin-dependent diabetes mellitus (NIDDM), insulin resistance, usually in adults, related to obesity, lack of exercise, diet, and genetic factors
• Secondary Diabetes: due to other conditions such as genetic defects, pancreatic disease, endocrinopathies or drug-induced
• Gestational Diabetes (GDM): glucose intolerance during pregnancy, usually resolves after delivery but increases risk of later diabetes

Diabetes Diagnosis Criteria

• Symptoms of diabetes plus random plasma glucose >200 mg/dL
• Fast blood glucose >126 mg/dL
• 2-Hour glucose > 200 mg/dL during an oral glucose tolerance test (OGTT)
• Hemoglobin A1c (HbA1c) > 6.5%, Confirmed on repeat measurement

Prediabetes

• Fasting glucose between 100-125mg/dL
• 2-hr GTT results between 140-199mg/dL
• HbA1c between 5.7-6.4%

GDM testing

• Moms screened at weeks 24-28
• 2-hr GTT, fasting, 1 hour, and 2 hours after 75-g glucose load
• Diagnosis: Fasting >92mg/dl, 1 hour >180mg/dl, and 2 hour >153mg/dl

Hypoglycemia

• Plasma glucose level falls below 60 mg/dL
• Release of glucagon, epinephrine, cortisol, and growth hormone
• Treatment varies with cause (small, frequent meals, low in carbs, high in protein)

Lab Findings for Hypoglycemia

• Low plasma glucose levels

Glucose

• Insulin level
• Galactosemia
• Caused by a deficiency in galactose-1-phosphate uridyltransferase
• Results in the inability to convert galactose to glucose or lactose to glucose
• Symptoms: failure to thrive, mental retardation, cataracts, and death

Laboratory Testing Considerations for Glucose

• Reference values depend on various factors, including the type of specimen, how it was collected
• Reference range for glucose (serum/plasma): 74-100 mg/dL. Whole blood is 65-95mg/dl
• Specimen collection methods (serum, plasma, whole blood)
• Results are 10-15% lower for whole blood than serum/plasma due to dilution by cells

Glucose - Specimen Collection and Testing

• CSF specimens: Analyzed ASAP; 60-70% of current blood glucose level
• 24-hour urine: A small amount of glucose is lost daily (<500 mg/24 hr).

Preservation

• Perform testing <1hr
• Separate plasma from cells <1hr
• Refrigerate (slows down glucose utilization by cells).

Additive

• Sodium fluoride inhibits glycolysis

Anticoagulant

• Potassium oxalate

Enzymatic methods

• Glucose Oxidase or Hexokinase
• Fasting Glucose Levels

FBS (Fasting Blood Sugar)

• Most frequently ordered screening test for glucose metabolism
• Fasting values of >126 mg/dL indicate diabetes
• FBS should be repeated on another day to confirm diagnosis
• Borderline diabetes may have a normal FBS; needs a challenge test to show abnormality.

Glucose Tolerance Test (GTT)

• Purpose: diagnose hyperglycemia/diabetes and evaluate symptoms of hypoglycemia
• Phlebotomist responsibilities
• Ensure consistency of tubes and specimen type
• Venipuncture or dermal puncture: Monitor patient.

Oral Glucose Tolerance Test (OGTT)and results

• Blood glucose level drawn before drinking glucose drink
• Glucose drink given
• Blood glucose level drawn again 2 hours later
• Glucose levels should return to normal levels after 2 hours
• If glucose levels are higher than 200 mg/dL, a fasting or random glucose level is needed to diagnose diabetes.

Glycosylated Hemoglobin (HbA1c)

• Glucose attaches non-enzymatically to hemoglobin molecules
• Provides a time-averaged glucose reading reflecting the mean daily blood glucose levels over the previous 2-3 months
• Advantages: reflects time-averaged glucose, not subject to temporary variability, doesn't need fasting
• Influenced by: Red blood cell lifespan, sickle cell disease, hemolytic diseases,

HbA1c Testing, Specimen and Methods

• Specimen: EDTA whole blood

Ketones

• Produced in the liver from stored lipids
• Metabolic byproducts of fatty acid breakdown.
• Include acetone, acetoacetic acid, and β-hydroxybutyric acid (78%)
• Increased in conditions like carbohydrate deprivation, starvation, fasting, prolonged vomiting
• Increased in diabetes
• Tested in fresh serum or urine

(Micro)albuminuria

• Early sign of nephropathy (kidney damage) due to diabetes
• Caused by progressive changes in the kidneys
• Microalbumin to creatinine ratio of >30 mg/g is diagnostic
• Should be confirmed over a period of 3-6 months
• Random or timed collections (24-hour or 4-hour)

Lactose Tolerance Test

• Used to assess lactose malabsorption or lactase deficiency.
• Patients given a lactose load, breath samples collected at timed intervals, and increased hydrogen levels indicate a diagnosis

Glucose and HbA1C Decision Levels

• Fasting glucose
• Two-hour oral glucose tolerance
• HbA1c

Lipids and Lipoproteins

• Lipids are essential for body function, including fatty acids, triglycerides, phospholipids, and cholesterol/cholesteryl esters.
• Lipoproteins: Chylomicrons, VLDL, LDL, and HDL are lipoproteins varying in size, density, lipid content, and associated apolipoproteins (Apo)
• Apolipoproteins (Apo) such as Apo A-I, Apo B, Apo B-100, Apo B-48, Apo CII, Apo D, and Apo E have specific functions and origins.
• Lipid metabolism: absorption, exogenous pathways (dietary lipids), endogenous pathways (liver-derived lipids), reverse cholesterol transport
• Factors impacting cholesterol levels must be identified
• Methods for cholesterol and triglyceride measurement, specimen choice, and potential interfering substances are crucial.
• Metabolic syndrome criteria and significance need to be understood.
• Atherosclerotic plaque formation: a complex process that needs to be defined.
• Clinical conditions: Dyslipidemia, hyperlipoproteinemia, hypercholesterolemia, hypertriglyceridemia, and hypolipoproteinemia
• Clinical conditions: Arteriosclerosis and xanthoma

Friedewald formula

• Calculation and limitations for clinical settings
• Non-HDL cholesterol clinical significance

This quiz covers the fundamental concepts of carbohydrates, including their structure, classification based on sugar units and carbon chains, and their significance as an energy source. Explore different types of carbohydrates like monosaccharides, disaccharides, and polysaccharides in this informative quiz.