DISORDERS OF CARBOHYDRATE METABOLISM Pg 38 Study Guide

Module Focus Carbohydrates constitute a major source of energy for the body. Carbohydrates must be broken down to monosaccharides (glucose) before they can be absorbed into the intestinal mucosa. Under normal circumstances, the blood glucose concentration for adults is approximately 70 – 110 mg/dl.

Diabetes mellitus is a disease caused by the absolute or relative deficiency of insulin & which causes elevated blood glucose levels (hyperglycemia) >10 million Americans suffer from this chronic metabolic disorder.

Carbohydrate Metabolism Ingested starch (carbohydrates) are broken down to glucose by salivary & pancreatic enzyme amylase. Amylase Carbohydrates glucose + other sugars (monosaccharides)

2) Glucose is absorbed in the GI tract and transported to the liver where it is either: Metabolized to lactic & pyruvic acids which enter the TCA cycle, if oxygen is present, and form CO2 ,H2O and ATP to provide energy Stored as GLYCOGEN for future use Converted to fat (once the storage capacity of glycogen is exceeded) and stored as triglycerides in adipose tissue in an unlimited amount!!!

IN FASTING STATES The glucose levels are maintained by the liver: Glycogen is broken down into glucose Fats and proteins can also be broken down and new glucose can actually be formed from noncarbohydrate sources. The bottom line is that the brain is dependent on glucose as its primary energy source.

EFFECTS OF HORMONES ON PLASMA GLUCOSE CONCENTRATION Insulin is the only hormone that decreases plasma glucose. The secretion of glucagon, epinephrine, cortisol, growth hormone and thyroxine cause an increase in plasma glucose and may be responsible for secondary diabetes

Basic Terminology Diabetes means “flowing through” which refers to the poluria or excessive amounts of urine excreted. Diabetes mellitus = insulin deficiency Diabetes insipidus = antidiuretic hormone (ADH) deficiency which causes excessive urination, but has nothing to do with insulin or glucose.

PATHOPHYSIOLOGY OF INSULIN DEFICIENCY Insulin deficiency causes the body to excessively utilize fats since it can’t use carbohydrates for energy. The excessive breakdown of fats results in the formation of ketone bodies which are acids.

KETONE BODIES Acetone Beta-hydroxybutyric acid Acetoacetic acid These ketone bodies produce a metabolic acidosis (ketoacidosis) which causes the blood pH to decrease, which is an acute medical emergency that may be fatal.

INSULIN DEFICIENCY Causes hyperglycemia ( blood glucose) Glycosuria or glucose in the urine will appear if the blood level exceeds the renal threshold (160 – 180 mg/dl) Because glucose is a relatively small molecule, it will be filtered by the glomeruli. However, the proximal tubules will reabsorb most of it back into the blood stream for future use. If blood levels are higher than 160 – 180, the proximal tubules cannot reabsorb anymore & will dump the excess into the urine.

INCREASED GLUCOSE LEVELS The increased glucose levels also cause an osmotic diuresis which results in water and electrolytes, such as sodium, being pulled out of cells and an excessive amount of urine is excreted. “polyuria” This leads to dehydration, hypotension, and an electrolyte imbalance which may be fatal.

POTASSIUM FYI Insulin causes potassium to enter the cells, leading to a decrease in plasma potassium. A decrease in insulin will cause an increase in plasma potassium (hyperkalemia) which may lead to cardiac arrythmias.

CHRONIC COMPLICATIONS OF DIABETES Diabetics are prone to infections Cataracts, retinal detachment, blindness Neuropathy Nephropathy (kidney problems) Angiopathy (high risk for heart attacks and strokes)

CLASSIFICATION OF DIABETES Type 1, Insulin Dependent – 10% of diabetics Type 2, Non-Insulin Dependent – 90% of diabetics Gestational – precipitated by pregnancy

Type 1 Synonyms: IDDM Insulin dependent Juvenile diabetes Ketosis-prone diabetes

Type 1 Characteristics Abrupt onset of symptoms Absolute insulin deficiency Rapid weight loss Ketosis-prone Polyuria (excessive urination) Polydipsia (excessive thirst or drinking of fluids) Patients are usually < 20 yrs old

Type 1 characteristics Osmotic diuresis (loss of fluids & electrolytes) Requires insulin injections Patients are prone to chronic complications Death usually results from coronary heart disease (CHD)

No known cause of Type 1 Idiopathic Autoimmune destruction of beta cells HLA chromosome #6 DQA & DQB genes Environmental (viruses, chemicals, or toxins may precipitate insulin deficiency) African or Asian

Type 1 Laboratory Findings Hyperglycemia ( blood glucose) Fasting Blood Glucose (FBS) = >126 mg/dl on 2 separate occasions Glucose appears in the urine (glycosuria) when the blood levels exceed the renal threshold of 160 – 180 mg/dl Ketoacidosis (metabolic acidosis) is an acute complication ( blood pH)  Cholesterol & triglyceride synthesis occurs

Type 2 Diabetes Synonyms: Non-insulin dependent NIDDM Adult/maturity Onset diabetes Non-ketosis prone diabetes 90% of diabetics have NIDDM

Type 2 Characteristics Insidious (vague) onset There is a relative insulin deficiency where the insulin receptor sites on the cells are decreased or “worn out” that may be due to production of insulin antibodies

Obesity is common, patients are >20 yrs old when diagnosed (usually around 40) Typically treated with weight reduction, dietary restrictions, or oral insulin Not usually prone to acute or chronic complications

Type 2 Lab Findings Hyperglycemia Ketones in the urine or blood are rarely found in NIDDM The patient usually is producing some insulin, but levels are too low to maintain a normal blood glucose

Causes of Type 2 Stronger genetic basis than Type 1 Excessive caloric intake, weight gain & obesity No relationship to viruses, no antibodies or HLA association

Gestational Diabetes Precipitated by pregnancy Newborn may be larger than normal May be an early warning sign that patient will eventually develop diabetes Often accompanied by family history Can cause congenital malformation if not diagnosed early

Diagnosis of GDM Perform a glucose challenge test at 24-28 wks. Administer a 50 gram glucose load Collect blood specimen one hour later

Other Causes Mild increases in plasma glucose may be caused by an increase in the secretion of one of the five hormones that increase plasma glucose. Cortisol Epinephrine Thyroxine (T4) Growth hormone Glucagon

Other Causes Genetic defects of beta cell function or insulin action Dugs, chemicals Infections Anti-insulin receptor antibodies

ADA Guidelines for Diagnosis FBS > 126, 2 separate occasions (recommended test) or Random glucose > 200 mg/dl + symptoms of diabetes OGTT, 2 hour = >200 mg/dl

ADA Guidelines for Diagnosis Fasting Blood Glucose (FBS) test Draw specimen after an overnight fast (8 hrs) Normal = approximately 70 – 110 mg/dl If >126 mg/dl on two separate occasions, diagnostic for diabetes mellitus & no further testing is required. 110 – 125 mg/dl = impaired tolerance

Random “Casual” Glucose Drawn anytime during day. >200 mg/dl accompanied by symptoms is diagnostic for diabetes mellitus

Critical Values If the patient’s blood glucose value is <40 mg/dl or > 500 – 600 mg/dl, this is considered a “critical” or panic value & the physician must be notified immediately.

Postprandial Glucose (20PP) Draw a fasting blood glucose specimen Administer 75 grams of glucose orally. Draw blood again 2 hours after the ingestion of the glucose.

The normal patient will return to fasting levels (70 – 110 mg/dl) at or before 2 hours. <140 at 2 hours is considered normal. >200 mg/dl at 2 hours or shortly after eating (random glucose) is considered diagnostic for diabetes mellitus. 140 – 199 = impaired glucose tolerance

Gestational Post-Challenge Glucose A standard oral glucose load is administered after an 8 hour fast. Draw patient specimen 1 – 2 hours after glucose load Normal =<150 mg/dl at 1 hour after 50 gms or <140 mg/dl at 2 hours after 75 gms

ORAL GLUCOSE TOLERANCE TEST (OGTT) This test is usually only performed if the FBS, 2 hour PP, or challenge test do not provide diagnostic information Patient Preparation: Patient should be on a normal diet for 3 days prior to test (No Dr. Atkins!) Patient should not be on any medications or under emotional stress Patient must be ambulatory.

OGTT Draw fasting blood glucose specimen Have adult patient ingest approx. 75 gms of glucose in a 5-minute period. Glucose solutions look like bottles of soda and come in several flavors. Do not even hint to the patient that the solution is sickeningly sweet or they may vomit!

SPECIMENS FOR OGTT Blood specimens are usually obtained at 30 minutes, 1 hour, 2 hours, and 3 hours after ingestion of the glucose solution.

Normal GTT The normal patient peaks at 30 – 60 minutes (typically around 150 mg/dl), and should return to fasting level or <120 mg/dl at or before 2 hours. All values should be < 200 mg/dl. Since the renal threshold of 160 – 180 has not been exceeded, all urine specimens should be negative for glucose.

Diabetic Normal Response

Diabetic GTT Results The diabetic will typically demonstrate glucose levels >200 mg/dl at 2 hours and one other time period during the tolerance. NOTE: Discontinue test if patient vomits. Do not perform GTT if patient’s fasting level is already well above normal.

Gestational Diabetes Results Fasting = >105 mg/dl 1 hour = >190 mg/dl 2 hour = >165 mg/dl 3 hour = > 145 mg/dl Or 2 or more values > 140 mg/dl

Glycated “Glyco-” Hemoglobin Also known as hemoglobin A1C Glucose attaches to the beta globulin chains over the life span (120 days) of the RBC. Therefore, the rate of glycohemoglobin formation reflects the concentration of glucose over 4 – 10 weeks.

Glycated hemoglobin Useful for the long-term management of the diabetic to prevent or arrest the development of complications. Diabetics can’t cheat like they do with a FBS test since this reflects their degree of control over the preceding 8 – 10 weeks! 5 – 8% is normal Diabetics are usually 10 – 22% if uncontrolled An EDTA whole blood specimen is utilized to perform the test.

< 100 mg/24hrs is normally excreted. URINE GLUCOSE TEST A chemical strip utilizing the glucose oxidase method is utilized. Urine specimens should be negative for glucose unless plasma levels exceed the renal threshold (as seen in diabetics) 160 – 180 mg/dl < 100 mg/24hrs is normally excreted.

Cerebral Spinal Fluid Glucose CSF glucose levels are approximately 2/3 of the patient’s serum levels. Normal CSF glucose = 40 – 70 mg/dl. CSF glucose is decreased in bacterial meningitis.

Fructosamine Glycosylated protein albumin The half-life of albumin is much shorter (2-3 weeks) than RBCs This tests provides a view of the control of the diabetic over the last 2-3 weeks

KETONES This test is included in the chemical strip used to test urine samples Urine samples are normally negative for ketones Excessive increases may be found in diabetic ketoacidosis caused by the incomplete metabolism of fats.

Home (Self) Monitoring When capillary whole blood is used instead of serum or plasma, reference values are usually 10%-15% lower Therefore, when patient has blood tested using serum, results will always be higher. Normal = approximately 65 – 95 mg/dl

Miscellaneous Tests Plasma Insulin levels are occasionally performed C-peptide Insulin levels are a direct reflection of pancreatic insulin secreting capacity This test is more reliable than insulin levels for assessing endogenous insulin secretory reserves. Diagnosing insulinomas

Microalbumin Low levels of albumin in urine prior to development of overt proteinuria Early indicator of renal complications

SPECIMEN COLLECTION Serum: Separate serum from clot within 30 – 60 minutes 7% of glucose is lost per hour due to the utilization of glucose by red & white cells Serum is stable for 8 hours at room temperature Serum is stable for 72 hours at 4 – 8C (refrigerator)

Sodium Fluoride Plasma Gray stopper vacuum tube Contains sodium fluoride which acts as a preservative to prevent glycolysis (breakdown of glucose) The tube also contains an anticoagulant to prevent clotting This is the best specimen if analysis cannot be performed immediately and/or plasma cannot be separated ASAP. Stable at RT for 24 hours. Disadvantages: more expensive and plasma not suitable for other chemistry tests that might be ordered.

Capillary Plasma or Serum If capillary plasma or serum is utilized, values are slightly higher than venous samples (3 mg/dL)

Whole Blood If whole blood is utilized (home monitoring devices), values are approximately 10 - 15% lower than venous serum or plasma specimens due to water content

BLOOD GLUCOSE METHODS Hexokinase = current reference method Glucose oxidase

Glucose + ATP G-6-Phos + ADP Hexokinase Method The enzyme Hexokinase transfers a phosphate group from ATP to glucose to form glucose-6-phosphate. Hexokinase Glucose + ATP G-6-Phos + ADP

Hexokinase Method The enzyme Glucose-6-phosphate dehydrogenase (G-6-PD) causes the reduction (addition of H+) of the coenzyme NAD to NADH. This reduction causes an increase in absorbance in the ultraviolet range (340 nm) The increase in absorbance is directly proportional to the amount of glucose in the sample.

Hexokinase Method G-6-PD Glucose-6-Phos + NAD Phosphogluconate + NADPH + H+

REMEMBER As NAD is reduced (gains a H+) NADH =  in Absorbance at 340 nm As NADH is oxidized (loses a H+) NAD =  in Absorbance at 340 nm HINT: If a hydrogen ion is added, there is an increase in absorbance. If a hydrogen ion is lost, there is a decrease in absorbance.

GLUCOSE OXIDASE METHOD The enzyme glucose oxidase reacts with glucose, water & oxygen to form gluconic acid and hydrogen peroxide(H2O2) Glucose oxidase Glucose + H2O + O2 gluconic acid + H2O2 For each molecule of glucose reacted, one molecule of oxygen is consumed. Therefore, the amount of oxygen consumed can be measured or the amount of H2O2 produced.

Measurement of H2O2 formed The amount of hydrogen peroxide produced can be measured by using the enzyme peroxidase and a color producing (chromagen) oxygen acceptor. Peroxidase H2O2 + reduced chromagen Oxidized chromagen The intensity of the colored complex (oxidized chromagen) is directly proportional to the concentration of the glucose in the sample.

Sources of Error Excessive levels of Acetaminophen and vitamin C (ascorbic acid) inhibit the enzymatic reaction and may cause falsely decreased glucose levels. Overall the method is relatively free from interferences.

HYPOGLYCEMIA Any condition in which the plasma glucose concentration falls below 40 – 50 mg/dl.

Hypoglycemia Symptoms: vary depending on the degree and rapidity of decrease Weakness, faintness Anxiety, mental confusion Hunger, cold sweat Loss of consciousness (<40 mg/dl) may cause brain damage

Classification Reactive (stimulus) Fasting/Spontaneous Overdose of insulin Decreased gluconeogenesis (ethanol ingestion) Postprandial (after a meal) Fasting/Spontaneous Insulinomas Sepsis Depleted glycogen stores

Diagnosis: a 5 to 6 hour glucose tolerance test may be performed. The patient’s plasma glucose may fall below normal between 4 – 6 hours. Test is insensitive Insulin levels C-peptide