Carbohydrate metabolism

• Glucose is the major energy substrate

• The body’s source of glucose are dietary carbohydrate and endogenous (principally hepatic) production by glycogenolysis (release of glucose stored as glycogen) and gluconeogenesis (glucose synthesis from, e.g., lactate, glycerol and most amino acids).

• Blood glucose concentration depends on the relative rates of influx of glucose into the circulation and its utilization.

•Blood glucose concentration is normally subject to rigorous control rarely falling below 70mg/dl or rising above 180 mg/dl in healthy individuals whether fasting or recently fed

• The maintenance of plasma glucose concentrations below about 180 mg/dl minimizes loss from the body as well as providing the optimal supply to the brain

The importance of extracellular glucose concentrations• Brain cells are very dependent on the extracellular glucose

concentration for their energy supply; hypoglycaemia is likely to

impair cerebral function. This is because they cannot:

1. Cannot Store glucose in significant amounts

2. Cannot Synthesize glucose

3. Cannot Metabolize substrates other than glucose and ketones. Plasma

ketone bodies concentrations are usually very low and are of little

importance as an energy source under physiological conditions

4. Cannot Extract enough glucose from the extracellular fluid at low

concentrations for their metabolic needs, because entry into brain

cells isn’t facilitated by insulin• The liver is the most important organ maintaining a constant energy

supply for other tissues, including the brain, under a wide variety of

conditions • The integration of the various processes and thus the control of blood

glucose concentration is achieved through the intensive action of

various hormones: these are insulin and the ‘counter regulatory’

hormones, namely glucagon, cortisol, catecholamines and growth

hormone.

Disorders of glucose homeostasis

• The major disorders of glucose homeostasis are diabetes mellitus Diabetes mellitus is characterized by glucose intolerance and thus a tendency to hyperglycaemia,

• Other Various conditions which can cause a pathologically low blood glucose concentration, that is, hypoglycaemia

Diabetes mellitus• Is a group of metabolic disorders of carbohydrate

metabolism in which glucose is underutilized, producing hyperglycaemia.

Aetiology and pathogenesisDiabetes mellitus is a common condition. There are two

distinct types: 1. In Type I (insulin-dependent diabetes mellitus-IDDM) there

is destruction of pancreatic cells and effectively no insulin secretion (autoimmune destruction of the pancreatic -cells).

2. In Type II (non insulin-dependent diabetes mellitus-NIDDM) either insulin is secreted in amounts insufficient to prevent hyperglycaemia or there is insensitivity to its actions.

Overall some 20% of patients are insulin dependent; most patients with NIDDM can be treated by diet, with or without hypoglycaemic drugs, for example, sulphonylureas and biguanides.

Major characteristics of insulin-dependent (IDDM) and non insulin-dependent (NIDDM) diabetes mellitus

FeatureIDDMNIDDMTypical age of onsetChildren, young

adultsMiddle-aged, elderly

Onset Acute (days or even weeks)

Gradual (over months)

Habitus (an individual’s general physical appearance)

LeanOften obese (40%)

Weight lossUsualUncommon

Ketosis-proneUsuallyUsually not

Plasma insulin concentration

Low or absentOften normal; may be

Family history of diabetes

Uncommon Common

Other classes of diabetes:

3. Diabetes mellitus associated with other conditions and syndromes:

e.g., Cushing’s disease, due to excessive cortisol production

This was formally known as secondary diabetes

4. Impaired glucose tolerance: impaired glucose tolerance is

diagnosed in individuals who have fasting blood glucose levels

less than those required for a diagnosis of diabetes mellitus, but

who have a plasma glucose response during the oral glucose

tolerance test judged to be between normal and diabetic

An OGTT is required to assign a patient to this class

5. Gestational diabetes mellitus: is carbohydrate intolerance of

variable severity with onset or first recognition during the present

pregnancy

The incidence of GDM is between 1 and 5%. Approximately 30%

of women with GDM develop diabetes mellitus within 20 years

after delivery, but carbohydrate may revert to and remain normal

after delivery

Pathophysiology and clinical features

• There are two aspects to the clinical manifestations of diabetes

mellitus:

1. Those related directly to the metabolic disturbances and

2. Those related to the long-term complications of the condition.

• The prevalence of the long-term complications increases with

duration of the disease and the risk of complications is greater if

glycaemic control is poor:

1. Nephropathy

2. Neuropathy {nerve damage}

3. Retinopathy {leading to blindness}

4. Arteriopathy {may result in stroke, gangrene or coronary disease}.

Abnormalities of lipoprotein metabolism occur frequently in

patients with diabetes mellitus and may predispose to

atherosclerosis

5. Infections which are common in diabetic patients and may

aggravate renal and peripheral vascular disease

• The hyperglycaemia of diabetes mellitus is mainly a result of

increased production of glucose by the liver and of decreased

removal of glucose from the blood.

• In the kidneys, filtered glucose is normally completely reabsorbed

in the proximal tubules, but a blood glucose concentrations much

above 200 mg/dL, reabsorption becomes saturated and glucose

appears in the urine.

• Glycosuria results in an osmotic diuresis, increasing water excretion

and raising the plasma osmolality, which in turn stimulates the thirst

centre.

• Osmotic diuresis and thirst cause the classical symptoms of

polyuria and polydipsia.

• Untreated, the metabolic disturbances may become deep with the

development of life-threatening ketoacidosis, non-ketotic

hyperglycaemia or lactic acidosis.

Diagnostic Criteria for Diabetes Mellitus1. Random plasma glucose ≥ 200 mg/dL (11.1 mmol/L) +

symptoms of diabetes2. Fasting plasma glucose ≥ 126 mg/dL (7.0 mmol/L)3. Two hour plasma glucose ≥ 200 mg/dL (11.1 mmol/L) during

an oral glucose tolerance test

Any of the three criteria must be confirmed on a subsequent day by any of the three methods

• Categories of Fasting Plasma Glucose (FPG).Normal fasting glucose FPG < 110 mg/dLImpaired fasting glucose FPG ≥ 110 mg/dL and < 126 mg/dLDiabetes likely FPG ≥ 126 mg/dL

• Categories of Oral Glucose ToleranceNormal glucose tolerance 2-h PG < 140 mg/dLImpaired glucose tolerance 2-h PG ≥ 140 mg/dL and < 200 mg/dLDiabetes likely 2-h PG ≥ 200 mg/dL

Metabolic complications of diabetes

• Patients with diabetes mellitus may develop one of several metabolic

complications needing emergency treatment: diabetic ketoacidosis and

hyperosmolal non-ketotic coma

Diabetic ketoacidosis

During fasting: when exogenous glucose is unavailable and the plasma

insulin concentration is therefore low, endogenous triglycerides are

converted to free fatty acids and glycerol by lipolysis. Both are

transported to the liver in plasma. Glycerol enters the hepatic

gluconeogenic pathway; the synthesised glucose can be released from

these cells, thus minimising the fall in glucose concentrations.

Most tissues, other than the brain, can oxidise fatty acids to acetyl CoA,

which can then be used as an energy source. When the rate of synthesis

exceeds its use the hepatic cells produce ketone bodies (acetoacetic

acid, -hydroxybutyric acid which can be decarboxylated to acetone.

These ketones can be used as energy source by brain and other tissues

at a time when glucose is in relatively short supply. Therefore ketosis

occurs when fat stores are the main energy source and may result from

fasting.

Inter-

tissue

relationshi

ps during

starvation

Diabetic ketoacidosis

• The development of ketosis requires changes in both adipose

tissue and the liver.

• In uncontrolled diabetes, the low insulin levels results in increased

lipolysis and decreased re-esterification increasing plasma free

fatty acids. There will be enhanced hepatic gluconeogenesis and

impaired glucose entry into cells. The increased glucagon/insulin

ratio enhances fatty acid oxidation in the liver. Thus, increased

hepatic ketone production.

• Excessive formation of ketone bodies results in increased blood

levels (ketonemia) and increased excretion in the urine

(ketonuria). This process is observed in conditions associated with

decreased availability of carbohydrates (such as starvation or

frequent vomiting) or decreased use of carbohydrates (such as in

diabetes mellitus and glycogen storage disease).

• Ketoacidosis may be the presenting feature of IDDM, or may

develop in a patient known to be diabetic who neglect to take his

insulin

Clinical features of diabetic ketoacidosis

• Thirst, polyuria, dehydration, hypotension, tachycardia and peripheral circulatory failure, ketosis, hyperventilation, vomiting, abdominal pain and drowsiness and coma.

Metabolic features: hyperglycaemia, glycosuria, non-respiratory acidosis, ketonaemia, uraemia, hyperkalaemia, hypertriglyceridaemia and haemoconcentration.

The deep, noisy respiration and the odour of acetone on the breath are classical features of diabetic ketoacidosis.

Hyperkalaemia is commonly present and is a result of the combined effects of decreased renal excretion and a shift of intracellular potassium (due to insulin lack, since insulin promotes cellular potassium uptake, and to acidosis and tissue catabolism). However, in spite of hyperkalaemia, there is always considerable potassium depletion due to increased urinary potassium loss in the presence of osmotic diuresis.

The plasma sodium concentration is usually decreased because of sodium depletion and the osmotically driven shift of water from intracellular compartment.

Diabetic and fasting ketoacidosis

• Diabetic ketoacidosis is differentiated from that of fasting

by hyperglycaemia and is usually more sever. Despite the

differences in plasma glucose concentration, the ketone

production in both cases is due to intracellular glucose

deficiency. In diabetic ketosis this is due to low insulin

activity.

• Ketosis always reflects excessive use of fat as an energy

source due to: intracellular glucose deficiency and low

insulin activity. The low insulin activity increases the rate

of production of gluconeogenic substrates by glycolysis

and proteolysis, and the rate of hepatic gluconeogenesis.

The resultant increased rate of glucose released into the

extracellular fluid is appropriate in starvation, but

aggravates the hyperglycaemia in diabetes mellitus.

Hyperosmolal non-ketotic comma

• The term hyperosmolal coma or precoma is usually

limited to a condition in which there is marked

hyperglycaemia but no detectable ketoacidosis.

• It has been suggested that insulin activity is

sufficient to suppress lipolysis but insufficient to

suppress hepatic gluconeogenesis or to facilitate

glucose transport into cells.

• It is commoner in older patients. Plasma glucose

concentrations may exceed 50mmol/L (900mg/dl).

• Glycosuria which lead to cerebral cellular

dehydration contributes to the coma and

hyperventilation.

Long-term complications in diabetes mellitus

1. Nephropathy: Microvascular changes characterized by

thickening of the capillary basement membrane, ultimately

leads to renal insufficiency or failure.

2. Neuropathy: any disease of the peripheral nerves, usually

causing weakness and numbness. Manifesting as sensory loss,

impotence, postural hypotension, constipation and diarrhoea.

Peripheral neuropathy may give rise to impaired perception of

pain and temperature (particularly in lower extremities).

Ischemia may cause skeletal muscle atrophy and motor

abnormalities.

3. Retinopathy {leading to blindness}, is a consequence of

microvascular changes. It might lead to retinal microaneurysm

and retinal detachment, secondary glaucoma and vision loss.

Premature cataracts are common. New formation of capillaries,

and haemorrhage.

Long-term complications in diabetes mellitus

4. Arteriopathy {may result in stroke, gangrene or coronary

disease}.

Atherosclerosis is a multistage process set in motion when cells lining

the arteries are damaged as a result of high blood pressure, smoking,

toxic substances in the environment, and other agents. Plaques

develop when high density lipoproteins (LDL) accumulate at the site

of arterial damage and platelets act to form a fibrous cap over this

fatty core

Prevention appears to be the primary means of attacking

atherosclerosis: through low-fat diets, regular vigorous exercise,

control of high blood pressure or diabetes, and avoidance of tobacco.

5. Infections which are common in diabetic patients and may

aggravate renal and peripheral vascular disease. Including

tuberculosis, pneumonia. Eruptive xanthomas occurs most often in

long-standing, poorly controlled diabetes mellitus.

Injury, infection, neuropathy, vascular disease or ischemia may lead

to gangrene

Glycated hemoglobin: Glycated hemoglobin:

Hemoglobin A1cHemoglobin A1c

• Under physiologic conditions

HbA is slowly and non-

enzymatically glycosylated

• The extent of glycosylation is

dependent on the plasma level

of particular hexoses

• The most abundant glycosylated

Hb is HbA1c which has glucose

unit that covalently linked to

amino group of N-terminal

valines of the beta chain

• In the case of Diabetes mellitus,

the amount of HbA1c will

increase

Glycated hemoglobin:

• Glucose reacts non-enzymatically with NH2-terminal amino acid

of the -chain of human haemoglobin, resulting in the formation

of haemoglobin A1c.

• This haemoglobin is formed slowly and continuously throughout

the 120 days life-span of the red cell. There is a two to three

fold increase in red cells of patients with diabetes mellitus.

• Formation of glycated haemoglobin is irreversible, it on both the

life span of the red blood cell (average 120 days) and the blood

glucose concentration.

• The amount of HbA1c therefore represents the integrated

values for glucose over the preceding 6 to 8 weeks and provides

an additional criterion for assessing glucose control. Values are

free of day-to-day glucose fluctuations and unaffected by

exercise or recent food ingestion. The interpretation of glycated

haemoglobin depends on the red blood cells having a normal life

span.

Glycated proteins

• Measurement of glycated proteins is useful in monitoring

long-term glucose control

• The amount of Hb A1c represents the integrated values

for glucose over the preceding 6 to 8 weeks and provides

an additional criterion for assessing glucose control.

• Not subject to the wide fluctuations observed when blood

glucose levels are assayed. Values are free of day-to-day

glucose fluctuations and unaffected by exercise or recent

food ingestion.

• Glycated protein levels, therefore, are a valuable addition

to blood glucose determinations in the assessment of

glycaemic control.

Glycated hemoglobin• Correlation of glycated haemoglobin to glucose level: there is a

lag time of several weeks between an increase of blood glucose and an increase of glycated haemoglobin. A similar lag time exists between the return of blood glucose to normal and the return of glycated haemoglobin values to normal.

• Glycated haemoglobin should be routinely monitored every 3 to 4 months.

Advantages over blood and urine glucose measurements:1. only one assay per month2. not influenced by hour-to-hour variations due to food,

exercise,...3. can be used in the detection of diabetes (good correlation with

glucose tolerance test).Reference intervals: values for glycated haemoglobins are

usually expressed as a percentage of total blood haemoglobin. Healthy subjects have values less than 6.5% in patients with poorly

controlled diabetes mellitus, values may extend to twice the upper limit of normal or more but rarely exceed 20%.

Role of the clinical lab in diabetes mellitus

• The clinical lab has a vital role in both the diagnosis and

management of diabetes mellitus

• The diagnosis of diabetes is made solely by the

demonstration of hyperglycaemia

Other assays, such as the OGTT, contribute to the

classification and characterization.

Management

• There are many aspects to the management of diabetes

mellitus:

1. Education of patients is vital; they will have diabetes for the

rest of their lives and must be responsible for their own

treatment with guidance from a physician.

2. Regular follow-up is essential to monitor treatment and

detect early signs of complications, particularly retinopathy

which can in many cases be treated successfully.

Management

• The aim of treatment is:

1. To alleviate symptoms and prevent the acute metabolic complications of diabetes can be attained with dietary control (restriction of carbohydrates, an increase in dietary fiber) with or without oral hypoglycaemic agents in patients with NIDDM, and with diet and insulin in patients with IDDM

2. To prevent the long-term complications

• Whatever the treatment, the fluctuations in blood glucose concentration that occur in most diabetic patients are still greater than those which occur in normal subjects.

Hypoglycaemia

• Hypoglycaemia is a blood glucose concentration below the fasting

range, but it is difficult to define specific limits.

• It is defined as a blood glucose concentration of less than 45mg/dl

Clinical features

• The clinical features of hypoglycaemia are the result of dysfunction

of the nervous system and the effects of catecholamines which are

released in response to the stimulus provided by the low blood

glucose.

• Clinical features include: weakness, tiredness, confusion, shakiness,

sweating, nausea, rapid pulse, light-headedness, hunger and epigastric

discomfort, convulsions and coma.

• The brain is totally dependent on blood glucose, and very low levels

of plasma glucose (less than 20 or 30 mg/dl) causes sever central

nervous system dysfunction.

• Restoration of plasma glucose usually produces a rapid recovery, but

irreversible damage may occur.

Causes of hypoglycaemia

Types of hypoglycaemia :

1. Hypoglycaemia that follows a stimulus (reactive

hypoglycaemia), including the stimulus of a meal

(post-prandial hypoglycaemia)

2. Hypoglycaemia that occurs during fasting

It is usually possible to distinguish between these

categories from the patient’s history

Hypoglycaemic syndromes

Reactive hypoglycaemia (stimulus)

1. Drug-induced hypoglycaemia: most patients with IDDM may

have occasional episodes of hypoglycaemia. Most frequently,

hypoglycaemia is related to a missed meal, increase in physical

activity or accidental insulin over dosage.

The diagnosis must depend on the blood glucose concentration, but if

there is any doubt, it is always safe to give glucose to a confused

or unconscious diabetic patient awaiting the result becoming

available.

Diabetic patients should always carry sugar and a means of

identification to facilitate treatment in emergency.

Hypoglycaemia due to drugs other than those used to treat diabetes

is uncommon. Children, but not adults, poisoned with salicylates

may develop sever hypoglycaemia. It has also been reported in

patients who have taken overdoses of paracetamol and, in these

cases, it is probably related to the severe liver damage that this

drug can cause.

Hypoglycaemic syndromes

Reactive hypoglycaemia (stimulus)

2. Post-prandial hypoglycaemia: in patients who

have undergone gastric surgery, hypoglycaemia

developing 90-150 min after a meal, particularly a

meal rich in sugar, is common. There is a rapid

passage of glucose into the small intestine and

release of hormones which stimulates insulin

secretion. The insulin response is excessive and

hypoglycaemia occurs as glucose absorption from

the gut falls down rapidly, rather than slowly as it

does when gastric emptying is normal.

Hypoglycaemic syndromes

Reactive hypoglycaemia (stimulus)

Alcohol and reactive hypoglycaemia:

hypoglycaemia may develop between 2-10 hours

after ingestion of large amounts of alcohol.

Hypoglycaemia is caused by suppression of

gluconeogenesis during metabolism of alcohol.

Insulin- and drug-induced reactive hypoglycaemia are

potentiated by alcohol.

Alcohol also increases insulin release in response to an

oral glucose load.

Hypoglycaemic syndromes

Fasting hypoglycaemia

• Symptoms occur typically at night or in the early morning, or

may be precipitated by a prolonged fast and strenuous

exercise.

• Fasting hypoglycaemia is rare but usually signals serious

underlying pathology and may be life-threatening.

• Autonomic symptoms usually begin at plasma glucose

concentrations below 45 mg/dl, and cerebral function

deteriorates when glucose is less than 25 mg/dl.

Insulinoma: insulinomas are tumours of the insulin-secreting -

cells of the pancreatic islets. C-peptide should be measured.

Although secreted in equimolar amounts with insulin, C-peptide

is cleared from the circulation more slowly, so that it may be

more reliable marker of endogenous insulin secretion than

insulin itself.

Hypoglycaemic syndromes

Fasting hypoglycaemia

Hepatic and renal disease: it may complicate very

sever hepatitis, hypoxic liver disease associated

with congestive cardiac failure or liver necrosis of

the whole liver is affected.

The kidneys are capable of gluconeogenesis to little

extent; they are also responsible for insulin

degradation. These facts may in part explain the

sever hypoglycaemia that is occasionally a feature

of terminal renal disease.

Treatment of hypoglycaemia

Hypoglycaemia should be treated by urgent iv.

administration of 10 to 20 ml of at least 10%, and in

adults 50%, glucose solution after withdrawal of a blood

sample for glucose determination.

If hypoglycaemia is suspected glucose should be given

immediately while waiting for the laboratory results.

It is less dangerous to give glucose to a patient with

hyperglycaemia than to give insulin to a patient with

hypoglycaemia

Determination of glucose in body fluids

Specimen collection and storage

• Plasma or serum is used for the majority of glucose determinations in

labs, whereas most methods of self-monitoring of glucose use whole

blood.

• Red blood cells in vitro continue to utilize glucose (it decreases serum

glucose by approximately 5% to 7% in 1 hour; 5 to 10 mg/dl), with the

result that unless a blood sample can be analyzed immediately, it is

essential to collect it into a tube containing sodium fluoride to inhibit

glycolysis.

• Fluoride ions prevent glycolysis by inhibiting enolase, an enzyme that

requires Mg+2. The inhibition is due to the formation of an ionic

complex consisting of Mg+2, inorganic phosphate and fluoride ions.

Fluoride is also weak anticoagulant because it binds calcium; however,

clotting may occur after several hours.

• Potassium oxalate is used as an anticoagulant in such ‘fluoride-oxalate’

tubes, and plasma obtained from this blood is thus unsuitable for the

measurements of potassium concentration.

• Many analytical procedures are used to measure blood glucose levels.

Almost all commonly used techniques are now enzymatic, and older

methods, such as colorimetric or oxidation-reduction techniques, are

rarely used.

Urinary albumin excretion

• Patients with diabetes mellitus are at high risk of suffering renal

damage.

• Approximately one third of patients with IDDM develop end-stage

renal disease requiring dialysis or transplantation.

• Persistent proteinuria, which is detectable by routine screening

tests (equivalent to a urinary albumin excretion rate 200g/min),

suggests the presence of diabetic nephropathy. This is usually

associated with long-standing disease and its occurrence less

than 10 years after the onset of diabetes is unusual.

• Once diabetic nephropathy occurs, renal function deteriorates

rapidly and renal insufficiency evolves.

• Treatment at this stage can slow down progression of disease, but

it cannot stop or reverse it. The presence of increased urinary

albumin excretion signals an increase in the transcapillary escape

rate of albumin and is therefore a marker of microvascular

disease.

Urinary albumin excretion

• Prospective studies have demonstrated that

increased urinary albumin excretion precedes and is

highly predictive of diabetic nephropathy, end-stage

renal disease, and proliferate retinopathy in IDDM.

• Variation in urine flow rate in an individual may be

corrected by expressing albumin as a ratio of

creatinine (i.e., albumin/ creatinine). At least three

separate samples should be assayed because of the

high intraindividual variation (30% to 40%) and

diurnal variation (higher during the day).

Urinary albumin excretion

Reference intervalsUrinary albumin excretion

g/min mg/24 hrAlbumin/Creatinine

Normal10150.01

IDDM20-20030-3000.02-0.2

Diabetic nephropathy

200300 0.2

The End