Blood Sugar

    When we refer to ‘blood sugar’, we actually mean the monosaccharide (simple sugar) glucose dissolved in the blood. Maintaining a stable blood glucose concentration is necessary in order to keep it high enough to ensure normal functioning of the brain, whilst also preventing the harmful consequences which can arise when the concentration is too high.

     The normal concentration of glucose is about 3.5- 5.5 mmol/litre (60-90 mg/L) and this is referred to as euglycaemia, or normal blood glucose. When blood glucose falls below 3.5 mol/litre(60 mg/L), hypoglycaemia is beginning to develop. If it falls to 2 mmol (35 mg)/L there is loss of consciousness (hypoglycaemic coma). A person would be diagnosed as having diabetes if their blood glucose after an overnight fast exceeded 7.0 mmol/litre: this is called hyperglycaemia, an abnormally high blood glucose.

    The figure shows a typical 24-hour profile of blood glucose concentration. The concentration can increase rapidly after consumption of simple sugars, especially glucose itself, either in a drink or in tablet form. This will provide a more rapidly available source of energy than would occur with starchy food.
 

Typical 24-hour profile of blood glucose concentration

Regulation of blood glucose level:

1-Hormonal regulation:

• Insulin hormone: it decrease glucose level through the following mechanisms:

    Carbohydrates:

1. Insulin ↑glucose uptake in muscle and adipose tissue by regulating glucose transporter (GLUT4) (N.B: glucose transporter in the liver (GLUT 2) is not regulated by insulin).
2. Oxidation of glucose
3. Inhibit gluconeogensis and glycogenolysis
4. Stimulate glycogenesis and lipogenesis.

Fat: stimulate lipogenesis and inhibit Ketogenesis by the following mechanism:

Inhibits hormone-sensitive lipase in adipocytes. Increases TG synthesis by increasing activity of acetyl CoA carboxylase; increases intracellular glucose concentration which increases glycerol 3-P; and increases lipoprotein lipase activity in adipose tissue.

Protein: protein synthesis↑

Regulation of plasma glucose concentration by insulin

• Anti insulin hormones:

They opposes the metabolic actions of insulin. The important metabolic effects include: they decrease the glucose transporter across the cells, ↑gluconeogenesis, ↑glycogenolysis and↓glycogen synthesis.

They are:

1. Glucagon hormone:

Carbohydrates:↑gluconeogenesis, ↑glycogenolysisand ↓glycogen synthesis

Fat:↑Ketogenesis

Protein:↓Hepatic protein synthesis and ↑protein catabolism in the liver

N.B: Glucagon DOES NOT affect muscle proteins.

Factors affecting glucagons secretion and actions

1. Catecholamines:

Regulation of energy metabolism by sympathetic nervous activity

and circulating epinephrine in response to a fall in plasma glucose levels

1. Growth hormone
2. Coticosteroids (cortisol)

2- Hepatic regulation:

Liver regulate the blood glucose level according to the fed stat:

• During fasting: it stimulates gluconeogenesis and glycogenolysis
• During feeding stat, it stmulate oxidation of glucose by activating the enzymes of glycolysis, also it stimulate glycogenesis and lipogenesis.

        3- Renal regulation:

Glucose is filtered through kidney, but if it exceed the renal threshold (180 mg/dl), it start to appear in urine

• Causes of low renal threshold leads to the appearance of glucose in urine as defect in the absorption of glucose from renal tubules and it is called diabetes innocence, also it appear in in 25% of pregnant women.
• High renal threshold > 180 mg/dl as in elderly and in renal damages.

Types of diabetes:

1. Diabetes mellitus
2. Diabetes insipidus: due to defect in antiduiretic hormone (ADH).
3. Diabetes innocence: due to defect in the absorption of glucose.
4. Bronze diabetes due to defect in iron absorption.