Diabetes mellitus

Definition

Diabetes mellitus is a condition in which the pancreas no longer produces enough insulin or when cells stop responding to the insulin that is produced, so that glucose in the blood cannot be absorbed into the cells of the body. Symptoms include frequent urination, lethargy, excessive thirst, and hunger. The treatment includes changes in diet, oral medications, and in some cases, daily injections of insulin.

Pathophysiology:

   When we consume food or drink containing carbohydrates, most of this will be either simple glucose  sucrose or starch  Thus, most of the carbohydrate we consume is available to the body as glucose, and so eating or drinking it will lead, after digestion and absorption, to a rise in blood glucose. The magnitude of this rise is controlled by the release of insulin from the pancreas. Insulin acts to stimulate the uptake of glucose from the blood into cells such as those of muscle and adipose tissue, its storage as glycogen (in muscle and liver), and its part in the synthesis of triglycerides, the stored form of fat (mainly in adipose tissue). The relatively slow rate of absorption of dietary carbohydrate (it can take 2-3 hours to absorb the carbohydrate from a normal breakfast), and the effects of insulin, ensure that blood glucose does not usually rise above 8 mmol/litre after meals in non-diabetic people.

   When blood glucose concentration is normal, the glucose which is filtered from the blood in the kidneys is reabsorbed back into the blood stream by the kidney tubules, and so none is lost in the urine. But if blood glucose exceeds about 12 mmol/litre, this causes more glucose to be filtered by the kidneys than they can reabsorb. It is called that it exceed the renal threshold of the kidney (180 mg/dl) and therefore lost in the urine. And, because glucose is a powerful osmotic agent, it draws water with it, causing large volumes of sweet urine to be excreted (characteristic of diabetes mellitus). The other undesirable consequence of a persistently elevated blood glucose is that a chemical reaction (glycation or glycosylation) can occur between glucose and proteins, including the important structural proteins in cell membranes, and this can damage the membranes, producing harmful effects. Thus the action of insulin to control blood glucose prevents these undesirable effects of hyperglycaemia, and also ensures that glucose is available for use by the body's tissues.
  The brain and the rest of the nervous system, and also the red blood cells, must receive a constant supply of glucose to function normally. In prolonged starvation it is possible for the brain to satisfy some of its energy requirements by using ketone bodies, but under normal circumstances the adult human brain needs approximately 6 gm per hour of glucose to function normally. After meals containing carbohydrate this is not a problem, as the absorbed carbohydrate provides a ready supply of glucose. However, if we have a high fat meal, or have an extended period between meals (e.g. fasting overnight), we have to provide glucose from within the body. This is done either by the breakdown of the glycogen stored in the liver, which releases glucose into the blood, or by making glucose from amino acids released from the body protein stores. This synthesis of glucose (gluconeogenesis) occurs mainly in the liver, and to a lesser extent in the kidneys. The stimulation of the liver to break down its glycogen store and make glucose from amino acids occurs as a result of the fall in plasma insulin which occurs in fasting, together with an increase in glucagon, which is another hormone released from the pancreas.
   After a meal the concentration of glucose rises, but this rise is limited by the hormone insulin, which is secreted by the pancreas to stimulate the uptake of glucose into tissues. Diabetes mellitus is the result of failure of the insulin mechanism.

  Failure to maintain blood glucose in the normal range leads to conditions of persistently high (hyperglycemia) or low (hypoglycemia) blood sugar. Diabetes mellitus, characterized by persistent hyperglycemia of several causes, is the most prominent disease related to failure of blood sugar regulation.  Though it is called "blood sugar" and sugars besides glucose are found in the blood, like fructose and galactose, only glucose levels are regulated via insulin and glucagon.