blood sugar

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. Blood glucose concentration in healthy people, after an overnight fast, will normally be between 3.5 and 5.5 mmol/litre and this is referred to as euglycaemia, or normal blood glucose. With more prolonged fasting it can go lower than 3.5, and in some individuals it can exceed 6 mmol/litre. A person would be diagnosed as having diabetes if their blood glucose after an overnight fast exceeded 7.0 mmol/litre: this is hyperglycaemia, an abnormally high blood glucose.

When we consume food or drink containing carbohydrates, most of this will be either simple glucose (a monosaccharide); sucrose (a disaccharide which contains equal amounts of glucose and fructose); or starch (which is a polysaccharide — a polymer of glucose). 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. 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.

When blood glucose concentration is normal, the glucose which is filtered from the blood in the kidneys is reabsorbed back into the bloodstream 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. Glucose is 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, which are products of fat breakdown, but under normal circumstances the adult human brain needs approximately 6 g 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 (known as 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.

In some circumstances the rate of use of blood glucose exceeds the rate at which it is released from the liver, and blood glucose concentration falls. When blood glucose falls below 3.5 mmol/litre, the condition of hypoglycaemia is beginning to develop. This condition occurs quite commonly in people with diabetes who are treated with insulin injections, but much less so in non-diabetics. However, hypoglycaemia can occur in healthy people if they undertake prolonged periods of quite high intensity exercise, such as ultra-distance running or cycling, without consuming carbohydrate. Another cause of hypoglycaemia in non-diabetic people is the consumption of about 50 g or more of alcohol (about 5–6 units) after either 24–36 hours of starvation or 2–3 hours of exercise to exhaustion. Starvation or exhaustive exercise will have caused liver glycogen to be depleted, so the liver needs to synthesize glucose to maintain the supply to the brain; but alcohol prevents the liver from performing this synthesis, causing blood glucose to fall and hypoglycaemia to develop.

When blood glucose is at hypoglycaemic levels, there are a number of characteristic effects on the brain. Reactions are slowed, the person has difficulty in concentrating and can feel light-headed, vision may be disturbed, and hunger is common. The autonomic nervous system reaction to hypoglycaemia causes sweating and trembling, and the person often becomes aware that their heart is beating more rapidly (they describe having palpitations). With mild degrees of hypoglycaemia (blood glucose between 3.5 and 3.0 mmol/litre) most people would be unaware of anything untoward occurring, although sensitive measurements of brain function would detect a slowing of reactions. As blood glucose falls further, the effects become more noticeable, but provided the symptoms are used to prompt the consumption of carbohydrate, these effects are rapidly reversed. If blood glucose falls to very low levels, unconsciousness can occur, but this is extremely rare, except for people with insulin-treated diabetes.

I. A. Macdonald

See also insulin; metabolism; starvation; sugars.