Diabetes Handbook Online

How the Body Uses Sugar

The human body eats to provide itself with fuel for maintaining all its functions, for growth and for energy. The three main parts of a human's diet are protein (such as meat), fat (such as butter) and carbohydrate (such as sugar or any starch that comes from potatoes, bread or cereals). When there is a defect in any of he body's capabilities of absorbing, and using, any of these three main constituents of food, then the effect of the disorder will be to alter the absorption of the others. Thus a protein-absorption abnormally affects the body's use of fats, a fat intolerance affects the carbohydrates, and diabetes, which is a carbohydrate-absorption abnormally, affects both the others. The human body cannot live and function on one type of foodstuff alone. The process of using up food in the body is called metabolism (from the Greek for 'a process of change') and fundamentally what is involved is the change or breakdown of the chemical constituents of food into energy and then the getting rid of waste by excretion as water, solids or carbon dioxide.

Carbohydrate in the digestive system bread, potatoes, cereal, sugar, cakes, sweets and other sources is first broken down into glucose by the digestive juices of the mouth (saliva), and then those of the stomach and the intestines. The glucose circulates in the blood and is carried throughout the body to every cell, from the brain to the growing toenail. Within two to three hours of the high blood-sugar level that develops after a meal, the excess of blood sugar is taken up by the cells and chemically changed once more. (Insulin is necessary for this transfer of glucose from blood to cell, but more of that later.) In the cell, the glucose is chemically altered by means of special substances called enzymes to either give energy or to be stored as glycogen. Burning up the glucose as a fuel for the cells energy releases water and carbon dioxide and these are later carried away by the bloodstream to be excreted via the kidney or the exhaled breath. Glycogen is the special form of converted glucose that can be used at a later time by the cell when its energy requirements increase. Different cells in different parts of the body use the blood glucose for different purposes. For example, the brain and nervous system cells cannot store it (and so use it immediately and constantly), nor can the special muscle cells of the heart. In the liver, by contrast, nearly all the circulating blood sugar delivered to it is converted into storage as glycogen or else into fat.

Carbohydrate metabolism

As everyone recognizes, fat is a long-term storage material, put away for use in times of deprivation. When glucose reaches the liver, therefore, it is altered again by the use of special enzymes to what are known as fatty acids, chemically called triglycerides. These re-circulate in the blood as fat in the abdomen, below the skin, in the breasts, thighs and buttocks. Thus anyone who overeats, or takes in more carbohydrate in their diet than their body cells need, gets fat. Similarly, if the body is deprived of energy from carbohydrate, or needs more that it can get in the diet, the body fat is broken down and the cycle of glucose metabolism is reversed. The fat becomes a fatty acid (this time a ketone), the liver converts the ketone back to glucose and the cell then absorbs the glucose. Therefore, in starvation or illness, the body loses its fat stores, and its weight and becomes thin.

In order to understand what happens in diabetes, it is important to remember that this cycle of change backwards and forwards is occurring all the time in the healthy human being. The body cannot withstand long periods without food; in general, death will occur from starvation in a matter of weeks regardless of the size of the person. Similarly, in even short periods of high energy requirement and reduced food intake the body will rapidly break down its stores of fat to ensure that a blood-glucose level is maintained for brain and heart-muscle function.

Ketones in the bloodstream the sign of fat being broken down appear very rapidly in illness, and if they are at an excessive level they will be excreted by the kidney, too. Ketones appear as acetone in the urine of the woman in labour an indication of how the physical effort she is making to give birth to her baby causes her body to mobilize all its energy sources. Ketones are detectable as a sweet scent (or nail-varnish type of smell) on the breath of the sick child and, in particular, they are obvious in the breath of badly controlled diabetics.


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