Digestion of sugars

The large molecules of the polysaccharides (starches and cellulose) present in foods are too big for us to digest directly. Our bodies hydrolyse (i.e. action of water in the presence of an enzyme) the starches and break them up into smaller molecules, e.g. glucose. Our systems cannot hydrolyse the complicated long chains of sugar units in cellulose, but those of grazing animals can. The small glucose molecules are then small enough to be absorbed by the villi on the surface of the small intestine and carried to the liver. The glucose can enter the blood system to be circulated and be readily available to the cells for energy release, for example at muscle endings, as required.
(1) If the body cells require instant energy (as in exercise), the glucose is supplied with oxygen from the blood and is immediately oxidized on the instructions of a facilitating ADP (adenosine di-phosphate) molecule to release energy: C6H12O6 þ 6O2 ! 6CO2 þ 6H2O þ energy One gram of carbohydrate gives 17 kJ of energy in the process of respiration. The carbon dioxide is removed from muscle endings by the blood and is transported into the lungs for exhalation.


(2) If our body does not need immediate energy the sugars and glucose are converted into a long chain molecule called glycogen in the liver for storage. Some is also stored in the muscles, brain and blood in readiness for instant use. Quick energy release is facilitated by insulin supplied from the pancreas. If the liver glycogen is used up and its quantities run low, our system says it is time to eat again. If we do not eat, the cells start to catabolize (break down) some of the body fats and proteins. Only if we are starving, or eat a series of low-carbohydrate meals, does this begin to have an adverse effect upon the body. Some slimming schemes use this method but they have to be carefully controlled to prevent permanent damage. Long-distance and marathon runners take in high-carbohydrate meals a day or so before the race. This gives them enough energy to prevent breakdown of body proteins.
(3) Glucose can also be used by the cells to synthesize amino acids, which in turn can be linked up to form proteins and are stored.
(4) If all the storage areas of glucose are full, it is used to make fats which can be stored in many places all over the body. Thus we can get ‘fat’ through eating too many sweet things.
(5) Sometimes when there is excess glucose in a diet it can be excreted in the urine. People with diabetes also have high blood glucose levels and often eliminate the excess glucose into the urine. The urine test is often used as the initial test to see if a person is a diabetic, along with a smell of rotten apples on their breath. This is followed up by other more accurate and specific tests.