We do not // can not digest cellulose. This is a carbohydrate used by plants to make their cell walls. It is also called roughage. If you do not eat foods materials which contain fibre you might end up with problems of the colon and rectum. The muscles of you digestive system mix food with the digestive juices and push food along the intestines by peristalsis; if there is no fibre in your diet these movements cannot work properly.
You must have carbohydrate, protein, fat, vitamins, minerals salts and fibre in the correct proportions. If there is not enough protein, you will not be able to grow properly and you will not be able to repair yourself i.e. wounds will not heal properly. If you do not have enough energy containing foods you will feel very tired, you will not have enough energy. If you have too much energy containing foods you will become overweight. If you think that you are overweight you might try taking more exercise to "burn off" some of the excess food which you ate at you last meal.
Vitamins are only required in very small quantities. There is no chemical similarity between these chemicals; the similarity between them is entirely biological.
Vitamin A: good for your eyes.
Vitamin B: about 12 different chemicals.
Vitamin C: needed for your body to repair itself.
Vitamin D: can be made in your skin, needed for absorption of Calcium.
Vitamin E: the nice one - reproduction?
Like carbohydrates, fats contain the elements Carbon, Hydrogen and Oxygen. Fats are used as a source of energy: they are also stored beneath the skin helping to insulate us against the cold. Do not think that by avoiding fat in your diet you will stay thin and elegant! If you eat too much carbohydrate and protein, you will convert some of it into fat, so you will put on weight. You must balance the amount of energy containing foods with the amount of energy that you use when you take exercise. You must have some fat in your diet because it contains fat soluble vitamins.
Carbohydrates are the most important source of energy. They contain the elements Carbon, Hydrogen and Oxygen. The first part of the name "carbo-" means that they contain Carbon. The second part of the name "-hydr-" means that they contain Hydrogen. The third part of the name "-ate-" means that they contain Oxygen. In all carbohydrates the ratio of Hydrogen atoms to Oxygen atoms is 2:1 just like water.
Proteins are required for growth and repair. Proteins contain Carbon, Hydrogen, Oxygen, Nitrogen and sometimes Sulphur. Proteins are very large molecules, so they cannot get directly into our blood; they must be turned into amino-acids by the digestive system. There are over 20 different amino-acids. Our bodies can turn the amino-acids back into protein. When our cells do this they have to put the amino-acids together in the correct order. There are many millions of possible combinations or sequences of amino-acids; it is our DNA which contains the information about how to make proteins. Our cells get their amino-acids from the blood.
These are also needed in small quantities, but we need more of these than we need of vitamins.
Iron: required to make haemoglobin.
Calcium: required for healthy teeth, bones and muscles.
Sodium: all cells need this, especially nerve cells.
Iodine: used to make a hormone called thyroxin.
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