Proteins are elements that play a part in building our cells. They are our main source of energy, necessary for all kinds of metabolic processes. Our skin, muscles, internal organs, hormones, enzymes, genes and all sorts of tissues are made of proteins.
Because of the major role they play, a diet deprived of proteins is likely to cause problems and various diseases. Food supplies us with animal proteins (meats, fishes, eggs and milk products) and proteins of vegetable origin (dried vegetables, cereals and soya).

A protein is made up from a chain of amino-acids. During digestion, the links between these are broken by enzymes, which results in a liberation of amino-acids. All of these are required for our organism to synthesize its own proteins.
Some amino-acids can be produced by the organism, but others can only be supplied by food. The latter are called 'essential amino-acids'. This notion of 'essential amino-acids' establishes the notion of 'biological value' of a protein.
Thus some vegetal proteins have a limited value, i.e. a poor fraction of amino-acids. It is therefore necessary to associate different foodstuffs in one's diet in order to obtain a good biological protein.
Examples of associations

Concerning animal proteins, they have a good biological value, and they are often found in the giblets or offals or meat.

Energetic equivalence of proteins: 1 g of protein = 4 calories = 17 KJ.

Food-fats or lipids are essential parts of our body. Half of our brain for instance is made of lipids. However, the need for lipids is not great.
Lipids are useful as a reserve of energy in case of sustained deficiency. Lipids bring us also vitamins A, E and K.
The sources for lipids are either animal (eggs, meats, fishes, cold cuts, butter, cream and cheese) or vegetal (oils, margarine, oleaginous fruits).
Lipids are made of fatty-acids. We can distinguish the saturated fatty-acids and the unsaturated fatty-acids.

Energetic value of lipids: 1 g of lipid = 9 calories = 38 KJ

Glucides play essentially an energetic role. They are our main source of energy through the provision of glucose, which is the fuel for all cell-types in the human body. For example, glucides are the only source of energy which the brain can utilize, as it is unable to store energy.
Sources for glucose in food are mainly of vegetable origin : leguminous plants, cereals, potatoes (starch), and also fruits, vegetables, sugar beet, and sugar cane. A small fraction of glucides comes from the animal world : milk products and liver (glycogen).
We can separate :

Energetic value of glucides: 1g of glucose = 4 calories = 17 KJ

Fibres are substances that stem from plants and bypass the digestion in the small bowel of a healthy human being. There are two types de fibres : those that can be broken down by the colic flora, and the insoluble fibres, that are hardly, if at all broken down. They stem from the vegetal terrestrial world (cereals, leguminous, fruits and vegetables), and from the see-world (seaweeds).
The feeding fibres play an important part in the nutritional balance and the metabolism : regulation of the intestinal transit, control on food-intake via their effect on satiety.

Energetic value of fibres: 1 g of fibres = 0 calorie

Minerals are not a source of energy, but they are incorporated in cell-structures (cell-membranes, bones, etc.). The only source for the body is from food, because the organism cannot create them by itself. Besides, minerals are necessary for the activity of enzymes and hormones. The main minerals and their effects are described below.

Very small amounts are present inside our body, but they play a major role in various metabolic reactions. The most significant are Iron, Zinc, Copper, Chromium, Selenium, Iodine, Manganesium and Cobalt.

They are necessary for a healthy life. A varying and balanced food supplies us with our needs for vitamins. Like minerals and oligo-elements, vitamins do not have a nutritional value. They are active molecules required in very small amounts, but a deficiency can cause severe diseases or dysfunctions.
Vitamins are separated in two categories :

One has to drink an average of 1.5 litres of water a day. Water is the only liquid essential to the body. This is important to ensure the hydration of our cells and the elimination of wastes through the kidney.
If you find it hard to drink those 1.5 litres, here is a trick that will help to cover your daily needs:

When you are following a diet, it is important to avoid weariness. Equivalences may help to alternate foods. Thus food-intake can vary according to individual tastes.

For example, one can replace:

- A pear by an apricot purée (without added sugar)
- A chicken leg by a fish filet