Macronutrients are those nutrients that provide energy for growth and to maintain metabolism.
They are divided in carbohydrates, proteins and fat.
Carbohydrates consists in carbon, hydrogen and oxygen atoms, according to the empirical formula (CH2O)n.
They comes from fruits, cereals, vegetables and tree (leaves, bark…). Plants are able to produce them combining water and carbon through chlorophyll photosynthesis.
According to the classical partition, they start from singular groups of particles, called monosaccharides (eg. fructose), that, combining each other, form disaccharides, oligosaccharides, and polysaccharides.
Usually carbohydrates are stored under sugar, starch of fiber form. The latter is not digestible by digestive enzymes but it’s very useful for maintaining a healthy digestive system by easing defecation and lowering the serum cholesterol.
They are stored in the liver and in the muscles in the form of glycogen and they are primarly a fuel for the human body, then a protein sparer (without them, the body will start to producee energy burning lean tissues), the main source of brain fuel and the main substrate of fat oxidation (without CHO, body will start to burn more fat, in a “dirty” way).
Liver is the main “tank” of glycogen, and, as example, after 1h of high intensity exercise, about 55% of stocks will be burned.
A sedentary person should have an intake of 40-50% of CHO per day in calories, amount to adjust at 60-70% for highly trained athletes.
Fiber should be 20-40g a day

Fat are biomolecules that are soluble in nonpolar solvents. They are categorized into several classes (Fatty acids, Glycerolipids, ecc…) and conduct a lot of important function, such as cells membranes, energy storage, signaling and carrier for fat-soluble vitamins.
They provide 9 kcal/g and they are jolly useful to supply energy to the body respecting other nutritional constraints. In this sense, in a diet the percentage of lipids must be seen as a consequence of other constraints, not a constraint to be respected.

Proteins are very complex organic substances and they are abundantly present in living organisms, representing more than half of the dry weight of animals and bacteria.\
Proteins are present in foods of animal origin (meat, milk, eggs, fish, etc.) and in those of vegetable origin (cereals, legumes etc.).
The animals construct the proteins necessary to the organism by transforming the proteins taken with the food into amino acids, and then recombine them in the proteins necessary following the information provided by their DNA, in protein synthesis.
Unlike what happens with carbohydrates and lipids, the body does not store protein stocks. About 65% of the protein content is present in the muscles.
On the sedentary protein requirement there is a remarkable agreement: for an adult about 0.83 g of protein per kg of body weight.
Things change when a correction is introduced for sports or work.
The most striking research is that of Tarnopolski from 1988. Collecting the expelled nitrogen with urine, faeces and sweat, it is possible to calculate the balance of the protein balance; the research examined three groups, sedentary, body builders and athletes. The result was that for body builders, the correct supplement to maintain balance had to be 1.2 g per kg of weight, while for bottom athletes it was 1.6 g.
iI can be estimated that proteins provide on average about 4 kcal / g.