Proteins are macromolecules made of one or more chains of amino acids. They have many functions, and basically they are the buildings blocks of our body. Let’s see why.

Proteins are essential nutrients, as they provide the amino acids used by the body to synthesises proteins for several vital roles:

• structural (skeleton, skin, fabrics and supporting tissues, cells)
• protective (barriers, immune system, anti-inflammatory)
• transportation and communication (plasma proteins, hormones, membrane receptors)
• enzyme (digestion, metabolism, homeostasis, synthesis)
• energy (energy source)

The amino acids necessary for the synthesis of proteins useful to man are 20, but they are not all the same: 8 of these are considered essential because the body cannot produce them and they must necessarily be taken with food (Figure 2).

Furthermore, it is essential to remember that each protein synthesised by the body has a composition of specific amino acids and when it is synthesised it requires the presence of all the amino acids that it is composed of: if even one of these is deficient, the synthesis of the protein is limited. In truth there is no specific dietary requirement for proteins, but these should be taken so as to provide all the amino acids necessary for synthesis by the body. In children amino acids such as cysteine, taurine, tyrosine, histidine and arginine are considered semi-essential, in that not all the synthesis mechanisms are fully developed.

On the basis of the characterisation of protein amino acids it is therefore possible to identify which foods have high biological protein value, and are therefore able to provide all essential amino acids.

Among the essential amino acids, methionine plays a key role in the growth of the individual. The proteins of plants are in general poor in sulphur amino acids such as methionine, on average 0.6 g/100 g of protein, while red meat, poultry and fish contain between 1 and 1.26 g of sulphur amino acids/100 g of protein.

More generally, plant proteins are considered to be of lower quality as they are unbalanced in the relationship between cysteine and methionine necessary for the growth, which should be in favour of methionine. From the total number of amino acids containing sulphur, red meat, poultry and fish have 30-40% of cysteine and 60-70% of methionine, while soy, beans, peas and lentils have 60% cysteine and 40% of methionine.

Another protein evaluation method recently developed by the scientific world is the DIAAS (Digestible Indispensable Amino Acid Score), which defines with a numeric index the protein quality of certain foods: the higher the value, the better the protein quality (Figure 4).

If it is clear therefore that the nutritional value of animal protein is high, it is also interesting to assess the protein content of food compared to the portion recommended by the new RDAs with respect to the caloric intake.

Fish and meat have the highest protein caloric efficiency (Protein/Kcal*100), i.e. per portion they bring a higher proportion of high quality protein, but with a reduced caloric intake (Figure 5): a remarkable advantage in terms of overweight and obesity prevention.

The vegetable proteins are often associated with a low content of saturated fat and are therefore recommended as an alternative to animal proteins. But if we wanted to cover our protein needs using only plant-based foods, we would have to assume between 3 to 5 times more calories than the calories obtained from foods of animal origin, particularly from lean cuts of meat or fish.

The Sustainable Meat Project