The last macronutrients to be considered are proteins, also known as protides. Proteins, in truth, have mainly a plastic and transport role within the human organism, while the energy role is only accessory. Proteins, in fact, serve more as raw material for the construction of particular biological tissues, such as muscles, or as vehicles for transport within the blood. Transferrin is a prime example of the latter function of proteins. It is a protein that has the function of binding to iron and transporting it within the blood. Iron, in fact, cannot circulate in free form in the blood, but must necessarily be bound to a transport protein. Perhaps due to the fact that they do not have a great influence on the energy metabolism of the human body, it is very difficult to gain weight due to a surplus of protein. When more protein is consumed than the body needs, in fact, the excess proteins are burnt immediately. Only in the case of a glucose deficit does the body, via gluconeogenesis, convert them into the latter sugar. Some say that consuming too much protein implies that it is first converted into glucose and then the latter is converted into storage fat. In reality, although this sometimes happens, it happens very rarely. Although, as mentioned, proteins have above all a plastic and transport role, they can also sometimes be used as an energy source. We have already discovered, in fact, that proteins, like carbohydrates, provide an average of 4kcal per gram. As far as the influence of this macronutrient on body composition is concerned, it can be said that it is predominantly pro-slimming and pro-muscle mass gain. The protein intake recommended for athletes seeking an increase in muscle mass is higher than that recommended for those with different needs. If, however, a certain threshold is exceeded, the influence of protein on fitness may change.

Biological value of proteins and their classification

  Now, before delving into the discussion of the influence that proteins have on body composition and, more generally, on the athlete’s performance capacity, a premise is in order. It must in fact be made clear that some proteins are better suited than others to fulfil their biological role. This is due to their constitutive structure. More specifically, proteins are made up of several units called amino acids (or amino acids). Well, these are not all the same. In nature, there are (at least) twenty proteinogenic amino acids, which can follow one another within the protein structure in the most varied ways, even giving rise to different geometric shapes. Now, of these, some can be produced by the body itself from precursors (i.e. other amino acids). In other words, the human body is able to take an amino acid that has been supplied to it through food and transform it into another amino acid that it needs. These latter types of amino acids are called non-essential. Then there are amino acids that, unlike the non-essential ones, must necessarily be taken from the diet because, although the human body needs them, it cannot produce them on its own. This is why these amino acids are called, precisely, essential. Finally, there are conditionally essential amino acids, i.e. they can be produced by the body on its own, but not in sufficient quantities to meet its needs. It is therefore clear that the protein sources to be preferred are those that contain mainly essential and conditionally essential amino acids. Precisely according to the amount of essential amino acids contained in proteins, these can be classified into:

• high biological value proteins: those that contain copious amounts of essential and conditionally essential amino acids;
• proteins of low biological value: those that, on the contrary, contain mainly non-essential amino acids.

Foods that contain proteins of low biological value, if not supplemented by those that contain qualitatively better ones, can also be supplemented by others whose proteins have the same low biological value, but with a complementary amino acid profile. In other words, foods whose proteins lack certain amino acids can be supplemented in the diet (not necessarily in the context of the same meal) with others that instead contain them, despite being deficient in those contained in the former. The most typical example of this type of association is found in common culinary mixtures between cereals and legumes (e.g. pasta and beans, rice and peas…).

The influence of proteins on DOMS

  Many athletes, particularly those practising bodybuilding, are subject to a particular type of muscular pain that appears later than the application of the training load: these are the so-called DOMS (acronym for Delayed Onset Muscle Soreness). These symptoms are due to the physiological muscle damage caused by exertion, which involves the release of calcium ions (Ca⁺ ) normally found in the intracellular environment and their invasion of the space outside the cells. Here they are usually enclosed within semi-permeable vesicles that progressively release them. When the concentration of calcium ions in the extracellular environment exceeds a certain threshold, one begins to experience those typical muscular pains that are often wrongly attributed to an accumulation of lactic acid. This normally occurs 24-48 hours after the load is applied. Well, in this respect, studies (Flakoll, 2004; Millard-Stafford, 2005; Shimomura, 2006) have proven that regular protein intake can reduce the extent of DOMS.

The influence of proteins on insulin

  Regarding the influence of protein on hormones, it is good to start by clarifying that, apart from carbohydrates, protein food intake also stimulates the endocrine pancreas to produce insulin, although much less than carbohydrates do. In fact, even a protein meal causes insulin spikes to be produced. This leads to the same problems and benefits concerning insulin secretion as seen in the section on carbohydrates, only in this case, they are of much reduced magnitude. Focusing, however, on the relationship of protein with insulin resistance, it is possible to say that protein usually reduces insulin resistance. In fact, there is evidence that a high-protein diet (i.e. a diet high in protein) promotes an increase in the amount of GLUT-4 receptors on muscle cells and a decrease in the same on adipocytes.

Proteins and testosterone

  Protein, however, also has its dark side: too much protein in the diet can cause a reduction in testosterone levels (the male sex hormone par excellence), which can sometimes result in fitness problems, especially for men. The reason for this influence of proteins on testosterone lies in one of its functions. It was mentioned earlier that proteins are made up of amino acids. Now, amino acids, like, indeed, proteins, mostly have a plastic function, but there is a class of amino acids that has mainly a trophic (i.e. nourishing) function: the branched amino acids, also known as BCAAs (acronym, in fact, for Branched Chains Amino Acids). They are transported from the blood stream into the cells, so that the cells are nourished by testosterone. But if there are a lot of BCAAs in the bloodstream, due to a high-protein diet, the need for a vehicle to carry them into the cells is lost (at least in part). Hence, the body reduces testosterone production. As mentioned, this is not beneficial for physical fitness: levels of strength and muscle mass, in fact, are closely dependent on the amount of testosterone. This, in fact, is the reason why men generally possess higher levels of strength and muscle mass than women. A reduction in strength will inevitably result in a worsening of all intensity athletic performance (power and speed). Moreover, albeit indirectly, a drop in testosterone levels can also adversely affect extensive (endurance) performance: the male hormone par excellence, in fact, is responsible, among other things, for stimulating the production of another hormone, called erythropoietin. The latter in turn is responsible for the production of red blood cells, i.e. those cells that carry oxygen through the blood to nourish cells. Well, a reduction in the quantity of red blood corpuscles will result in a reduced supply of oxygen for the peripheral cells, which will no longer have one of the essential raw materials for energy production through aerobic metabolism (that, precisely, which uses oxygen), leading to a worsening of extensive performance. To sum up, then, it is a chain reaction whereby:

1. an excess of protein leads to a lower production of testosterone;
2. the drop in testosterone levels leads to a drop in erythropoietin levels as well;
3. the decrease in erythropoietin levels leads to a reduction in the amount of red blood cells;
4. a lower amount of red blood cells implies a lower availability of oxygen for peripheral cells, which, as a consequence, will produce less energy during extensive performance.

Proteins and IGF-1

  As is well known, testosterone is an anabolic hormone, so a deficiency of it necessarily implies low muscle mass. Despite the deleterious effect that protein can sometimes have on testosterone, it should nevertheless tend to be considered a nutrient that promotes anabolism of muscle and bone mass (Cayol, 1997). This is due to its influence on insulin-like growth factor 1, or IGF-1 (Dawson; Hughs, 2004). This, in fact, is, like testosterone, an anabolic hormone, and since protein has a stimulating function in the production of this hormone, protide intake will, in this way, promote the addition of new muscle and bone mass.

Proteins and blood pH

  Another sore point about protein is its deleterious influence on blood pH. The pH is the concentration of free protons (hydrogenions) in the blood (remember that protons are those subatomic particles with a positive charge). More in detail, as we have already seen, all the functions of the human organism must constantly be kept in equilibrium (homeostasis): blood glucose cannot be too high or too low; body temperature must always be around 37°C, without rising or falling; blood pressure must be neither too high nor too low, etc. Among all these parameters that must necessarily be kept in balance, on pain of loss of health, there is also the pH of the blood. Well, excessive protein consumption can shift the body’s delicate balance of blood pH towards acidity. As is often the case, our organism can rely on defence mechanisms to protect itself. In this case, one of the defence mechanisms consists of taking alkaline buffers (calcium ions) from muscle fibres and bone tissue to be poured into the blood in order to balance the acidity caused by protein excess. Now, the removal of these buffers from the muscles presupposes that the muscle fibres are destroyed and this inevitably results in loss of muscle mass. And while this will lead to a loss of strength and, consequently, a reduction in the motor skills associated with it, it will also cause potential further fattening. As seen before, in fact, myocytes compete with adipocytes for the uptake of fatty acids and glucose from the blood. The more muscle mass one has, the less likely one is to gain weight, and vice versa. As for bone mass, this too is destroyed by taking alkaline buffers, which has a negative impact on both health and fitness. The term osteoporosis should already be enough to warn us. In truth, the deleterious effect of proteins on muscle and bone mass is rather controversial. Many believe that the acidifying effect of proteins on the blood does not imply the destruction of muscle tissue. In fact, there are other mechanisms the body uses to get rid of excess acidity, mechanisms that do not involve cellular damage. I believe the truth lies somewhere in between: the human body is only able to buffer excess acidity without destroying muscle and bone within certain limits. If the protein intake exceeds a certain limit (a very high one that is difficult to reach with a normal diet), then muscle and bone mass loss occurs. There is a known disease, and this is not an opinion but a fact, called rabbit starvation, which affected the first explorers who ventured into the American continent. These men fed themselves for many days solely on rabbit meat, which is known to be very lean and high in protein. Well, this disease led them, as a result of the excess blood acidity, first to a progressive loss of muscle and bone mass, and then to death. But this should not frighten: as mentioned, it is highly unlikely that a normal diet could lead someone to become ill in this way.