Nutrition - Proteins

Comments · 506 Views

Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue, and can also serve as a fuel source. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition. Proteins differ from one another primarily in their sequence of amino acids.


Proteins are essential nutrients for the human body.

Proteins are large biomolecules consisting of one or more long chains of amino acids.

Amino acids are classed in two main categories which are; essential and nonessential amino acids.

Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue, and can also serve as a fuel source. As a fuel, proteins provide as much energy density as carbohydrates: 4 kcal (17 kJ) per gram; in contrast, lipids provide 9 kcal (37 kJ) per gram. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition.

Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acids. Proteins are among the most abundant organic molecules in living systems and are way more diverse in structure and function than other classes of macromolecules. A single cell can contain thousands of proteins, each with a unique function.

Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific three-dimensional structure that determines its activity.

Once formed, proteins only exist for a certain period of time and are then degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan is measured in terms of its half-life and covers a wide range. They can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable.

Although protein structures, like their functions, vary greatly, all proteins are made up of one or more chains of amino acids. In nutrition, these amino acids are classed in two main categories which are; essential and nonessential amino acids.

Essential Amino Acids:

An essential amino acid is an amino acid that cannot be synthesized by the organism, and thus must be supplied in its diet. The nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.

Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress. These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine.

Nonessential Amino Acids:

Five amino acids are dispensable in humans, meaning they can be synthesized in the body and thus deemed nonessential in the human diet. These five are alanine, aspartic acid, asparagine, glutamic acid and serine.

Proteins are the chief actors within the cell, they carry out the duties specified by the information encoded in genes.

The best-known role of proteins in the cell is as enzymes.

Many proteins are involved in the process of cell signaling and signal transduction.

Structural proteins confer stiffness and rigidity to otherwise-fluid biological components.


Proteins are the chief actors within the cell, they carry out the duties specified by the information encoded in genes. With the exception of certain types of RNA, most other biological molecules are relatively inert elements upon which proteins act. Proteins make up half the dry weight of an Escherichia coli cell, whereas other macromolecules such as DNA and RNA make up only 3% and 20%, respectively. The set of proteins expressed in a particular cell or cell type is known as its proteome.

The chief characteristic of proteins that also allows their diverse set of functions is their ability to bind other molecules specifically and tightly. The region of the protein responsible for binding another molecule is known as the binding site and is often a depression or "pocket" on the molecular surface. This binding ability is mediated by the tertiary structure of the protein, which defines the binding site pocket, and by the chemical properties of the surrounding amino acids' side chains. Extremely minor chemical changes such as the addition of a single methyl group to a binding partner can sometimes suffice to nearly eliminate binding; for example, the aminoacyl tRNA synthetase specific to the amino acid valine discriminates against the very similar side chain of the amino acid isoleucine.

Proteins can bind to other proteins as well as to small-molecule substrates. When proteins bind specifically to other copies of the same molecule, they can oligomerize to form fibrils; this process occurs often in structural proteins that consist of globular monomers that self-associate to form rigid fibers. Protein–protein interactions also regulate enzymatic activity, control progression through the cell cycle, and allow the assembly of large protein complexes that carry out many closely related reactions with a common biological function. Proteins can also bind to, or even be integrated into, cell membranes. The ability of binding partners to induce conformational changes in proteins allows the construction of enormously complex signaling networks. As interactions between proteins are reversible, and depend heavily on the availability of different groups of partner proteins to form aggregates that are capable to carry out discrete sets of function, study of the interactions between specific proteins is a key to understand important aspects of cellular function, and ultimately the properties that distinguish particular cell types.

The best-known role of proteins in the cell is as enzymes, which catalyze chemical reactions. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Enzymes carry out most of the reactions involved in metabolism, as well as manipulating DNA in processes such as DNA replication, DNA repair, and transcription. Some enzymes act on other proteins to add or remove chemical groups in a process known as posttranslational modification. About 4,000 reactions are known to be catalyzed by enzymes. The rate acceleration conferred by enzymatic catalysis is often enormous—as much as 1017-fold increase in rate over the unanalyzed reaction in the case of orotate decarboxylase (78 million years without the enzyme, 18 milliseconds with the enzyme).

The molecules bound and acted upon by enzymes are called substrates. Although enzymes can consist of hundreds of amino acids, it is usually only a small fraction of the residues that come in contact with the substrate, and an even smaller fraction—three to four residues on average—that are directly involved in catalysis. The region of the enzyme that binds the substrate and contains the catalytic residues is known as the active site.

Dirigent proteins are members of a class of proteins that dictate the stereochemistry of a compound synthesized by other enzymes.

Cell signaling and ligand binding
Many proteins are involved in the process of cell signaling and signal transduction. Some proteins, such as insulin, are extracellular proteins that transmit a signal from the cell in which they were synthesized to other cells in distant tissues. Others are membrane proteins that act as receptors whose main function is to bind a signaling molecule and induce a biochemical response in the cell. Many receptors have a binding site exposed on the cell surface and an effector domain within the cell, which may have enzymatic activity or may undergo a conformational change detected by other proteins within the cell.

Antibodies are protein components of an adaptive immune system whose main function is to bind antigens, or foreign substances in the body, and target them for destruction. Antibodies can be secreted into the extracellular environment or anchored in the membranes of specialized B cells known as plasma cells. Whereas enzymes are limited in their binding affinity for their substrates by the necessity of conducting their reaction, antibodies have no such constraints. An antibody's binding affinity to its target is extraordinarily high.

Many ligand transport proteins bind particular small biomolecules and transport them to other locations in the body of a multicellular organism. These proteins must have a high binding affinity when their ligand is present in high concentrations, but must also release the ligand when it is present at low concentrations in the target tissues. The canonical example of a ligand-binding protein is hemoglobin, which transports oxygen from the lungs to other organs and tissues in all vertebrates and has close homologs in every biological kingdom. Lectins are sugar-binding proteins which are highly specific for their sugar moieties. Lectins typically play a role in biological recognition phenomena involving cells and proteins. Receptors and hormones are highly specific binding proteins.

Transmembrane proteins can also serve as ligand transport proteins that alter the permeability of the cell membrane to small molecules and ions. The membrane alone has a hydrophobic core through which polar or charged molecules cannot diffuse. Membrane proteins contain internal channels that allow such molecules to enter and exit the cell. Many ion channel proteins are specialized to select for only a particular ion; for example, potassium and sodium channels often discriminate for only one of the two ions.

Structural proteins
Structural proteins confer stiffness and rigidity to otherwise-fluid biological components. Most structural proteins are fibrous proteins; for example, collagen and elastin are critical components of connective tissue such as cartilage, and keratin is found in hard or filamentous structures such as hair, nails, feathers, hooves, and some animal shells. Some globular proteins can also play structural functions, for example, actin and tubulin are globular and soluble as monomers, but polymerize to form long, stiff fibers that make up the cytoskeleton, which allows the cell to maintain its shape and size.

Other proteins that serve structural functions are motor proteins such as myosin, kinesin, and dynein, which are capable of generating mechanical forces. These proteins are crucial for cellular motility of single celled organisms and the sperm of many multicellular organisms which reproduce sexually. They also generate the forces exerted by contracting muscles and play essential roles in intracellular transport.

Vegetarian sources of proteins include, legumes, nuts (above), and seeds among others.

Meat (above), products from milk, eggs, soy, and fish are sources of complete protein.

Chicken is a good source of protein for those avoiding red meats.

Fish is another rich source of proteins.

Diary products are good sources of complete proteins.


Protein can be found in a wide range of food. The best combination of protein sources depends on the region of the world, access, cost, amino acid types and nutrition balance, as well as acquired tastes. Some foods are high in certain amino acids, but their digestibility and the anti-nutritional factors present in these foods make them of limited value in human nutrition. Therefore, one must consider digestibility and secondary nutrition profile such as calories, cholesterol, vitamins and essential mineral density of the protein source. On a worldwide basis, plant protein foods contribute over 60 percent of the per capita supply of protein, on average. Meat, products from milk, eggs, soy, and fish are sources of complete protein.

Whole grains and cereals are another source of proteins. However, these tend to be limiting in the amino acid lysine or threonine, which are available in other vegetarian sources and meats. Examples of food staples and cereal sources of protein, each with a concentration greater than 7 percent, are (in no particular order) buckwheat, oats, rye, millet, maize (corn), rice, wheat, sorghum, amaranth, and quinoa.

Vegetarian sources of proteins include legumes, nuts, seeds and fruits. Legumes, some of which are called pulses in certain parts of the world, have higher concentrations of amino acids and are more complete sources of protein than whole grains and cereals. Examples of vegetarian foods with protein concentrations greater than 7 percent include soybeans, lentils, kidney beans, white beans, mung beans, chickpeas, cowpeas, lima beans, pigeon peas, lupines, wing beans, almonds, Brazil nuts, cashews, pecans, walnuts, cotton seeds, pumpkin seeds, hemp seeds, sesame seeds, and sunflower seeds.

Food staples that are poor sources of protein include roots and tubers such as yams, cassava and sweet potato. Plantains, another major staple, are also a poor source of essential amino acids. Fruits, while rich in other essential nutrients, are another poor source of amino acids. The protein content in roots, tubers and fruits is between 0 and 2 percent. Food staples with low protein content must be complemented with foods with complete, quality protein content for a healthy life, particularly in children for proper development.

A good source of protein is often a combination of various foods, because different foods are rich in different amino acids. A good source of dietary protein meets two requirements:

• The requirement for the nutritionally indispensable amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) under all conditions and for conditionally indispensable amino acids (cysteine, tyrosine, taurine, glycine, arginine, glutamine, proline) under specific physiological and pathological conditions

• The requirement for nonspecific nitrogen for the synthesis of the nutritionally dispensable amino acids (aspartic acid, asparagine, glutamic acid, alanine, serine) and other physiologically important nitrogen-containing compounds such as nucleic acids, creatine, and porphyrins.

Healthy people eating a balanced diet rarely need protein supplements. Except for a few amino acids, most are readily available in human diet. The limiting amino acids are lysine, threonine, tryptophan and sulfur-containing amino acids.

Protein powders – such as casein, whey, egg, rice and soy – are processed and manufactured sources of protein. These protein powders may provide an additional source of protein for bodybuilders. The type of protein is important in terms of its influence on protein metabolic response and possibly on the muscle's exercise performance. The different physical and/or chemical properties within the various types of protein may affect the rate of protein digestion. As a result, the amino acid availability and the accumulation of tissue protein is altered because of the various protein metabolic responses.

A lack of protein in the diet can lead to muscle wasting.

Edema causes fluid to accumulate in the body.

Dry, sparse hair that falls out easily or changes color or texture is a sign of low protein intake.

Gastrointestinal distress.

Moodiness and anxiousness.

Protein deficiency can also lead to Poor sleep and insomnia.


Every cell, tissue and organ in your body contains the macronutrient protein, which provides your body with energy. The digestive process breaks down protein in food into amino acids that repair and replenish the body. Protein helps build muscles, produce new cells, regulate hormones and enzymes, heal wounds and promote immune function. Low dietary protein is most common in developing countries due to inadequate access to protein-rich foods. However, it can also affect people in developing countries who make poor dietary selections. Insufficient dietary protein can result in many negative side effects.

Muscle Wasting
A lack of protein in the diet can cause muscle soreness, weakness and cramping. Protein supports muscle growth and strength. A lack of protein in your diet reduces muscle strength, muscle function and decreases lean body mass. You also lose body fat, because protein provides structure for adipose tissues. The wasting away of muscle and fat tissue is known as cachexia.

Edema causes fluid to accumulate in the tissues and cavities of the body. Edema most often affects the abdomen, hands, ankles and feet. Protein helps regulate and maintain a proper fluid and electrolyte balance within the body. Not getting enough dietary protein can affect your body’s fluid and electrolyte balance, causing swelling and edema.

Skin and Nail Alterations
A lack of protein in the diet can cause changes in your skin and nails. Protein enables cell regeneration, produces new cells and replaces dead ones. Therefore, if you do not consume adequate amounts of protein, your skin may become very light and burn easily when exposed to sunlight. You may experience cracking, flaking, dryness and rashes of the skin. Delayed wound healing and ulcers are signs of low protein intake. Protein aids nail formation. Protein deficiency can cause white bands or brownish spots on the nails.

Hair Loss
Dry, sparse hair that falls out easily or changes color or texture is a sign of low protein intake. Hair contains 90 percent protein. Protein deficiency results in thinning hair or hair loss. According to the Institute of Medicine, adult females require 46 grams of protein per day; adult males need 56 grams of protein.

Your immune system needs protein to protect your body and defend against foreign bodies such as bacteria and viruses. When your body does not have the right amount of protein, the number of new white blood cells decreases. This results in a weakened immune system and increased risk of infection.

Gastrointestinal Distress
Low protein stores cause you to feel lethargic, fatigued and weak. You may also experience headaches, nausea, diarrhea, soreness of the stomach and even fainting. Protein helps transport nutrients within the body, delivering and releasing them where they are needed. When protein cannot perform this function, it disrupts the body’s homeostasis. This may result in a loss of appetite, irritability, insomnia, apathy, and the inability to stay warm.

Moodiness and anxiousness
Amino acids are the building blocks for neurotransmitters which control your mood. Proteins help the brain synthesize hormones like dopamine and serotonin that help bring on positive feelings like calm, excitement and positivity.

Poor Sleep
Poor sleep and insomnia can sometimes be linked to unstable blood sugar levels, a rise in cortisol and a decrease in serotonin production. Blood sugar swings during the day carry over through the night. Carbohydrates require much more insulin than fat or protein does. Eating foods with protein before bed can help with tryptophan and serotonin production, and they have a minimal effect on blood glucose levels; in fact, protein slows down the absorption of sugar during a meal.

Brain Fog
Protein is needed to support many aspects of healthy neurological functioning. Brain fog, poor concentration, lack of motivation and trouble learning new information can be signs that you’re low in neurotransmitters you need to focus including dopamine, epinephrine, norepinephrine, and serotonin. Neurotransmitters are synthesized in the brain using amino acids, and studies show that balanced diets with enough protein can boost work performance, learning and motor skills.

Slow Heal
A low protein diet can raise your risk for muscle loss, falling, slow bone healing, bone weakness, fractures and even osteoporosis. Protein is needed for calcium absorption and helping with bone metabolism. Studies show that older adults with the greatest bone losses are those with a low protein intake of about 16–50 grams per day. Research also shows that a diet high in amino acids can help treat muscle loss due to aging.

Over-consumption of protein risks kidney stone formation from calcium in the renal circulatory system.


The body is unable to store excess protein. Dietary protein is converted to individual amino acids by the digestive process, which are then absorbed. When amino acids are in excess of needs, the liver takes up the amino acids and subjects them to deamination, a process that converts the nitrogen from the amino acids into ammonia, further processed in the liver into urea via the urea cycle. Excretion of urea is performed by the kidneys. Other parts of the amino acid molecules can be converted into glucose and used for fuel. When food protein intake is periodically high or low, the body tries to keep protein levels at an equilibrium by using the "labile protein reserve" to compensate for daily variations in protein intake. However, unlike body fat as a reserve for future caloric needs, there is no protein storage for future needs.

Research has supported a theory that excessive intake of protein increases calcium excretion in urine, occurring to compensate for the pH imbalance from oxidation of sulfur amino acids. The research is inconclusive as to whether this calcium excretion from bone resorption contributes to osteoporosis. A regular intake of calcium stabilizes this loss.

Another issue arising from over-consumption of protein is a higher risk of kidney stone formation from calcium in the renal circulatory system.

An epidemiological study from 2006 has found no relationship between total protein intake and blood pressure; it did, however, find an inverse relationship between vegetable protein intake and blood pressure.



Each person is unique in terms of their exact protein needs; your body weight, gender, age, and level of activity or exercise all determine how much protein is best for you, and your needs likely vary a bit day to day.

• According to the USDA, the recommended daily minimum intake of protein for adults who are at an average weight and activity level is: 56 grams per day for men, and 46 grams per day for women.

• However these are considered minimum amounts, so they might be too low if you’re very active, pregnant or ill.

• These amounts are equal to eating about 0.36 grams of protein for every pound that you weigh, however some people find that they feel better when they increase their protein intake and aim to eat about 0.5 grams of protein for every pound.

• This higher recommendation would translate to a woman who weighs 150 pounds eating about 75 grams of protein daily, and a man who weighs 180 pounds eating about 90 grams.

• If all the math seems confusing, remember that most experts recommend consuming about 20–30 percent of your overall calories from protein foods.

Read more