Nutrition Without the BS (Part III — Protein)

The last macronutrient the human body needs is protein. Proteins are made of 20 different amino acids, linked together by peptide bonds. We can make a distinction between essential and non-essential amino acids. Essential amino acids (9 of them) need to be part of our diet, and the non-essential amino acids can be synthesized from the essential amino acids in our diet.

Digestion and Absorption

Polypeptides are chains of amino acids. Proteins are made of one or more polypeptide molecules. Polypeptides are broken down into individual amino acids in the GI tract.

Digestion and absorption are almost complete by the time the food reaches the lower part of the small intestine and whatever remains will go into the colon and will leave the body undigested. About 5% of the dietary protein leaves the body undigested.

Amino acids are used by the muscles and liver.

Functions of protein in the body

There are over 100,000 different proteins in the body. They can be active outside the cell (called extracellular) or inside the cell (intracellular) and have multiple functions in cells and tissues. The role of a protein is determined by the amino acid sequence and the folding of the polypeptide chain. The persistent faulty incorporation of one incorrect amino acid in the polypeptide chain can severely disrupt the function of a protein and is at the basis of many genetic diseases.

Proteins can fulfill a variety of functions in the body:

• Building material: Many proteins have an important structural role inside and outside the cell. The main structural and most abundant protein in the body is collagen. Skin, tendons, cartilage, bone, and connective tissues contain a lot of collagen. Collagen acts as a scaffold that gives strength and structure to cells and tissues. Defects in collagen synthesis lead to a severe disease called osteogenesis imperfecta, which is characterized by very brittle bones that easily fracture.
• Enzymes: Enzymes are proteins that speed up biochemical reactions. Every cell has thousands of different enzymes that are responsible for many different reactions, including those that are required to break down fatty acids and glucose to generate energy.
• Transporters: Transporters assist with import or export of different molecules across the cell membrane.
• Hormones: Hormones are messengers that circulate in the blood. They are released from a particular tissue into the bloodstream to signal to distant tissues. For example, insulin is released by the pancreas and travels to the muscle and fat tissue to promote glucose uptake. Chemically, hormones are either polypeptides or steroids.
• Antibodies: Antibodies circulate in the body and are involved in immune defense against pathogens such as bacteria and viruses. They are also referred to as immunoglobulins and are secreted by plasma cells.
• Regulation of fluid balance: Proteins play a major role in making sure that water in the body is appropriately distributed across the bloodstream (intravascular compartment), the space between cells (intercellular compartment), and inside cells (intracellular compartment).

Protein Turnover and Nitrogen Balance

Digestion and absorption of dietary protein result in the uptake of amino acids into the bloodstream. The amino acids in the blood are taken up into tissues and used to synthesize body proteins. This process is called protein synthesis. In a healthy adult that is neither gaining nor losing weight, protein synthesis is balanced by protein breakdown (protein degradation). The amino acids released by the protein breakdown end up in the bloodstream and can be reused for protein synthesis. The balance between protein synthesis and degradation is called protein turnover. It means that we continuously break down body proteins into amino acids and re-use the amino acids to make body proteins again.

Every one of the more than 100,000 proteins in the body has a different turnover rate. Some proteins are degraded within minutes after they are newly synthesized. Other proteins are stable for many months. Whether a protein has a slow or a fast turnover rate depends on its function. Structural proteins (like collagen) usually have a slow turnover rate, meaning that they are very stable, whereas many enzymes involved in energy metabolism have a fast turnover rate.

Nitrogen Balance

The nitrogen balance describes the difference between nitrogen intake (via diet) and nitrogen excretion (via urine and a small portion via stools). Amino acids are degraded in the liver.

Nitrogen balance reflects the protein balance because almost all of the nitrogen in our diet is part of dietary protein. Adults are normally in nitrogen balance, which means that the total content of protein and amino acids in the body does not change.

People are in a state of positive nitrogen balance when protein intake exceeds protein excretion. A positive nitrogen balance results in a net gain of protein in the body. Children and pregnant women, as well as patients recovering from a severe illness or trauma, are in a state of positive nitrogen balance.

People are in a state of negative nitrogen balance when protein excretion exceeds protein intake. A negative nitrogen balance results in a net loss of protein from the body. A negative nitrogen balance is never normal. People that suffered substantial trauma or burns or have another type of illness (e.g. cancer or infectious disease) are in a state of negative nitrogen balance. A negative nitrogen balance can also be due to low protein intake.

Breakdown of amino acids generates energy in the form of ATP (adenosine triphosphate). When amino acids are broken down, the amino (=nitrogen containing) portion of the molecule is converted into urea.

The carbon part of the amino acid is further broken down into carbon dioxide and water. The carbon part of some amino acids can also be converted into glucose. These amino acids are called gluconeogenic amino acids.

The carbon part of the remaining amino acids cannot be converted into glucose. These amino acids are called ketogenic amino acids The possibility to convert amino acids into glucose is especially important during prolonged fasting to maintain blood glucose levels. Under those conditions, body proteins are broken down and liberate amino acids that are used to make glucose.

Leptin

The adipose tissue plays a central role in the long-term regulation of food intake. When people gain body fat, leptin release from fat tissue is increased. The increased levels of leptin in the blood reach the hypothalamus where they stimulate satiety cells. Increased satiety leads to a reduction in food intake, which in turn causes decreased energy storage, closing the circle.

When people lose body fat, leptin released from fat tissues is decreased. Reduced level of leptin in the blood stimulates hunger cells in the hypothalamus, provoking hunger. Hunger causes people to consume food, which in turn leads to increased energy storage, again closing the circle.

The reduction in leptin levels during weight loss is believed to be one of the reasons why people have difficulty maintaining weight.

Next: What we really know about obesity.