Protein quality and recommendations

For optimal synthesis of body protein, all the aminoacids need to be present in adequate quantities and proper proportions. Any essential aminoacid deficit will drastically limit the synthesis of much needed protein.

You need to learn about the concept of limiting protein. The limiting protein is the least abundant aminoacid in a dietary protein source (for example for wheat the limiting protein is the lysine). By combining different dietary protein sources you will provide with a wide array of aminoacids.

Protein quality has two variables. First is the digestibility (what percent of the dietary protein is absorbed into the body), second being the aminoacids composition (the higher the similarity of aminoacids composition and the average body protein, the higher is the quality, so, obviously, animal proteins are better than plant proteins, unless you combine various sources). A meal or a diet should have a mixture of different  proteins. A mixture of plant-based proteins will have a more favorable aminoacids composition than one individual plant. This is called protein complementation - combining more proteins to achieve higher protein quality (grains are low in lysine, and beans are low in methionine, but together they are well balanced).

Methods to measure protein quality:

1. Biological value (BV) - measure of the protein percentage absorbed from a food to be incorporated into the body.

2. Protein efficiency ratio (PER) - weight gain divided by its intake of a particular protein during test period. PER was the official method before getting replaced by PDCAAS (protein digestibility corected aminoacid score). AAS (aminoacid score) is calculated for any particular protein comparing the level of limitating aminoacid in the protein towards the level of the same aminoacid in a reference protein. PDCAAS for milk is 1, for isolated soy protein is 1, for wheat is 0.4, for beans is 0.6-0.7, but wheat combined with beans is almost 1. PDCAAS is easy to calculate , based on the human aminoacids requirements.

DIAAS (digestible indispensable aminoacid score) - the diference is that, while for PDCAAS indigestability of a particular protein is measured in stool, for DIAAS is measured in terminal ileum, being more accurate.

The recommended protein intake is 0.8 grams per kilogram weight (56grams for a 70kg adult). Most people have more than RDA (required daily ammount), as protein is 10-25% of our total energy intake. You will have above 20% if you eat mostly meat, eggs and dairy. Endurance and strenght athletes eat 1.2-1.7 grams per kilogram (and many do not know that you can achieve this through diet alone, without any supplementation). According to the Journal Of Sport Sciences, it is ideal to consume them as soon after exercise, for them to be optimally used.

Beware: even if it is unproven, there are some potential health effects of protein that are promoted by media.

- High protein intake to be avoided by patients with kidney disease.
-No conclusive evidence to link proteins to cardiovascular diseases, cancer or osteoporosis.
-No conclusive evidence that there is a relation between protein intake, energy intake and body-weight.
-Between animal and plant-based proteins, the animal food is bad because of the package (more fats, salt and carbs), not because the proteins from the aminal sources are bad.

Next posts will be about energy requirements and energy balance. Have a wonderful day!
G.

Protein turnover and nitrogen balance

Today i will speak a bit about protein synthesis and protein degradation. The protein balance, also known as nitrogen balance, is the balance between protein input and protein output. The Protein turnover consist mainly in changing the ingested proteins into aminoacids, and then using the aminoacids resulted to synthesize the proteins needed in our body. For some proteins the turnover is fast (less than 2 hours), for others can be slow (up to 12 months - structural proteins). But on average in 12 months our body is completely renewed.

When we talk about protein balance, we are thinking at the input and output in terms of carbon = energy and urea = nitrogen. Because most of the nitrogen in the body is on protein form, measuring the nitrogen intake and excretion results in finding the nitrogen balance, which is more or less equal with the protein balance.

In normal conditions the protein input is equal with the protein output. In some cases the input is bigger than the output, we are talking about a positive balance (this will happen in pregnancy, growing-up children, gaining weight through bodybuilding or in recovering patients). Negative balance (when the input is lower than the output) happen in case of illness, losing weight, low protein intake, burns or heavy trauma.

On average we will have 80 grams of dietary protein and 70 grams of residual protein, 150 grams of protein in total. The output will be 150 grams also, mainly urea and undigested proteins via stools with the carbon used for energy.

The aminoacid will divide in urea (the nitrogen part) and carbon dioxide CO2 and water (from the carbon part). Some aminoacids carbon part can be converted into glucose (gluconeogenic aminoacids). If they cannot be converted, they are called ketogenic aminoacids. The gluconeogenesis process will start during prolonged fasting to maintain the blood sugar levels. But except for this case, most people will increase the protein degradation if they increase the protein intake.

Protein functions

The proteins are used for a multitude of functions in the cells and in the tissues. The function of a protein is determined by two factors: the aminoacids sequence and the folding of the polypeptide chain. Thinking about this, a persistent faulty incorporation of one incorrect aminoacid in the polypeptide chain can cause a severe disruption of the protein function, being the basis of many genetic diseases.

The main protein functions in our body are:

1. Building material. They got an important structural role outside and inside the cell. The main structural protein is collagen, which is also the most abundant, being found in skin, tendons, cartilage, bone, connective tissue and giving structure and strength to tissues and cell. Defects in collagen synthesis can result in brittle bones (osteogenesis imperfecta). 

2. Enzymes. Are defined as proteins that speed up a biochemical reaction, having a catalyst function, by lowering the activation energy barrier needed to be overcome for a reaction to happen. Every cell has more than one thousand enzymes responsible for different reactions (like those required to generate energy by breaking down glucose or fatty acids).

3. Transporters (totally unrelated to Jason Statham movie). They offer assistance with the transport of different molecules across the cell membrane (in and out).

4. Hormones. They are messengers circulating via blood stream, released from a specific tissue into the blood to signal something to distant tissues ( like the insulin released by pancreas to reach muscle and fat tissue to promote glucose uptake). Chemically speaking, the hormones are either polypeptides or steroids.

5. Antibodies. They are involved in defending the body against pathogens such are viruses and bacteria, and are called immunoglobulins, secreted by plasma cells, with the role of creating an immune system.

6. Regulation of fluid balance. They are taking care of the water being appropriately distributed across the blood stream (intravascular), in the space between the cells (intercellular) and inside the cells (intracellular).

Next post will be about protein turnover and nitrogen balance, and maybe some more extra bits. Have a nice day. See you soon.

Hope

She was leaning forward, ready for a kiss. But he put a finger on her lips, whispering:
"Not now. Find me later"

Opening her eyes, Maria realized that she is still in her home, on her sofa. And then, her eyes could see the address card laying in the middle of the bowl, on her coffee table. It was not just a dream after all.

It was a black, metallic, little square of paper, with golden letters. Queen Industries, followed by an address." It was not a dream after all. Or i just fall asleep and wake up in the middle of a full Marvel nightmare movie." she was thinking.

With a quick move, she took the card and throw it into the recycle bin. All was good again.

Protein absorption and digestion

Let's talk about the digestion of protein, through polypeptides, all the way to the final product, the aminoacids.

The digestion of the protein starts in the stomach, where the enzyme called pepsin will break the polypeptides into smaller parts. The highly acid medium will help this to happen. The digestion continue in the duodenum and upper small intestine, where the pancreas will secrete another enzyme called chymotripsin, with the role of breaking the small polypeptides into even smaller polypeptides. The final work is done in the lower small intestine, where carboxypeptidase and aminopeptidase enzymes will break everything into aminoacids, and they will be absorbed into the blood stream. About 5% of the proteins will leave the body undigested, through feces.

The single molecules of aminoacids are taken up into the blood stream and distributed across the human body, used to synthesize the so much needed proteins (like albumin in liver or muscle proteins in muscle) and they can have extracellular functions, such as hormones and cell adhesion, or intracellular, such as generating fuel, cell structure, signal transduction. You can find more than 100.000 proteins in our body.

The protein digestion starts in the stomach, when the acid environment causes proteins to unfold, allowing pepsin to access the dietary protein easier. The pepsin enzyme is produced by the Chief cells lining the stomach, and it is present as an inactive pro-enzyme called pepsinogen, being activated by the high acidity. The pepsin will cleave the peptide bonds, creating small polypeptide from the protein. The process is further going into the small intestine (upstream), with the polypeptides being cleaved into even smaller polypeptides under the influence of trying and chymotripsin (which are also created by pancreas as inactive pro-enzymes, being activated when they reach the intestine). In the downstream on the small intestine, the polypeptides are finally transformed into individual aminoacids, under the action of aminopepsidase and carboxypepsidase (both of them produced by the intestinal cells). Those last mentioned enzymes will remove single aminoacids from each end of the peptide. The single aminoacids are taken to the liver, after they were being absorbed via portal circulation, and to the rest of the body, used as building block for the synthesis of the body proteins. In the liver, they are mainly used to synthesize the main serum protein called albumin.

In the next post I will talk about the protein functions (enzymes, structural, hormones, transporter, antibodies and so on).