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Most people associate protein with strength and meat with protein. So is eating tons of meat efficient in making you stronger? Not really. Meat is one good source of protein but so are eggs, milk, and grains & vegetables. Learn about protein quality and available protein sources in your diet.

Amino Acids

Amino means “nitrogen containing”. Amino acids are the building blocks of proteins. There are a total of 20 amino acids; 9 are essential (body can’t synthesize them) amino acids and the remaining 11 amino acids are non-essential (body can synthesize them).

Essential Amino Acids
Non-Essential Amino Acids

Histidine

Alanine

Isoleucine

Arginine

Leucine

Asparagine

Lysine

Aspartic Acid

Methionine

Cysteine

Phenylalanine

Glutamic Acid

Threonine

Glutamine

Tryptophan

Glycine

Valine

Proline

Serine

Tyrosine

 

Amino acids are connected to each other via condensation reactions, forming peptide bonds. When numerous amino acids join together then they form a polypeptide, which essentially are called proteins.

Steps in Protein Synthesis

Protein synthesis occurs at the cellular level by ribosomes, which actually make proteins for intracellular and extracellular purposes. Proteins for intracellular purposes are synthesized by free-floating ribosomes in the cytoplasm, whereas proteins for extracellular purposes are synthesized inside the rough endoplasmic reticulum. 

  1. DNA in nucleus contains information necessary for protein synthesis
  2. The transcription of the DNA information results in a message (called mRNA), which contains the “blueprint” for protein synthesis
  3. mRNA exists the nucleus and moves to ribosomes, where protein synthesis occurs based on the information from the mRNA
  4. Transfer RNA (tRNA) transport amino acids, which are building blocks for proteins, to the ribosomes
  5. The message contained in the mRNA is being translated (translation) and amino acids are linked together to form proteins

Digestion & Absorption of Protein

When you eat foods that contain protein then the body doesn’t store that food protein directly but breaks it down to amino acids from which the body then can make its own protein.

Proteins are partially broken down in the stomach via the enzyme pepsin before they move into the small intestine, where they are further broken down and digested.  The amino acids can be used to provide energy or to make other needed compounds. All amino acids that are not immediately being used are transported to the liver via capillaries.

Protein Quality

The quality of protein is based on two factors:

  1. Digestibility
  2. Amino Acid Composition

In order to provide the amino acids for protein synthesis, the body breaks down the protein from food sources into amino acids. The protein’s food sources influences its digestibility and hence rate of availability. In general, animal proteins (90 – 99%) have a higher digestibility than plant proteins (70 – 90%), soy protein (>90%) being the exception.

In order to synthesize protein, the body needs to have all the amino acids that are needed available at once. The liver can produce any non-essential amino acids but the diet has to supply any essential amino acids, otherwise the body breaks down its own protein (e.g. muscle protein) to obtain them. In other words, the more essential amino acids the protein provides, the higher its quality.

Apart from soy protein, plant protein from vegetables, nuts, seeds, grains, and legumes are lower in quality because they lack one or more essential amino acids. Consuming a combination of the aforementioned vegetable proteins enhances the quality of proteins but it is not very convenient.

Food Sources for High-Quality Protein

  • Meat
  • Fish
  • Poultry
  • Soy Products
  • Cheese
  • Eggs
  • Yogurt
  • Milk

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