Proteins and Amino Acids: Sources and Requirements

Proteins and Amino Acids: Dietary Sources, Allowances, Deficiency, and Risks

Proteins are polymer chains made of amino acids linked together by peptide bonds.
The most important aspect and defining characteristic of protein, from a nutritional standpoint, is its amino acid composition.
1g of protein = 4kcal.
Physiology:
- Proteins are broken down in the stomach during digestion by enzymes known as proteases into smaller polypeptides to provide amino acids for the body.

Amino acids can be divided into either essential amino acids or non-essential amino acids.
The essential amino acids, which must be obtained from food sources, are leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine, and histidine.
Non-essential amino acids can be made by the body from other amino acids.
- They are arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine.

Whole protein foods (such as milk, meat, fish, egg, and vegetables).
A variety of protein powders (such as casein, whey, egg, rice, and soy).
- These provide an additional source of protein for exercising muscles.
Whole grains and cereals (buckwheat, oats, rye, millet, maize (corn), rice, wheat, bulgar, sorghum, amaranth, and quinoa).
- However, these tend to be limiting in the amino acid lysine or threonine.
Legumes (more complete sources of protein than whole grains and cereals):
- Examples 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, sesame seeds, and sunflower seeds.
- On a worldwide basis, plant protein foods contribute over 60 percent of the per capita supply of protein, on average.
- In North America, animal-derived foods contribute about 70% of protein sources (meat, milk products, eggs, and fish).
- 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.

Can lead to mental retardation and kwashiorkor.
Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs.
Marasmus – inadequate energy intake overall (including protein).
Kwashiorkor – inadequate protein intake with adequate calorie intake.

The body is unable to store excess protein.
Protein is digested into amino acids, which enter the bloodstream.
Excess amino acids are converted to other usable molecules by the liver in a process called deamination.
- Deamination converts nitrogen from the amino acid into ammonia.
- Ammonia is then converted by the liver into urea (in the urea cycle).
- Excretion of urea is performed by the kidneys.
- Excess protein may also increase renal calcium excretion; therefore, calcium supplementation should be given.

When there is excess protein intake, amino acids can be converted to glucose or ketones, in addition to being oxidized for fuel.