What actually makes something a “protein”?

Is collagen a protein?

I recently read an interesting piece on collagen, pointing out that although it’s dominated by just a few amino acids — mainly glycine, proline, and hydroxyproline — none of them are classed as "essential".

Which on the face of it suggests its maybe not very useful?

But dig deeper and it turns out glycine, in particular, is not always readily made in sufficient amounts by the body and is a major component of skin and connective tissue.

Amino Acids and protein status

That got me wondering what amino acids actually need to be present for something to qualify as “protein”?

There are 23 amino acids used by the human body.


Nine are essential — meaning we must get them from food - we can't make them.

But there is no minimum number of different amino acids required for something to be labelled a "protein".

So collagen absolutely is a protein, even though:


• It’s dominated by glycine, proline and hydroxyproline


• It lacks several essential amino acids

What makes a 'complete protein'?

A complete protein, however:


• Contains all 9 essential amino acids


• In proportions adequate for human needs

Those nine are:


Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.

So collagen is a protein, but not a complete protein.

In a nutshell: 
You couldn’t live on it — but it may still be useful for maintaining healthy skin, joints and tendons.

Complete proteins by food type

For interest naturally complete protein foods:

Animal-based (almost all complete):


• Eggs (often used as the reference standard)


• Meat (beef, pork, lamb)


• Poultry


• Fish & seafood


• Dairy (milk, yogurt, cheese, whey, casein)

Animal proteins tend to have:


• High digestibility


• High leucine (important for muscle protein synthesis)

Plant-based complete proteins include:


• Soy (tofu, tempeh, edamame)


• Quinoa


• Buckwheat


• Amaranth


• Chia seeds


• Hemp seeds


• Spirulina

References

Essential amino acids & protein requirements

FAO/WHO Protein and Amino Acid Requirements in Human Nutrition
https://www.fao.org/4/y5686e/y5686e00.htm

Collagen amino acid composition

PubMed: Roles of Dietary Glycine, Proline and Hydroxyproline in Collagen Synthesis
https://pubmed.ncbi.nlm.nih.gov/28929384/

Glycine may be conditionally essential

PubMed: Glycine Metabolism and Its Alterations in Obesity and Metabolic Diseases
https://pubmed.ncbi.nlm.nih.gov/30856015/

PubMed: The Role of Glycine in Human Health
https://pubmed.ncbi.nlm.nih.gov/20093739/

Collagen as an incomplete protein

USDA FoodData Central (collagen/gelatin protein composition data)
https://fdc.nal.usda.gov/

Harvard T.H. Chan School of Public Health – Protein
https://nutritionsource.hsph.harvard.edu/protein/

Leucine and muscle protein synthesis

PubMed: Leucine Regulates Translation Initiation of Protein Synthesis in Skeletal Muscle After Exercise
https://pubmed.ncbi.nlm.nih.gov/14978094/

PubMed: Leucine as a Treatment for Muscle Wasting
https://pubmed.ncbi.nlm.nih.gov/25444557/

Complete proteins and plant protein quality

Harvard T.H. Chan School of Public Health – Protein
https://nutritionsource.hsph.harvard.edu/protein/

British Nutrition Foundation – Protein
https://www.nutrition.org.uk/healthy-sustainable-diets/protein/

Good sources of protein and amino acid quality

National Institutes of Health (Protein Fact Sheet)
https://www.ncbi.nlm.nih.gov/books/NBK594226/