Written by Associate Nutritionist Spela Horjak 

Checked by Registered Dietitian Nishti

As more and more people are shifting towards plant-based diets, the question of protein quality is one that pops up quite often. Plant protein is often referred to as ‘incomplete’ or ‘lower quality’ making it sound inferior in comparison to animal sources of protein. 

But is this really something we should be worried about?

Read on to find out.

The truth about plant-based protein quality

  • Protein is made up of chains of amino acids which are the building blocks of protein. Nine of those are essential amino acids (EAAs), meaning the body cannot produce them so they need to be obtained through dietary protein.

    These amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and histidine (1).

    Protein effectiveness is judged by its portfolio of amino acids and how well it is digested and absorbed in the body (2).

Complete vs Incomplete Protein

When a protein contains all nine EAAs in adequate amounts, it is referred to as ‘complete protein’. All animal sources of protein (meat, fish, dairy, eggs) contain high levels of all nine.

There is a common misconception that plant-based sources of protein are ‘incomplete’ in that they are often missing at least one of the essential amino acids, making them suboptimal. The truth is all plants contain all nine essential amino acids but some are more abundant than others.

Amino Acids in Plant Proteins

Many plant-based proteins contain a sufficient amount of all amino acids, making them complete sources: soya & soya products, Quorn mince (made of mycoprotein), quinoa, buckwheat, peanuts, chia seeds, hemp seeds, and algae.

Most plant-based protein-rich foods contain adequate amounts of at least eight of the nine EAAs. Others may have one or two limiting amino acids (normally lysine or methionine), however, this can be made up for by eating a wide variety of foods with various amino acid profiles.  

  • Legumes – low in methionine, high in lysine
  • Grains, cruciferous vegetables, green vegetables – low in lysine, high in methionine

We can get enough lysine by eating 2-3 portions of legumes a day (80g each). Foods that are high in methionine are many nuts, seeds, and grains.  


Protein digestibility 

Protein digestibility describes how well a particular protein can be digested and absorbed for use in the body.

Plant proteins are less digestible when compared to animal proteins (3).

For this reason, the Vegetarian Resource Group recommends increasing the daily protein intake for vegans and vegetarians from 0.75g to 0.9g per kg (0.34g/pound) of body weight (4).


Examples of protein requirements for various body weights:

Body weight in kg (lb)

Protein intake (g)

Protein in calories

60 (132)



70 (154)



80 (176)



To demonstrate the difference in protein digestibility, here’s a table showing the digestibility of various animal and plant protein sources (adapted from (5)).

Protein source

True digestibility (%)



Wheat flour, white


Milk, cheese


Soya protein isolate


Peanut butter


Meat, fish


Rice, polished


Wheat flour, whole


Soy flour








  • Some plant protein sources are digested better or equally as animal proteins, while others have a slightly lower score. This is why vegetarians and vegans should aim for a slightly higher protein intake to make up for the fact that we don’t digest the whole plant.
  • These differences, however, aren’t significant enough to worry about protein deficiency among vegans and vegetarians. For someone that has a specific goal they are working towards e.g. vegan athletes, they will need to be more intentional about the type and amount of plant-based protein ingested.

The myth of combining protein

The old school of thought suggested that various plant-based protein sources had to be combined in order to obtain a full profile of amino acids in sufficient amounts at each meal since proteins don’t get stored in the body like some other nutrients.

For example – legumes (that are lower in methionine) were to be combined with rice (that is lower in lysine) so that together they can form the ‘complete’ protein.

We now understand that combining sources of protein in each meal isn’t strictly necessary, however, it still makes sense to include a wide variety of foods in our diet so it’s a good thing to aim for.

Proteins are broken down into amino acids and stored in a pool of free amino acids from which any protein that is needed is synthesised (8) so as long as we eat a varied diet and eat enough, the body will have access to all amino acids it needs.

The ‘Protein Package’

Research has highlighted the health benefits of a diet where most protein comes from plant-based sources.

There is typically a huge difference in the overall dietary pattern when comparing vegetarians and non-vegetarians (7). For example, non-vegetarians consume more saturated fat which is associated with an increased risk of heart disease. There is significantly less saturated fat in plant protein in comparison to vegan protein (mainly in coconut and palm oil). There is also no cholesterol in plants.

On the other hand, vegetarians consume more fiber in the form of fruits, vegetables, beans, legumes, and nuts which lowers the risk of heart disease, stroke, bowel cancer, and Type 2 diabetes (6). Fiber is only present in plant foods, and there is no fiber in animal foods.

This shows us the importance of the ‘protein package’ that the protein comes in – referring to the other nutrients that are obtained through eating plant-based sources of protein.

Here is a longer list of plants containing protein:

Soya products

Soya beans, tofu, tempeh, soya milk, soya mince, edamame, soy flour


Beans, dry peas, chickpeas, split peas


Almonds, peanuts, cashews, hazelnuts, pistachios


Pumpkin, hemp, sunflower, flax, sesame, chia seeds


Teff, quinoa, wheat, rice, millet, oats, buckwheat, corn

Starchy vegetables

Potatoes, sweet potatoes, beetroot, butternut squash

Fruits & vegetables

All; highest are spinach, kale, broccoli, asparagus, brussels sprouts, green peas, artichokes, avocado, mushrooms

Meat replacements

Seitan, textured vegetable protein, mock meats, Quorn (made of mycoprotein)


Plant protein quality should no longer be a topic of concern as scientists and health professionals now agree that getting a sufficient amount of protein on a vegan or vegetarian diet is easily achieved when the diet is varied and enough calories are consumed. Not to mention the added benefits that come along with the vitamins, minerals, antioxidants, and fiber present in plant proteins. Need help with your diet or your health? To book a no-obligation consultation please visit the booking page. 



Joye I. Protein Digestibility of Cereal Products. Foods. 2019 Jun 8;8(6):199. doi: 10.3390/foods8060199. PMID: 31181787; PMCID: PMC6617089.


Hoffman JR, Falvo MJ. Protein – Which is Best? J Sports Sci Med. 2004 Sep 1;3(3):118-30. PMID: 24482589; PMCID: PMC3905294.


Tome, D. (2013) Digestibility issues of vegetable versus animal proteins: Protein and amino acid requirements— functional aspects. Food and Nutrition Bulletin, vol. 34, no. 2 Available at : https://journals.sagepub.com/doi/pdf/10.1177/156482651303400225 [Accessed 05 September 2022]


Vegetarian Resource Group



WHO Protien and amino acid requirements in human nutrition



NHS How to get more fibre into your diet https://www.nhs.uk/live-well/eat-well/digestive-health/how-to-get-more-fibre-into-your-diet/


Rizzo NS, Jaceldo-Siegl K, Sabate J, Fraser GE. Nutrient profiles of vegetarian and nonvegetarian dietary patterns. J Acad Nutr Diet. 2013 Dec;113(12):1610-9. doi: 10.1016/j.jand.2013.06.349. Epub 2013 Aug 27. PMID: 23988511; PMCID: PMC4081456.


British Nutrition Foundation The science of protein



Nico S. Rizzo, Joan Sabaté, Karen Jaceldo-Siegl, Gary E. Fraser; Vegetarian Dietary Patterns Are Associated With a Lower Risk of Metabolic Syndrome: The Adventist Health Study 2. Diabetes Care 1 May 2011; 34 (5): 1225–1227


NHS Metabolic syndrome



Qi XX, Shen P. Associations of dietary protein intake with all-cause, cardiovascular disease, and cancer mortality: A systematic review and meta-analysis of cohort studies. Nutr Metab Cardiovasc Dis. 2020 Jun 25;30(7):1094-1105. doi: 10.1016/j.numecd.2020.03.008. Epub 2020 Mar 17. PMID: 32451273.