The main concern shared by people wanting to adopt a more plant-based way of eating is whether they will still meet their protein requirements when eating less or no meat. Rest assured that it is entirely possible to meet all your protein requirements and build muscle on a plant-based diet. The Academy of Nutrition and Dietetics state vegetarian diets that include a variety of plant products provide the same protein quality as diets that include meat (1).
The biological value or quality of a protein is determined based on both its amino acid composition, and how well it is digested. Therefore, not all proteins are created equal. Regardless of whether a protein comes from a plant, or animal source, the types of amino acids, as will its digestibility will vary between foods. Let’s explore these concepts a little further.
Amino Acid Composition
All proteins are made up of amino acids. When a protein is consumed it breaks down into amino acids and these are used by the body for different processes, like building muscle. If you happened to read our earlier protein guide, you may recall that a complete protein contains all of the 9 Essential Amino Acids (EAAs) that your body needs for optimal function. Typically, all animal proteins – meat, dairy, eggs and fish – are complete proteins or ‘high quality’. Plant foods more often than not have a lower percentage of at least one essential amino acid, commonly lysine. Legumes, don’t fall too far short of being a ‘complete proteins’ but have a lower methionine content.
Protein Digestibility
Protein digestibility refers to how much of the absorbed protein is consumed by the organism.
The measure of a protein’s quality is referred to as its PDCAAS (Protein Digestibility Corrected Amino Acid Score). The score ranges from 0-1 with 1 being the highest quality, and 0 being the lowest. Our body can’t digest protein with a low PDCAAS very well.
Digestibility is related to the presence of EAAs. If food is lacking in one of them - we talk about a limiting amino acid. Animal foods are more digestible because they are complete and contain all the EAAs, which improves the utilisation of these components so that the body can manufacture its own proteins. Animal protein have very high digestibility, with eggs and milk protein being the easiest to absorb.
Those consuming a plant-based diet have somewhat higher protein requirements as plant proteins are slightly less digestible when compared with animal proteins. Since they generally have one limiting essential amino acid, the use of plant protein by the body rarely is rarely over 85%.
Here is a ranking of common food ranked by PDCAAS score.
| Source Of Protein | PDCAAS |
|---|---|
| Whey Protein | 1 |
| Cow's Milk | 1 |
| Caseine (Milk Protein) | 1 |
| Eggs | 1 |
| Mycoprotein (From Fungi) | 0.99 |
| Beef | 0.92 |
| Pea Protein | 0.89 |
| Sacha Inchi Powder | 0.87 |
| Chickpeas / Soybeans | 0.78 |
| Black Beans | 0.74 |
| Vegetables | 0.73 |
| Legumes | 0.70 |
| Hemp Seeds | 0.66 |
| Fresh Fruit | 0.64 |
| Cooked Peas | 0.59 |
| Peanuts | 0.52 |
| Rice | 0.50 |
| Wheat | 0.42 |
| Wheat Gluten | 0.25 |
The digestibility of a protein is also in part determined by how processed the food is. The less processed the food, the less digestible its protein is. Generally speaking, plant proteins are digested at rates of 70-90% whereas soy, casein and egg have values of around 97% and above (2). Protein requirements are therefore around 10-20% higher for plant-based eaters to account for this.
Should I worry about complete protein? Can I get all EAAs from plants?
For a protein to be used by the body, it is necessary that the protein contains all the EAAs in the required proportions. Animal protein are of high biological value because they are complete. Vegetable protein are of lower quality because they contain EAAs but some of them in small amounts.
To avoid these shortcomings when following a plant-based diet, various plant foods have to be combined. Today it is thought that the liver has the ability to store EAAs throughout the day so complementing plant proteins do not need to be consumed in the same meal so long as a variety of foods are consumed across the day. Eating complete proteins is therefore not essential, you’ve just got to eat a little more tactically. People who consume a well-balanced diet will consume all of the EAAs throughout the course of the day without too much extra effort. Eating a variety of different plant foods like grains, legumes and vegetables, will make sure you’re getting all of the EAAs your body needs.
Nutritional Value of Animal Protein
While animal proteins are highly regarded for their quality, they have also been associated with higher intakes of saturated fats and cholesterol, particularly when they come from processed meat. While fat is an essential nutrient for the body, saturated fat should be consumed in small amounts as it increases LDL cholesterol - also known as ‘bad cholesterol’. LDL cholesterol carries cholesterol from the liver to other parts of the body where it risks building up and causing a blockage in the blood vessels which increases the likelihood of strokes and heart attacks. On the flip side, animal protein has been shown to result in greater gains in lean body mass in the elderly when compared to plant protein (KK). Foods that are rich in animal protein are generally high in vitamin B12, vitamin D, heme-iron, zinc and DHA (an omega-3 essential fatty acid), some of these nutrients are often lacking in plant foods.
Nutritional Value of Plant Protein
Plant proteins are associated with a frequent reduction in intakes of saturated fats and cholesterol. People on plant-based diets tend to have lower Body Mass Indexes (BMIs), diabetes and obesity risk, favourable lipid profiles, and reduced risk of oesophageal, lung, stomach and colorectal cancers. Plant proteins are often accompanied by carbohydrates and fibre, which are important energy sources as well as having a significant role in maintaining a healthy gut environment and managing cholesterol levels. Plant foods like legumes, nuts and seeds also contain the essential fatty acids omega-3 and omega-6, and are rich in vitamins and minerals including vitamin C, potassium, magnesium and calcium.
