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 (9).
As we touched on earlier, 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 lycine. Legumes, don’t fall too far short of being a ‘complete proteins’ but have a lower methionine content.
Protein digestibility refers to how much of the absorbed protein is consumed by the organism. One of the main measures 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 utilization 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%.
Should I worry about complete protein? Can I get all EAAs from plants?
The old nutritional way of thinking leads us to believe plant-based foods were not complete proteins as they do not contain all of the EAAs. This resulted in a way of eating involving ‘complementing proteins’, where it was thought different plant-proteins must be consumed in the same meal to ensure all EAAs were consumed e.g. brown rice and beans should be eaten together. What we know today is that in fact plant proteins DO contain all of the EAAs, however other than soy, one or two of the EAAs are generally limited.
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 (11). 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 (12). 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.
The bottom line
Not all proteins are created equally. Typically, all animal proteins are complete proteins and more digestible while plant foods more frequently have a lower percentage of at least one EAA. Eating complete proteins is not essential, you’ve just got to eat a little more tactically by consuming a variety of protein sources across the day. Those consuming a plant-based diet have somewhat higher protein requirements as plant proteins are slightly less digestible. Animal proteins are rich sources of important nutrients vitamin B12, vitamin D, heme-iron, zinc and omega-3 essential fatty acid but have been associated with higher risk of coronary heart disease. Plant proteins 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. They are associated with lower BMIs, diabetes, obesity risk and colorectal cancers.
Chapter 6: How to choose my plant-based protein powder?
Chapter 7: Why is pea protein the best plant protein?
Chapter 9: Where should I start?