Are ‘Incomplete’ Proteins a Myth?

Protein is made up of building blocks called amino acids. Although hundreds of amino acids exist in nature, only 20 are needed to make all of the protein found in your body. These can be split into three main categories (1):

• Essential amino acids. This category consists of nine amino acids that your body cannot make. Your diet is the only way you can get these.
• Non-essential amino acids. This category includes the remaining 11 amino acids, which your body can typically make from the 9 essential amino acids.
• Conditionally essential amino acids. These amino acids are typically considered non-essential but become essential during adolescence, pregnancy, or under certain conditions, such as trauma or illness.

Foods that contain good amounts of all nine essential amino acids are generally considered sources of “complete” protein, whereas those that don’t are labeled as “incomplete” protein.

SUMMARY

Protein is made from amino acids, which can be categorized as essential, non-essential, or conditionally essential. Protein-rich foods are typically categorized as “complete” or “incomplete” based on the amount of essential amino acids they contain.

Contrary to popular belief, most foods — both animal- and plant-based ones — contain all nine essential amino acids. The difference lies in the amounts of them they offer.

For instance, meat, fish, eggs, and dairy contain high levels of all nine essential amino acids. On the other hand, plants tend to contain low amounts of at least one or two essential amino acids, depending on the category to which they belong.

For example, legumes and veggies tend to be low in methionine and cysteine, while grains, nuts, and seeds tend to be low in lysine (2).

In practical terms, this means that following a diet providing too little of either food group may cause you to get insufficient amounts of essential amino acids.

This is why animal-based foods are typically considered “complete” sources of protein, while most plant-based foods are considered “incomplete.”

The exceptions are soy, quinoa, amaranth, buckwheat, and nutritional yeast, as well as hemp and chia seeds. These plant foods offer good amounts of all nine essential amino acids and are considered “complete” sources of plant protein.

SUMMARY

With a few exceptions, most plant-based foods are typically viewed as “incomplete” sources of protein. In contrast, animal-based foods are considered “complete” proteins.

Many people believe that due to vegetarian and vegan diets’ low content of animal-based protein, they often lack sufficient amounts of “complete” protein.

However, apart from a few exceptions, this is very seldom the reality.

Currently, there’s no evidence of protein deficiency among vegetarians or vegans, except perhaps in the small percentage who eat too few calories or follow monotonous or restricted eating patterns, such as fruitarian or potato-based diets (3).

Nonetheless, the protein found in plants can be slightly more difficult for your body to absorb, compared with the protein in meat and other animal-based foods.

This is why vegetarians and vegans are sometimes encouraged to eat slightly more protein than meat eaters — that is, around 0.5 grams per pound (1 gram per kg) per day (4).

That said, the current evidence suggests that this difference in absorption is likely too minimal to cause vegetarians or vegans to get insufficient amounts of essential amino acids from their diet (3).

In short, as long as a plant-based diet remains rich enough in calories and offers a good variety of protein sources, there’s little reason to worry about getting too little “complete” protein on a vegetarian or vegan diet.

SUMMARY

Vegetarians and vegans should have little difficulty getting enough “complete” protein from their diet — that is, as long as it remains varied and rich enough in calories.

Protein-rich foods that contain good amounts of all nine essential amino acids are typically considered “complete” sources of protein, while those that don’t are labeled as “incomplete” sources.

This causes most plant foods to be viewed as “incomplete” proteins, perpetuating the myth that plant-based diets may not offer the right amount or type of protein.

That said, as long as a plant-based diet contains a good variety of food groups and enough calories, there’s little reason for vegetarians or vegans to worry about “complete” or “incomplete” proteins.

Note that this applies to healthy individuals of average weight. However, nutrient needs may vary depending on your activity level, body weight, and health status. If you’re losing weight or lack energy, consult a medical professional or registered dietitian.