What are the most likely essential amino acids to be deficient in a vegan diet? Where to find them?
A vegan diet is based only on vegetal foods. If we analyze the amino acid content of different food protein sources (animal and plant proteins), lysine is consistently at a much lower concentration in all major plant-food protein groups than in animal foods (1, 2). Lysine is one of the essential amino acids, thus lysine is most likely to be the first limiting amino acid in diets that are based predominantly on cereal grains (2).
For this reason the American Dietetic Association, in its 2009 position paper on vegetarian diets, recommends an increase of consumption of beans and soy products in order to satisfy the essential amino acids requirements in vegan diets (3).
All essential amino acids originate from plants (and microbes), and all plant proteins have all essential amino acids. Eating a whole food plant-based diet also known as a proper vegan diet or a proper plant-based diet, one can obtain absolutely all essential amino acids. Furthermore, our body has an amino acid reservoir from which it daily sends about 90 grams of amino acids to combine with amino acids from a meal and synthesize complete protein. (1) Even a banana has all essential amino acids. (2)
Being deficient of essential amino acids on a proper plant-based diet where one daily consumes foods from each category such as whole grains, legumes, fruits, nuts, and green leafy vegetables is unheard of.
People who are deficient in essential amino acids are also deficient in other nutrients and people who are deficient are likely either starving or in war zones.