Why is soybean protein considered a complete plant protein?

Soybean protein is considered a complete plant protein because, unlike most other plant-based foods, it contains all nine essential amino acids in proportions that align well with human nutritional requirements.

In human nutrition, amino acids are the building blocks of proteins. While the body can synthesize some, there are nine "essential" amino acids—including histidine, lysine, and methionine—that must be obtained through the diet. Most plant sources are considered "incomplete" because they are low in one or more of these specific amino acids, which are known as limiting amino acids. Soybeans are a notable exception to this rule, providing a high-quality protein profile similar to that of animal-derived foods like meat, fish, and dairy products.1