Why did nutrition studies challenge the belief that plant foods cannot provide enough protein?

The belief that plant foods cannot provide sufficient protein is often based on the fact that most individual plant sources are considered "incomplete" proteins, meaning they lack adequate levels of one or more of the nine essential amino acids required by the human body.12

However, nutrition studies have challenged the idea that this makes plant-based diets inadequate by highlighting the principle of protein complementarity. While individual plant foods may be low in specific amino acids—such as lysine, methionine, or tryptophan—a varied vegetarian diet can easily meet human requirements by combining different sources. For example, when legumes (which are high in lysine but low in methionine) are consumed alongside grains (which have complementary strengths), their combined amino acid profiles provide a complete nutritional balance.2

Research confirms that as long as these foods are balanced over the course of a day, the body can effectively synthesize the necessary proteins, rendering the concern over "incomplete" plant proteins largely irrelevant for those consuming a varied diet.2