Many cultures in the world do not eat meat as a primary protein source. Often we find dishes of rice and beans (e.g., rice and black beans, rice and tofu, rice and lentils) in the diet of these cultures. Explain why these combinations of foods are found to meet their protein requirement.

Proteins are made up of amino acids, and there are 20 different types that can be combined to make a protein. Nine of the 20 are essential amino acids, meaning they cannot be made by our bodies and must be obtained from food.

Different foods contain different combinations of amino acids. Most animal-based foods, like meat, fish, and eggs, contain all nine essential amino acids and are known as "complete proteins". However, most plant-based foods do not contain all the essential amino acids.

This is where the concept of "protein combining" comes in. By eating certain combinations of plant-based foods, you can get all of the essential amino acids your body needs. For example, grains (like rice) are low in an essential amino acid called lysine, while legumes (like beans, lentils, and tofu) are high in lysine. So, by eating rice and beans together, you're getting all of the essential amino acids just as you would from eating meat.

Therefore, cultures that do not eat meat as a primary protein source often rely on these combinations of foods to meet their protein requirements.