How many essential amino acids are there?

The correct choice, indicating that there are nine essential amino acids, reflects the established understanding of human nutrition. Essential amino acids are those that the body cannot synthesize on its own and must be obtained through diet. The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

This requirement for nine essential amino acids plays a crucial role in various bodily functions, such as protein synthesis, tissue repair, and the production of hormones and enzymes. Proper dietary intake of these amino acids is vital for maintaining health, particularly for individuals whose dietary patterns may lack certain proteins.

The other choices suggest fewer essential amino acids, but this does not align with the current scientific consensus on human nutrition. An understanding of these nine essential amino acids is foundational for those studying nutrition and dietary planning.