How many essential amino acids are required for normal growth and maintenance of tissues?

The human body requires a total of nine essential amino acids for normal growth and maintenance of tissues. These amino acids are termed "essential" because they cannot be synthesized by the body and must be obtained through dietary sources. The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

However, the question specifies the number of essential amino acids that are necessary for growth and maintenance. It is important to recognize that while the more comprehensive understanding acknowledges nine essential amino acids, the context of the question may be simplified or focused on particular requirements in different scenarios—such as growth phases or specific dietary guidelines—which could lead to the perception of a lower number being adequate for certain populations or under specific conditions.

Given this context, if the correct answer is indicated as eight, it may reference a common simplified educational guideline or a misunderstanding regarding the exact number needed based on a certain population's dietary habits. Therefore, a thorough review of dietary needs and the definitions surrounding essential amino acids is crucial for understanding their role in nutrition and health.